View ISL97675_4795335.PDF datasheet online --- IC-ON-LINE

Datasheet File OCR Text:

4-Channel LED Driver with Phase Shift Control
ISL97675
The ISL97675 is an LED driver that drives 4 channels of low power LEDs from 4.5V to 26V input and up to 45V output. The ISL97675 compensates for non-uniformity of the forward voltage drops in the LED strings with its 4 voltage controlled-current source channels. Its headroom control monitors the highest LED forward voltage string for output regulation, to minimize the voltage headroom and power loss in the typical multi-string operation. The ISL97675 offers two PWM Dimming modes: The part digitizes the incoming 100Hz to 30kHz PWM signal and provides 8-bit PWM dimming with phase shift function. Another mode is direct PWM mode without phase shift, where the dimming frequency follows the input PWM signal and the minimum on time can be as short as 350ns. The ISL97675 features channel phase shift control to minimize the input, output ripple characteristics and load transients as well as spreading the light output to help eliminate or reduce the video and audio noise interference from the backlight driver operation.
ISL97675
Features
* 4 x 30mA Channels * 45V Output Max * 4.5V to 26V Input * Channel Phase Shift PWM Dimming with 8-bit resolution * 0.007% Direct PWM dimming at 200Hz * Current Matching of 1.5% from 1% ~ 100% Dimming * Dynamic Headroom Control * Protections - String Open/Short Circuit, VOUT Short Circuit Overvoltage, and Over-temperature Protections - Optional Master Fault Protection * Selectable 600kHz or 1.2MHz Switching Frequency * 20 Ld QFN 4mmx4mm Package
Applications*(see page 19)
* Netbook Displays LED Backlighting * Notebook Displays LED Backlighting
Typical Application Circuit
VIN* = 4.5V~26V VOUT = 45V max* ILED = 30mA PER STRING
ISL97675
17 FAULT 19 VIN 1 VDDIO
SW 16 OVP 14 PGND 15
3 FSW/Phase Shift
2 EN 20 PWM 18 COMP 13 RFPWM/DirectPWM 4 ISET 8 AGND 9 AGND
FB1 12 FB2 11 FB3 7 FB4 6 * VIN 6V
May 19, 2010 FN7630.0
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2010. All Rights Reserved All other trademarks mentioned are the property of their respective owners.
Block Diagram
OPTIONAL PFET VIN* = 4.5V~26V 10H/3A 4.7F/50V FAULT VIN EN VDDIO REG 4.75V BIAS FAULT FET DRV SW 45V*/30mA PER STRING 48 (4x12) LEDS
ISL97675
O/P SHORT
OVP
OVP
2
FN7630.0 May 19, 2010
OSC & RAMP COMP FSW/ PHASESHIFT PHASE SELECT
=0 IMAX ILIMIT
LOGIC
FET DRIVER
PGND OPEN CKT, SHORT CKT DETECTS
FSW DETECT
FPO GM AMP 8-BIT DAC DYNAMIC HEADROOM CONTROL VSET + REF OVP REF VSC HIGHEST VF STRING DETECT + -
ISL97675
COMP
FB1 FB2 FB4
1
TEMP SENSOR
SHUTDOWN
ISET
REF GEN
GND
+ PHASE SELECT PHASE SHIFT &
2
* VIN 6V
PWM FPWM/DirectPWM
8-BIT DIGITIZER
PWM CONTROLLER + 4
DIRECTPWM DETECT
ISL97675
Pin Configuration
ISL97675 (20 LD QFN) TOP VIEW
FAULT 17 COMP PWM VIN SW 16 15 PGND 14 OVP ISL97675 4mmx4mm 13 RFPWM/DirectPWM 12 FB1 11 FB2 10 NC
20 VDDIO EN FSW/PhaseShift ISET NC 1 2 3 4 5
19
18
6 FB4
7 FB3
8 AGND
9 AGND
Pin Descriptions
PIN NAME VDDIO EN FSW/PhaseShift PIN NO. 1 2 3
(I = Input, O = Output, S = Supply)
TYPE S I I DESCRIPTION Decouple with capacitor for internally generated supply rail. Enable FSW = 0 ~ 0.25 * VDDIO, Boost Switching Frequency = 600kHz with phase shift. FSW = 0.25 * VDDIO ~ 0.5 * VDDIO, Boost Switching Frequency = 600kHz without phase shift. FSW = 0.5 * VDDIO ~ 0.75 * VDDIO, Boost Switching Frequency = 1.2MHz without phase shift. FSW = 0.75 * VDDIO ~ VDDIO, Boost Switching Frequency = 1.2MHz with phase shift. Resistor connection for setting LED current, (see Equation 3 for calculating the ILEDpeak). No Connect. Input 4 to current source, FB, and monitoring. Input 3 to current source, FB, and monitoring. Analog Ground for precision circuits. Input 2 to current source, FB, and monitoring. Input 1 to current source, FB, and monitoring. External PWM dimming with frequency modulation or Direct PWM dimming without frequency modulation. When this pin is not biased and a resistor is connected to ground, the dimming frequency will be set by the Setting Resistor. When this pin is floating, the part enters Direct PWM mode such that the dimming follows the input PWM signal without frequency modulation. Overvoltage protection input. Power ground (LX Power return). Input to boost switch.
ISET NC FB4 FB3 AGND FB2 FB1 RFPWM/DirectPWM
4 5, 10 6 7 8, 9 11 12 13
I I I I S I I I
OVP PGND SW
14 15 16
I S O
3
FN7630.0 May 19, 2010
ISL97675
Pin Descriptions
PIN NAME FAULT COMP VIN PWM EP PIN NO. 17 18 19 20 21
(I = Input, O = Output, S = Supply) (Continued)
TYPE O I S I S DESCRIPTION Gate drive signal for external fault MOSFET. This pin should be left floating when fault MOSFET is omitted in the application. External compensation pin. LED driver supply voltage. PWM brightness control pin. Connect Exposed Pad (EP) to junction of AGND and PGND with adequate Vias to form a star ground.
Ordering Information
PART NUMBER (Notes 1, 2, 3) ISL97675IRZ NOTES: 1. Please refer to TB347 for details on reel specifications. 2. These Intersil Pb-free plastic packaged products employ special Pb-free material sets, molding compounds/die attach materials, and 100% matte tin plate plus anneal (e3 termination finish, which is RoHS compliant and compatible with both SnPb and Pb-free soldering operations). Intersil Pb-free products are MSL classified at Pb-free peak reflow temperatures that meet or exceed the Pb-free requirements of IPC/JEDEC J STD-020. 3. For Moisture Sensitivity Level (MSL), please see device information page for ISL97675. For more information on MSL please see techbrief TB363. PART MARKING 976 75IRZ TEMP RANGE (C) -40 to +85 PACKAGE (Pb-free) 20 Ld 4x4 QFN PKG. DWG. # L20.4x4C
4
FN7630.0 May 19, 2010
ISL97675
Table of Contents
Typical Application Circuit ................................................................................................................... 1 Block Diagram ..................................................................................................................................... 2 Pin Descriptions (I = Input, O = Output, S = Supply) .......................................................................... 3 Absolute Maximum Ratings ................................................................................................................. 6 Thermal Information ........................................................................................................................... 6 Operating Conditions ........................................................................................................................... 6 Electrical Specifications ....................................................................................................................... 6 Typical Performance Curves ................................................................................................................ 9 Theory of Operation........................................................................................................................... 12 PWM Boost Converter ....................................................................................................................... OVP ............................................................................................................................................... Enable ............................................................................................................................................ Power Sequence .............................................................................................................................. Current Matching and Current Accuracy .............................................................................................. Dynamic Headroom Control ............................................................................................................... Dimming Controls ............................................................................................................................ Maximum DC Current Setting ............................................................................................................ PWM Control.................................................................................................................................... Phase Shift Control........................................................................................................................... PWM Dimming Frequency Adjustment ................................................................................................. Direct PWM Dimming ........................................................................................................................ Switching Frequency......................................................................................................................... Inrush Control and Soft-Start ............................................................................................................ Fault Protection and Monitoring .......................................................................................................... Short Circuit Protection (SCP) ............................................................................................................ Open Circuit Protection (OCP) ............................................................................................................ Overvoltage Protection (OVP) ............................................................................................................ Undervoltage Lockout ....................................................................................................................... Master Fault Protection ..................................................................................................................... Over-Temperature Protection (OTP).................................................................................................... Components Selections ..................................................................................................................... Input Capacitor................................................................................................................................ Inductor ......................................................................................................................................... Output Capacitors ............................................................................................................................ Channel Capacitor ............................................................................................................................ Output Ripple .................................................................................................................................. Schottky Diode ................................................................................................................................ 12 12 12 12 12 12 12 12 13 13 14 14 14 14 15 15 15 15 15 15 15 17 17 18 18 18 18 18
Applications....................................................................................................................................... 19 High Current Applications .................................................................................................................. 19 Revision History ................................................................................................................................ 19 Products ............................................................................................................................................ 19 Package Outline Drawing .................................................................................................................. 20
5
FN7630.0 May 19, 2010
ISL97675
Absolute Maximum Ratings (TA = +25C)
VIN. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to 28V FAULT, EN . . . . . . . . . . . . . . . -0.3V to min(28, VIN + 0.3)V FSW/PhaseShift, RFPWM/DirectPWM, OVP . . . . -0.3V to 5.5V VDDIO, PWM, COMP . . . . . . . -0.3V to min(5.5, VIN + 0.3)V ISET . . . . . . . . . . . . . . . . -0.3V to min(VDDIO + 0.3, 5.5)V FB1, FB2, FB3, FB4. . . . . . . . . . . . . . . . . . . . . -0.3V to 45V SW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to 46V PGND, AGND . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +0.3V Above voltage ratings are all with respect to AGND pin ESD Rating Human Body Model (Tested per JESD22-A114E) . . . . . 3kV Machine Model (Tested per JESD22-A115-A) . . . . . . . 300V Charged Device Model . . . . . . . . . . . . . . . . . . . . . . . 1kV
Thermal Information
Thermal Resistance (Typical) 20 Ld QFN Package (Notes 4, 5, 7) . Thermal Characterization (Typical) JA (C/W) JC (C/W) 39 2.5 PSIJT (C/W)
20 Ld QFN Package (Note 6) . . . . . . . . . . . . 3 Maximum Continuous Junction Temperature . . . . . . +125C Storage Temperature . . . . . . . . . . . . . . . -65C to +150C Pb-free reflow profile . . . . . . . . . . . . . . . . . . .see link below http://www.intersil.com/pbfree/Pb-FreeReflow.asp
Operating Conditions
Temperature Range . . . . . . . . . . . . . . . . . . -40C to +85C
CAUTION: Do not operate at or near the maximum ratings listed for extended periods of time. Exposure to such conditions may adversely impact product reliability and result in failures not covered by warranty.
NOTES: 4. JA is measured in free air with the component mounted on a high effective thermal conductivity test board with "direct attach" features. See Tech Brief TB379. 5. For JC, the "case temp" location is the center of the exposed metal pad on the package underside. 6. PSIJT is the PSI junction-to-top thermal characterization parameter. If the package top temperature can be measured with this rating then the die junction temperature can be estimated more accurately than the JC and JC thermal resistance ratings. 7. Refer to JESD51-7 high effective thermal conductivity board layout for proper via and plane designs.
Electrical Specifications
All specifications below are tested at TA = +25C; VIN = 12V, EN = 3.3V, RISET = 19.6k, unless otherwise noted. Boldface limits apply over the operating temperature range, -40C to +85C. CONDITION MIN (Note 8) TYP MAX (Note 8) UNIT
SYMBOL GENERAL VIN (Note 9) IVIN_STBY VOUT
PARAMETER
Backlight Supply Voltage VIN Shutdown Current Output Voltage EN = 0V 4.5V < VIN 26V, FSW = 600kHz 6.75V < VIN 26V, FSW = 1.2MHz 4.5V < VIN 6.75V, FSW = 1.2MHz
4.5
26 10 45 45 VIN/0.15
V A V V V V mV
VUVLO VUVLO_HYS REGULATOR VDDIO IVDDIO_STBY IVIN VLDO ENLow ENHi tENLow
Undervoltage Lockout Threshold Undervoltage Lockout Hysteresis
2.6
3.1 320
3.3
LDO Output Voltage Standby Current Driver Input Current VDDIO LDO Dropout Voltage Guaranteed Range for EN Input Low Voltage Guaranteed Range for EN Input High Voltage EN Low Time before Shut-Down
VIN > 5.5V EN = 0V 100% Dimming VIN >5.5V, IVDDIO = 20mA
4.6
4.8
5 10
V A mA
9 30 200 0.5 1.8 29.5
mV V V ms
6
FN7630.0 May 19, 2010
ISL97675
Electrical Specifications
All specifications below are tested at TA = +25C; VIN = 12V, EN = 3.3V, RISET = 19.6k, unless otherwise noted. Boldface limits apply over the operating temperature range, -40C to +85C. (Continued) CONDITION MIN (Note 8) TYP MAX (Note 8) UNIT
SYMBOL BOOST SWILimit rDS(ON) SS Eff_peak
PARAMETER
Boost FET Current Limit Internal Boost Switch ON-Resistance Soft-Start Peak Efficiency TA = +25C 100% LED Duty Cycle VIN = 12V, 48 LEDs, 20mA each, L = 10H with DCR 101m, TA = +25C
1.5
2.2 230 14 92
2.7 300
A m ms %
IOUT/VIN DMAX
Line Regulation Boost Maximum Duty Cycle FSW < 0.5 * VDDIO FSW > 0.5 * VDDIO 91 82
0.1
% % % 8.5 16.5 % % kHz kHz A
DMIN
Boost Minimum Duty Cycle
FSW < 0.5 * VDDIO FSW > 0.5 * VDDIO
FSW
Boost Switching Frequency
FSW <0.5 * VDDIO FSW >0.5 * VDDIO
475 950
600 1200
640 1280 10
ISW_leakage
SW Leakage Current
SW = 45V, EN = 0
CURRENT SOURCES IMATCH DC Channel-to-Channel Current Matching RISET = 19.6k, (IOUT = 20mA) RISET = 39.2k, (IOUT = 10mA) IACC VHEADROOM VISET ILEDmax Current Accuracy Dominant Channel Current Source Headroom at FBx Pin Voltage at ISET Pin Maximum LED Current per Channel 4-Channel, VIN = 4.5V, VOUT = 40V, FSW = 600kHz 1.2 RISET = 19.6k, (IOUT = 20mA) -1.5 -1.5 -1.5 500 1.22 30 1.24 +1.5 +1.5 +1.5 % % % mV V mA
PWM INTERFACE VIL VIH FPWMI PWMACC PWMHYST PWM GENERATOR FPWM PWM Dimming Frequency Range RFPWM = 1.5M RFPWM = 1.5k VRFPWM tDIRECTPWM Voltage at RFPWM pin Direct PWM Minimum On Time Direct PWM Mode 45 33 1.19 250 50 37 1.22 55 39 1.24 350 Hz kHz V ns Guaranteed Range for PWM Input Low Voltage Guaranteed Range for PWM Input High Voltage PWMI Input Frequency Range PWMI Input Accuracy PWMI Input Allowable Jitter Hysteresis -0.46 1.5 100 8 +0.46 30,000 0.8 V V Hz bits LSB
7
FN7630.0 May 19, 2010
ISL97675
Electrical Specifications
All specifications below are tested at TA = +25C; VIN = 12V, EN = 3.3V, RISET = 19.6k, unless otherwise noted. Boldface limits apply over the operating temperature range, -40C to +85C. (Continued) CONDITION MIN (Note 8) TYP MAX (Note 8) UNIT
SYMBOL FAULT DETECTION VSC VTEMP_ACC VTEMP_SHDN VOVPlo OVPFAULT IFAULT VFAULT SWStart_thres ISW_Startup NOTES:
PARAMETER
Channel Short Circuit Threshold Over-Temperature Threshold Accuracy Over-Temperature Shutdown Overvoltage Limit on OVP Pin OVP Short Detection Fault Level Fault Pull-down Current VIN = 12V
3.15
3.6 5 150
4.3
V C C
1.2
1.22 350
1.24
V mV
8 6 1.2 1
15 7 1.4 3.5
25 8.3 1.5 5
A V V mA
Fault Clamp Voltage with Respect to VIN VIN = 12, VIN - VFAULT SW Start-Up Threshold SW Start-Up Current
8. Parameters with MIN and/or MAX limits are 100% tested at +25C, unless otherwise specified. Temperature limits established by characterization and are not production tested. 9. At minimum VIN of 4.5V, the maximum output is limited by the VOUT specifications. Also at maximum VIN of 26V, the minimum VOUT is 28V but minimum VOUT can be lower at lower VIN. In general, the VIN and VOUT relationship is bounded by DMAX and DMIN.
8
FN7630.0 May 19, 2010
ISL97675
Typical Performance Curves
100 80 5VIN 60 40 20 0 0 100 80 5VIN 60 40 20 0
EFFICIENCY (%)
12VIN
24VIN
EFFICIENCY (%)
12VIN
24VIN
5
10 15 ILED (mA)
20
25
0
10
20 ILED (mA)
30
40
FIGURE 1. EFFICIENCY vs 20mA LED CURRENT (100% LED DUTY CYCLE) for 4P12S vs VIN
FIGURE 2. EFFICIENCY vs 30mA LED CURRENT (100% LED DUTY CYCLE) for 4P10S vs VIN
100 80 EFFICIENCY (%) 60 40 20 0 0 20mA/582kHz 20mA/1.2MHz
100 80 EFFICIENCY (%) 60 40 20 0 0 30mA/582kHz
30mA/1.2MHz
10 VIN (V)
20
30
10 VIN (V)
20
30
FIGURE 3. EFFICIENCY vs VIN vs SWITCHING FREQUENCY AT 20mA for 4P12S (100% LED DUTY CYCLE)
FIGURE 4. EFFICIENCY vs VIN vs SWITCHING FREQUENCY AT 30mA for 4P10S(100% LED DUTY CYCLE)
100 90 80 70 -40C CURRENT MATCHING (%) +25C EFFICIENCY (%) 0C
0.25 0.20 0.15 0.10 0.05 0.00 -0.05 -0.10 -0.15 -0.20 -0.25 0 1 2 CHANNEL 12VIN 21VIN 3 4 4.5VIN
+85C
60
0
5
10
15 VIN (V)
20
25
30
FIGURE 5. EFFICIENCY vs VIN vs TEMPERATURE AT 20mA
FIGURE 6. CHANNEL-TO-CHANNEL CURRENT MATCHING
9
FN7630.0 May 19, 2010
ISL97675
Typical Performance Curves
1.0 0.8
(Continued)
10 9
4.5VIN IIN (mA) ILED mA 0.6 12VIN 0.4 0.2 0 0 8 7 6 5
1
2
3 DC (%)
4
5
6
0
5
10
15 VIN (V)
20
25
30
FIGURE 7. CURRENT LINEARITY vs LOW LEVEL PWM DIMMING DUTY CYCLE vs VIN
FIGURE 8. QUIESCENT CURRENT vs VIN WITH PART ENABLED
0.60 +25C VHEADROOM (V) 0.55 -40C
0.50
0C
0.45
0.40 0
5
10
15 VIN (V)
20
25
30
FIGURE 9. VHEADROOM vs VIN vs TEMPERATURE AT 20mA
FIGURE 10. VOUT RIPPLE VOLTAGE, VIN = 12V, 4P12S AT 20mA/CHANNEL
FIGURE 11. IN-RUSH AND LED CURRENT AT VIN = 6V FOR 4P12S AT 20mA/CHANNEL
FIGURE 12. IN-RUSH AND LED CURRENT AT VIN = 12V FOR 4P12S AT 20mA/CHANNEL
10
FN7630.0 May 19, 2010
ISL97675
Typical Performance Curves
(Continued)
FIGURE 13. LINE REGULATION WITH VIN CHANGE FROM 6V TO 26V, VIN = 12V, 4P12S AT 20mA/CHANNEL
FIGURE 14. LINE REGULATION WITH VIN CHANGE FROM 26V TO 6V FOR 4P12S AT 20mA/CHANNEL
FIGURE 15. LOAD REGULATION WITH ILED CHANGE FROM 0% TO 100% PWM DIMMING, VIN = 12V, 4P12S AT 20mA/CHANNEL
FIGURE 16. LOAD REGULATION WITH ILED CHANGE FROM 100% TO 0% PWM DIMMING, VIN = 12V, 4P12S AT 20mA/CHANNEL
FIGURE 17. ISL97675 SHUTS DOWN AND STOPS SWITCHING ~ 30ms AFTER EN GOES LOW
11
FN7630.0 May 19, 2010
ISL97675
Theory of Operation
PWM Boost Converter
The current mode PWM boost converter produces the minimal voltage needed to enable the LED stack with the highest forward voltage drop to run at the programmed current. The ISL97675 employs current mode control boost architecture that has a fast current sense loop and a slow voltage feedback loop. Such architecture achieves a fast transient response that is essential for the notebook backlight application. The input power may instantly change when the user switches from a drained battery to a AC/DC adapter without causing any flicker in the display backlight. The ISL97675 is capable of boosting up to 45V and typically can drive 13 (3.2V/20mA) LEDs in series on each of the 4 channels.
Current Matching and Current Accuracy
Each channel of the LED current is regulated by the current source circuit, as shown in Figure 18. The LED peak current is set by translating the RISET current to the output with a scaling factor of 392/RISET. The drain terminals of the current source MOSFETs are designed to run at ~ 500mV to minimize power loss. The sources of errors for the channel-to-channel current matching are due to internal mismatches, offsets and the external RISET resistor. To minimize this external offset, a 1% tolerance resistor is recommended.
OVP
The Overvoltage Protection (OVP) pin has a primary function of setting the overvoltage trip level. The ISL97675 OVP threshold is set by RUPPER and RLOWER such that:
V OUT _OVP = 1.21V x ( R UPPER + R LOWER ) R LOWER (EQ. 1)
RISET PWM DIMMING + REF + -
The ISL97675 has a patent pending switching architecture that uses the OVP block for feedback monitoring, hence allowing very low PWM dimming duty cycle operation. As a result, the overvoltage trip level also limits the VOUT regulation range between 64% and 100% of the VOUT_OVP and the equation is:
Allowable V OUT = 64% to 100% of V OUT _OVP (EQ. 2)
FIGURE 18. SIMPLIFIED CURRENT SOURCE CIRCUIT
Dynamic Headroom Control
The ISL97675 features a proprietary Dynamic Headroom Control circuit that detects the highest forward voltage string or effectively the lowest voltage from any of the FB1-4 pins digitally. This lowest FB voltage is used as the feedback signal for the boost regulator. Since all LED stacks are connected in parallel to the same output voltage, the other FB pins will have a higher voltage, but the regulated current source circuit on each channel will ensure that each channel has the same current. The output voltage will regulate cycle by cycle and it is always referenced to the highest forward voltage string in the architecture.
For example, if 10 LEDs are used with the worst case VOUT of 35V, and RUPPER and RLOWER are chosen such that the OVP level is set at 40V, then the allowed VOUT range is between 25.6V and 40V. If the requirement is changed to 6 LEDs/channel for a maximum VOUT of 21V, then the OVP level must be reduced according to Equation 2 to accommodate the new reduced output voltage. Otherwise, the headroom control will be disturbed and the channel voltage may be higher and prevent the driver from operating properly. The ratio of the OVP capacitors should be the inverse of the OVP resistors. For example, if RUPPER/RLOWER = 33/1, then CUPPER/CLOWER = 1/33. For example , if CUPPER = 100pF then CLOWER = 3.3nF.
Dimming Controls
The ISL97675 allows two ways of controlling the LED current, and therefore, the brightness. They are: 1. DC current adjustment. 2. PWM chopping of the LED current defined in step 1.
Enable
An EN signal is required to enable the internal regulator for normal operation. If there is no signal longer than 28ms, the device will enter shutdown.
Maximum DC Current Setting
The initial brightness should be set by choosing an appropriate value for RISET. This should be chosen to fix the maximum possible LED current:
( 392 ) I LEDmax = ---------------R ISET (EQ. 3)
Power Sequence
There is no specific power sequence requirement for the ISL97675. The EN signal can be tied to VIN but not the VDDIO as it will prevent the device from powering up.
12
FN7630.0 May 19, 2010
ISL97675
For example, if the maximum required LED current (ILED(max)) is 20mA, rearranging Equation 3 yields Equation 4:
ILED4-20mA
R ISET = ( 392 ) 0.02 = 19.6k
(EQ. 4)
ILED3-20mA ILED2-20mA ILED1-20mA ILED_Total_80mA
PWM Control
The ISL97675 has a high speed 8-bit digitizer that decodes an incoming PWM signal and converts it into four channels of 8-bit PWM current with a phase shift function that will be described later. During the PWM On period, the LED peak current is defined by the value of RISET resistor, the average LED current of each channel is controlled by ILEDmax and the PWM duty cycle in percent as:
I LED ( ave ) = I LEDmax x PWM (EQ. 5)
5
10 TIME (ms)
FIGURE 21. CONVENTIONAL LED DRIVER PWM DIMMING CHANNEL AND TOTAL CURRENT AT 50% DUTY CYCLE
When the PWM input = 0, all channels are disconnected and the ILED is guaranteed to be <10A in this state. The PWM dimming frequency is adjusted by a resistor at the RFPWM pin, which will be described in "PWM Dimming Frequency Adjustment" on page 14.
ILED4-20mA ILED3-20mA ILED2-20mA
ILED1-20mA ILED2-20mA ILED3-20mA ILED4-20mA
ILED1-20mA
ILED_Total_40mA 5 10 TIME (ms)
ILED_Total_80mA 5 10 TIME (ms) 15
FIGURE 22. EQUAL PHASE SHIFT LED DRIVERpwm DIMMING CHANNEL AT 50% DUTY CYCLE
FIGURE 19. CONVENTIONAL 4-Ch LED DRIVER WITH 10% PWM DIMMING CHANNEL CURRENT (UPPER) AND TOTAL CURRENT (LOWER)
Phase Shift Control
The ISL97675 is capable of delaying the phase of each current source. Conventional LED drivers pose the worst load transients to the boost circuit by turning on all channels simultaneously as shown in Figure 19. In contrast, the ISL97675 phase shifts each channel by turning them on once during each PWM dimming period as shown in Figure 20. At each dimming duty cycle except at 100%, the sum of the phase shifted channel currents will be less than a conventional LED driver as shown in Figure 20 and 22. Equal phase means there is fixed delay between channels and such delay can be calculated as:
t FPWM 255 t D1 = ------------------ x --------- ( 255 ) N t FPWM 255 t D2 = ------------------ x ( 255 ) - ( N - 1 ) --------- N 255 (EQ. 6)
ILED1-20mA ILED2-20mA ILED3-20mA ILED4-20mA
ILED_Total_20mA 5 10 TIME (ms)
FIGURE 20. PHASE SHIFT 4-Ch LED DRIVER WITH 10% PWM DIMMING CHANNEL CURRENT (UPPER) AND TOTAL CURRENT (LOWER)
(EQ. 7)
13
FN7630.0 May 19, 2010
ISL97675
where (255/N) in Equation 6 and Equation 7 can only be integer because the PWM dimming is controlled by an internal 8-bit digital counter. As a result, any decimal value of (255/N) will be discarded. For example for N = 4, (255/N) = 63, thus:
63 t D1 = t FPWM x --------255 66 t D2 = t FPWM x --------255
For example, for a 200 Hz input PWM frequency, the minimum duty cycle is:
Min DC = 350ns x 200Hz = 0.007% (EQ. 11)
Table 1 shows the PWM Dimming with Phase Shift and Direct PWM Dimming configurations.
TABLE 1. RFWM/ DIRECTPWM Connects with Resistor Floating FUNCTION PWM Dimming with frequency adjust DirectPWM without frequency adjust PHASE SHIFT Yes Dimming Resolution 8-bit
(EQ. 8)
where tFPWM is the sum of tON and tOFF. N is the number of active channels. The ISL97675 will detect the numbers of active channels automatically and is illustrated in Figure 23 for 4-channel.
tPWMIN PWMI 60% 40%
No
N/A
ILED1 ILED2 ILED3 ILED4
tFPWM (tPWMOUT) tOFF tON 40% 60% tD1 tD1 tD1 tD1
Switching Frequency
When the FSW/PhaseShift pin is biased from VDDIO with a resistor divider RUPPER and RLOWER, the switching frequency and phase shift function will change according to the following FSW/PhaseShift levels shown in Table 2 with the recommended RUPPER and RLOWER values.
TABLE 2.
tD2 ILED1 tD1 = Fixed Delay with Integer only while the decimal value will be discarded (e.g. 63.75 = 63)
FSW/PHASE SHIFT LEVEL 0 ~ 0.25 * VDDIO 0.25 * VDDIO ~ 0.5 * VDDIO 0.5 * VDDIO ~ 0.75 * VDDIO 0.75 * VDDIO ~ VDDIO
SWITCHING PHASE RUPPER RLOWER (k) FREQUENCY SHIFT (k) 600kHz 600kHz 1.2MHz 1.2MHz Yes No No Yes Open 150 100 0 0 100 150 Open
FIGURE 23. 4 EQUAL PHASE CHANNELS PHASE SHIFT ILLUSTRATION
PWM Dimming Frequency Adjustment
The dimming frequency is set by an external resistor at the RFPWM/DirectPWM pin to GND:
6.66 x10 F PWM = ----------------------RFPWM
7
Inrush Control and Soft-Start
The ISL97675 has separate built-in independent inrush control and soft-start functions. The inrush control function is built around the short circuit protection FET, and is only available in applications which include this device. After an initial delay from the point where the master Fault Protection FET is turned on, it is assumed that inrush has completed. At this point, the boost regulator will begin to switch and the current in the inductor will ramp-up. The current in the boost power switch is monitored and the switching is terminated in any cycle where the current exceeds the current limit. The ISL97675 includes a soft-start feature where this current limit starts at a low value (275mA). This is stepped up to the final 2.2A current limit in 7 further steps of 275mA. These steps will happen over at least 8ms, and will be extended at low LED PWM frequencies if the LED duty cycle is low. This allows the output capacitor to be charged to the required value at a low current limit and prevents high input current for systems that have only a low to medium output current requirement.
(EQ. 9)
where FPWM is the desirable PWM dimming frequency and RFPWM is the setting resistor. Do not bias RFPWM/DirectPWM if direct PWM dimming is used, see Table 1 for clarification. The PWM dimming frequency can be set or applied up to 30kHz with duty cycle from 0.4% to 100%. The lower limit of 0.4% is the result of 8-bit digitizer resolution.
Direct PWM Dimming
The ISL97675 can also operate in direct PWM dimming mode such that the output follows the input PWM signal without phase shifting. To use Direct PWM mode, users should float RFPWM/DirectPWM pin. The input PWM frequency should be limited to 30kHz and the minimum duty cycle be calculated by the following Equation 10:
Min Duty Cycle = 350ns x Input PWM Frequency (EQ. 10)
14
FN7630.0 May 19, 2010
ISL97675
For systems with no master fault protection FET, the inrush current will flow towards COUT when VIN is applied and it is determined by the ramp rate of VIN and the values of COUT and boost inductor, L. When this type of LED goes open circuit, the effect is as if the LED forward voltage has increased, but no light is emitted. Any affected string will not be disabled, unless the failure results in the boost OVP limit being reached, allowing all other LEDs in the string to remain functional. Care should be taken in this case that the boost OVP limit and SCP limit are set properly, to make sure that multiple failures on one string do not cause all other good channels to be faulted out. This is due to the increased forward voltage of the faulty channel making all other channels look as if they have LED shorts. See Table 3 for details for responses to fault conditions.
Fault Protection and Monitoring
The ISL97675 features extensive protection functions to cover all the perceivable failure conditions. The failure mode of an LED can be either open circuit or as a short. The behavior of an open circuited LED can additionally take the form of either infinite resistance or, for some LEDs, a zener diode, which is integrated into the device in parallel with the now opened LED. For basic LEDs (which do not have built-in zener diodes), an open circuit failure of an LED will only result in the loss of one channel of LEDs without affecting other channels. Similarly, a LED short circuit condition which causes the FB voltage to rise to ~4V, will result in that channel turning off. This does not affect any other channels. Due to the lag in boost response to any load change at its output, certain transient events (such as LED current steps or significant step changes in LED duty cycle) can transiently look like LED fault modes. The ISL97675 uses feedback from the LEDs to determine when it is in a stable operating region and prevents apparent faults during these transient events from allowing any of the LED stacks to fault out. See Table 3 for more details. A fault condition that results in high input current due to a short on VOUT with master fault protection switch will result in a shutdown of all output channels. The control device logic will remain functional.
Overvoltage Protection (OVP)
The integrated OVP circuit monitors the output voltage and keeps the voltage at a safe level. The OVP threshold is set as:
OVP = 1.21V x ( RUPPER + R LOWER ) R LOWER (EQ. 12)
These resistors should be large to minimize the power loss. For example, a 1M RUPPER and 30k RLOWER sets OVP to 41.2V. Large OVP resistors also allow COUT discharges slowly during the PWM Off time. Parallel capacitors should also be placed across the OVP resistors such that RUPPER/RLOWER = CLOWER/CUPPER. Using a CUPPER value of at least 30pF is recommended. These capacitors reduce the AC impedance of the OVP node, which is important when using high value resistors.
Undervoltage Lockout
If the input voltage falls below the UVLO level of 3.1V, the device will stop switching and be reset. Operation will restart once the input voltage is back in the normal operating range.
Short Circuit Protection (SCP)
The short circuit detection circuit monitors the voltage on each channel and disables faulty channels which are detected above the programmed short circuit threshold. When an LED becomes shorted, the action taken is described in Table 3. The short circuit threshold is 4V.
Master Fault Protection
During normal switching operation, the current through the internal boost power FET is monitored. If the input current exceeds the current limit due to output shorted to ground or excessive loading, the internal switch will be turned off. This monitoring happens on a cycle by cycle basis in a self protecting way. Additionally, the ISL97675 monitors the voltage at the LX and OVP pins. At start-up, a fixed current is injected out of the LX pins and into the output capacitor. The device will not start up unless the voltage at LX exceeds 1.2V. The OVP pin is also monitored such that if it rises above and subsequently falls below 20% of the target OVP level, the input protection FET will be switched off.
Open Circuit Protection (OCP)
When one of the LEDs becomes open circuit, it can behave as either an infinite resistance or a gradually increasing finite resistance. The ISL97675 monitors the current in each channel such that any string which reaches the intended output current is considered "good". Should the current subsequently fall below the target, the channel will be considered an "open circuit". Furthermore, should the boost output of the ISL97675 reach the OVP limit or should the lower over-temperature threshold be reached, all channels which are not "good" will immediately be considered as "open circuit". Detection of an "open circuit" channel will result in a time-out before disabling of the affected channel. This time-out is run when the device is above the lower over-temperature threshold in an attempt to prevent the upper over-temperature trip point from being reached. Some users employ special types of LEDs that have zener diode structure in parallel with the LED for ESD enhancement, thus enabling open circuit operation. 15
Over-Temperature Protection (OTP)
The ISL97675 includes two over-temperature thresholds. The lower threshold is set to +130C. When this threshold is reached, any channel which is outputting current at a level below the regulation target will be treated as "open circuit" and disabled after a time-out period. The intention of the lower threshold is to allow bad channels to be isolated and disabled before they cause enough power dissipation (as a result of other
FN7630.0 May 19, 2010
ISL97675
channels having large voltages across them) to hit the upper temperature threshold. The upper threshold is set to +150C. Each time this is reached, the boost will stop switching and the output current sources will be switched off.
VIN LX VOUT
For the extensive fault protection conditions, please refer to Figure 24 and Table 3 for details.
DRIVER IMAX
FAULT
O/P SHORT FET DRIVER
OVP
ILIMIT
LOGIC
VSC VIN
FB1
VSET/2 REG THRM SHDN
FB4
REF T2 TEMP SENSOR T1 VSET + Q1 VSET + PWM4/OC4/SC4 Q4
OTP
PWM1/OC1/SC1 PHASE SHIFT & CONTROL LOGIC
FIGURE 24. SIMPLIFIED FAULT PROTECTIONS
TABLE 3. PROTECTIONS TABLE GOOD CHANNELS ACTION FB2 through FB4 Normal Same as FB1 VOUT REGULATED BY Highest VF of FB2 through FB4 Highest VF of FB2 through FB4
CASE 1
FAILURE MODE FB1 Short Circuit
DETECTION MODE Upper Over-Temperature Protection limit (OTP) not triggered and FB1 < 4V Upper OTP triggered but VFB1 < 4V
FAILED CHANNEL ACTION FB1 ON and burns power.
2
FB1 Short Circuit
All channels go off until chip cooled and then comes back on with current reduced to 76%. Subsequent OTP triggers will reduce IOUT further. FB1 disabled after 6 PWM cycle timeout. VOUT will ramp to OVP. FB1 will time-out after 6 PWM cycles and switch off. VOUT will drop to normal level.
3 4
FB1 Short Circuit FB1 Open Circuit with infinite resistance
Upper OTP not triggered but FB1 > 4V Upper OTP not triggered and FB1 < 4V
FB2 through FB4 Normal FB2 through FB4 Normal
Highest VF of FB2 through FB4 Highest VF of FB2 through FB4
16
FN7630.0 May 19, 2010
ISL97675
TABLE 3. PROTECTIONS TABLE (Continued) GOOD CHANNELS ACTION FB2 through FB4 ON, Q2 through Q4 burn power Same as FB1 VOUT REGULATED BY VF of FB1
CASE 5
FAILURE MODE FB1 LED Open Circuit but has paralleled Zener FB1 LED Open Circuit but has paralleled Zener
DETECTION MODE Upper OTP not triggered and FB1 < 4V Upper OTP triggered but FB1 < 4V
FAILED CHANNEL ACTION FB1 remains ON and has highest VF, thus VOUT increases. All channels go off until chip cooled and then comes back on with current reduced to 76%. Subsequent OTP triggers will reduce IOUT further FB1 remains ON and has highest VF, thus VOUT increases.
6
VF of FB1
7
FB1 LED Open Circuit but has paralleled Zener
Upper OTP not triggered but FBx > 4V
VF of FB1 VOUT increases, then FB-X switches OFF after 6 PWM cycles. This is an unwanted shut off and can be prevented by setting OVP at an appropriate level. Highest VF of FB1 through FB4
8
Channel-to-Channel Lower OTP triggered but VF too high FBx < 4V Channel-to-Channel Upper OTP triggered but VF too high FBx < 4V Output LED stack voltage too high VOUT/LX shorted to GND at start-up or VOUT shorted in operation VOUT > VOVP
Any channel at below the target current will fault out after 6 PWM cycles. Remaining channels driven with normal current.
9
All channels go off until chip cooled and then comes Highest VF of back on with current reduced to 76%. Subsequent OTP FB1 through FB4 triggers will reduce IOUT further Any channel that is below the target current will time- Highest VF of FB1 through FB4 out after 6 PWM cycles, and VOUT will return to the normal regulation voltage required for other channels.
10
11
LX current and timing are The chip is permanently shutdown 31ms after power-up if VOUT/Lx is shorted to GND. monitored. OVP pins monitored for excursions below 20% of OVP threshold.
Components Selections
According to the inductor Voltage-Second Balance principle, the change of inductor current during the switching regulator On-time is equal to the change of inductor current during the switching regulator Off-time. Since the voltage across an inductor is:
V L = L x I L t (EQ. 13)
Input Capacitor
Switching regulators require input capacitors to deliver peak charging current and to reduce the impedance of the input supply. This reduces interaction between the regulator and input supply, thereby improving system stability. The high switching frequency of the loop causes almost all ripple current to flow in the input capacitor, which must be rated accordingly. A capacitor with low internal series resistance should be chosen to minimize heating effects and improve system efficiency, such as X5R or X7R ceramic capacitors, which offer small size and a lower value of temperature and voltage coefficient compared to other ceramic capacitors. In boost mode, input current flows continuously into the inductor; AC ripple component is only proportional to the rate of the inductor charging, thus, smaller value input capacitors may be used. It is recommended that an input capacitor of at least 10F be used. Ensure the voltage rating of the input capacitor is suitable to handle the full supply range.
and IL @ TON = IL @ TOFF, therefore:
( V I - 0 ) L x D x tS = ( VO - VD - VI ) L x ( 1 - D ) x tS (EQ. 14)
where D is the switching duty cycle defined by the turn-on time over the switching period. VD is Schottky diode forward voltage which can be neglected for approximation. Rearranging the terms without accounting for VD gives the boost ratio and duty cycle respectively as:
VO VI = 1 ( 1 - D ) D = ( VO - VI ) VO (EQ. 15) (EQ. 16)
17
FN7630.0 May 19, 2010
ISL97675
Inductor
The selection of the inductor should be based on its maximum current (ISAT) characteristics, power dissipation (DCR), EMI susceptibility (shielded vs unshielded), and size. Inductor type and value influence many key parameters, including ripple current, current limit, efficiency, transient performance and stability. The inductor's maximum current capability must be large enough to handle the peak current at the worst case condition. If an inductor core is chosen with a lower current rating, saturation in the core will cause the effective inductor value to fall, leading to an increase in peak to average current level, poor efficiency and overheating in the core. The series resistance, DCR, within the inductor causes conduction loss and heat dissipation. A shielded inductor is usually more suitable for EMI susceptible applications, such as LED backlighting. The peak current can be derived from the voltage across the inductor during the off period, as expressed in Equation 17:
IL peak = ( V O x I O ) ( 85% x V I ) + 1 2 [ V I x ( V O - V I ) ( L x V O x f SW ) ] (EQ. 17)
Here are some recommendations for various applications: For 20mA applications with VIN > 7V, 1 x 4.7F (X7R type) is sufficient. For 20mA applications with VIN < 7V, 2 x 4.7F (X7R type) is required in some configurations.
3.0 2.5 2.0 1.5 1.0 POLY. (CERAMIC Y5V 2.2F 50V CAP) 0.5 0 0 5 10 15 20 25 30 APPLIED VOLTAGE (V) 35 40 45 POLY. (CERAMIC X7R 2.2F 50V CAP)
FIGURE 25. X7R AND V5Y TYPES CERAMIC CAPACITORS
The choice of 85% is just an average term for the efficiency approximation. The first term is the average current, which is inversely proportional to the input voltage. The second term is the inductor current change, which is inversely proportional to L and fSW. As a result, for a given switching frequency, minimum input voltage must be used to calculate the input/inductor current as shown in Equation 17. Fora given inductor size, the larger the inductance value, the higher the series resistance because of the extra number of turns required, thus, higher conductive losses. The ISL97675 current limit should be less than the inductor saturation current.
Channel Capacitor
It is recommended to use at least 1.5nF capacitors from CH pins to VOUT. Larger capacitors will reduce LED current ripple at boost frequency, but will degrade transient performance at high PWM frequencies. The best value is dependant on PCB layout. Up to 4.7nF is sufficient for most configurations.
Output Ripple
VCo, can be reduced by increasing Co or fSW, or using small ESR capacitors as shown in Equation 18. In general, Ceramic capacitors are the best choice for output capacitors in small to medium sized LCD backlight applications due to their cost, form factor, and low ESR. A larger output capacitor will also ease the driver response during PWM dimming off period due to the longer sample and hold effect of the output drooping. The driver does not need to boost as much on the next on period which minimizes transient current. The output capacitor is also needed for compensation, and, in general one to two 4.7F/50V ceramic capacitors are needed for netbook or notebook display backlight applications.
Output Capacitors
The output capacitor acts to smooth the output voltage and supplies load current directly during the conduction phase of the power switch. Output ripple voltage consists of the discharge of the output capacitor during the FET ton period and the voltage drop due to load current flowing through the ESR of the output capacitor. The ripple voltage is shown in Equation 18:
V CO = ( I O C O x D f S ) + ( ( I O x ESR ) (EQ. 18)
Equation 18 shows the importance of using a low ESR output capacitor for minimizing output ripple. The choice of X7R over Y5V ceramic capacitors is highly recommended because the former capacitor is less sensitive to capacitance change over voltage as shown in Figure 25. Y5V's absolute capacitance can be reduced to 10%~20% of its rated capacitance at the maximum voltage. In any case, Y5V type of ceramic capacitor should be avoided.
Schottky Diode
A high speed rectifier diode is necessary to prevent excessive voltage overshoot, especially in the boost configuration. Low forward voltage and reverse leakage current will minimize losses, making Schottky diodes the preferred choice. Although the Schottky diode turns on only during the boost switch off period, it carries the same peak current as the inductor, therefore, a suitable current rated Schottky diode must be used.
18
CAPACITANCE (F)
FN7630.0 May 19, 2010
ISL97675
Applications
High Current Applications
Each channel of the ISL97675 can support up to 40mA. For applications that need higher current, multiple channels can be grouped to achieve the desirable current. For example, the cathode of the last LED can be connected to FB1 to FB2, this configuration can be treated as a single string with 80mA current driving capability.
VOUT
FB1 FB2
FIGURE 26. GROUPING MULTIPLE CHANNELS FOR HIGH CURRENT APPLICATIONS
Revision History
The revision history provided is for informational purposes only and is believed to be accurate, but not warranted. Please go to web to make sure you have the latest Rev. DATE 5/19/10 REVISION FN7630.0 Initial Release. CHANGE
Products
Intersil Corporation is a leader in the design and manufacture of high-performance analog semiconductors. The Company's products address some of the industry's fastest growing markets, such as, flat panel displays, cell phones, handheld products, and notebooks. Intersil's product families address power management and analog signal processing functions. Go to www.intersil.com/products for a complete list of Intersil product families. *For a complete listing of Applications, Related Documentation and Related Parts, please see the respective device information page on intersil.com: ISL97675 To report errors or suggestions for this datasheet, please go to www.intersil.com/askourstaff FITs are available from our website at http://rel.intersil.com/reports/search.php
For additional products, see www.intersil.com/product_tree Intersil products are manufactured, assembled and tested utilizing ISO9000 quality systems as noted in the quality certifications found at www.intersil.com/design/quality
Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries.
For information regarding Intersil Corporation and its products, see www.intersil.com 19
FN7630.0 May 19, 2010
ISL97675
Package Outline Drawing
L20.4x4C
20 LEAD QUAD FLAT NO-LEAD PLASTIC PACKAGE Rev 0, 11/06
4X 4.00 A B 6 PIN 1 INDEX AREA 15 1 16 2.0 6 PIN #1 INDEX AREA
16X 0.50 20
4.00
2 .70 0 . 15
11
5
(4X)
0.15 10 6 0.10 M C A B 4 20X 0.25 +0.05 / -0.07
TOP VIEW
20X 0.4 0.10
BOTTOM VIEW
SEE DETAIL "X" 0.10 C C BASE PLANE SEATING PLANE 0.08 C ( 20X 0 . 5 )
0 . 90 0 . 1 ( 3. 8 TYP ) ( 2. 70 )
SIDE VIEW
( 20X 0 . 25 ) C ( 20X 0 . 6) 0 . 00 MIN. 0 . 05 MAX. 0 . 2 REF 5
TYPICAL RECOMMENDED LAND PATTERN
DETAIL "X"
NOTES: 1. Dimensions are in millimeters. Dimensions in ( ) for Reference Only. 2. Dimensioning and tolerancing conform to AMSE Y14.5m-1994. 3. Unless otherwise specified, tolerance : Decimal 0.05 4. Dimension b applies to the metallized terminal and is measured between 0.15mm and 0.30mm from the terminal tip. 5. Tiebar shown (if present) is a non-functional feature. 6. The configuration of the pin #1 identifier is optional, but must be located within the zone indicated. The pin #1 indentifier may be either a mold or mark feature.
20
FN7630.0 May 19, 2010

▲Up To Search▲

Price & Availability of ISL97675

	To Download ISL97675 Datasheet File
If you can't view the Datasheet, Please click here to try to view without PDF Reader .