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ISL8107
Data Sheet December 11, 2007 FN6605.0
Single-Phase Pulse-Width Modulation (PWM) Controller with Integrated High-Side Gate Drivers
The ISL8107 is a single phase, non-synchronous buck controller with an integrated high-side MOSFET driver. The controller operates from 9V to 75V input voltage range. The internal 1.192V reference voltage has a tolerance of 1% over the industrial temperature range. The ISL8107 employs voltage-mode control with feedforward compensation to provide optimal transient response and a constant loop gain over a wide input voltage range. The switching frequency can be programmed from 100kHz to 600kHz by external resistor and capacitor. The switching frequency can be synchronized to an external clock signal through the SYNC pin. The ISL8107 features programmable soft-start, hiccup mode for short circuit protection, and over-temperature protection. The overcurrent protection is implemented using MOSFET's rDS(ON) sensing. This approach simplifies the implementation without deteriorating the converter efficiency.
Features
* Operates from +9V to +75V Bias Supply Voltage * +1.192V Internal Voltage Reference - 1% tolerance Over the Industrial Temperature Range * Programmable Switching Frequency 100kHz to 600kHz * Simple Single-Loop Voltage Mode Control * Voltage Feedforward Compensation * Switching Frequency Synchronization * Lossless, Programmable Overcurrent Protection - Uses MOSFET's rDS(ON) - Hiccup Mode * Integrated N-Channel MOSFET Gate Drivers * Programmable Soft-Start * Over-Temperature Protection * QFN (5x5) Package - QFN compliant to JEDEC PUB95 MO-220 QFN - Quad Flat No Leads - Product Outline - QFN Near Chip Scale Package Footprint; Improves PCB Efficiency, Thinner in Profile * Pb-free (RoHS compliant)
Pinout
ISL8107 (16 LD 5X5 QFN) TOP VIEW
UGATE PHASE PVCC BOOT
Applications
* Test and Measurement Instruments * Routers and Switchers * Point of Load Modules
12 EN/SS 11 PGND
16 VFF OCSET VCC SYNC 1 2 3 4 5 RT/CT
15
14
13
* Distributed DC/DC Power Architecture * Industrial Applications * Telecom/Datacom Applications
10 PGOOD 9 6 SGND 7 FB 8 COMP VBG
Ordering Information
PART NUMBER (Note) ISL8107IRZ PART MARKING ISL8107 IRZ TEMP. RANGE (C) -40 to +85 -40 to +85 PACKAGE (Pb-Free) PKG. DWG. #
16 Ld 5x5 QFN L16.5x5B 16 Ld 5x5 QFN L16.5x5B Tape and Reel
ISL8107IRZ-T* ISL8107 IRZ
*Please refer to TB347 for details on reel specifications. NOTE: 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.
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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. 2007. All Rights Reserved All other trademarks mentioned are the property of their respective owners.
Block Diagram
VCC
INT. BIAS VOLTAGE
INTERNAL REGULATOR
INTERNAL REGULATOR
PVCC
POWER-ON RESET (POR) INT. BIAS VOLTAGE OCFLT + EN/SS 0.77V + 104A OCSET
2
SGND VBG PGOOD + OR + 1.03V REFERENCE VREF = 1.192V 1.345V FB
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SOFT-START AND FAULT LOGIC
BOOT
ISL8107
GATE CONTROL LOGIC
UGATE
PHASE
+ + + GM EA PWM
OSCILLATOR PGND
COMP
RT/CT
SYNC
VFF
ISL8107 Typical Application
VIN 9V TO 75V DBOOT CF1 PVCC VCC CF2 RT RT/CT CT UGATE PHASE COCSET Q1 OCSET ROCSET CBOOT LOUT VFF BOOT CHFIN CBIN
RFilter
CF2
VOUT
SYNC
PGND
D1 C3
CHFOUT
CBOUT
R3
PGOOD VBG
ISL8107
FB R1 RO C1 C2 C4 OPTIONAL SGND R2
COMP CBG EN/SS CSS
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ISL8107 Typical Application
VIN 9V TO 75V RT DBOOT CF1 PVCC RT/CT CT OCSET ROCSET CBOOT Q1 LOUT VFF BOOT CHFIN CBIN
CF2
COCSET VCC < VFF VCC CF2 UGATE PHASE
VOUT
D1 SYNC PGND C3 PGOOD VBG COMP CBG C1 C2 R2 RO R3
CHFOUT
CBOUT
ISL8107
FB R1
EN/SS CSS SGND
C4
OPTIONAL
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ISL8107
Absolute Maximum Ratings
Input Voltage (VCC,VFF) to GND . . . . . . . . . . . . . . . . . . . . . . .100V BOOT to GND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .105V ENSS pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6V FB, COMP, SYNC pins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8V ESD Rating Human Body Model (Per MIL-STD-883 Method 3015.7) . . . . .2kV Machine Model (Per EIAJ ED-4701 Method C-111) . . . . . . . .200V Charged Device Model (Per EOS/ESD DS5.3, 4/14/93) . . . . .1kV ESD Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Class 2
Thermal Information
Thermal Resistance (Typical) JA (C/W) JC (C/W) QFN Package (Notes 1, 2) . . . . . . . . 33 3 Maximum Junction Temperature . . . . . . . . . . . . . . . . . . . . . +150C Maximum Storage Temperature Range . . . . . . . . . .-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 Supply Voltage Range . . . . . . . . . . . . . . . . . . . . . . . . . . 9.0V to 75V
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: 1. 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. 2. For JC, the "case temp" location is the center of the exposed metal pad on the package underside. 3. Limits should be considered typical and are not production tested.
Electrical Specifications
PARAMETER VCC SUPPLY Bias Voltage Range Bias Supply Current VCC Shutdown Current
Recommended Operating Conditions, unless otherwise noted. SYMBOL TEST CONDITIONS MIN TYP MAX UNITS
9.0 IVCC IVCC_SD UGATE open VCC = 48V, ENSS = 0V -
2 40
75 3 60
V mA A
INTERNAL LINEAR REGULATOR (PVCC) Output Voltage Maximum Output Current Short Current Protection POWER-ON RESET POR Threshold POR Threshold Hysteresis OSCILLATOR Total Variation on Set Frequency Frequency Range SYNC Frequency Range Ramp Amplitude Min OFF Time REFERENCE VOLTAGE Feedback Voltage Accuracy VFB -1.0 1.192 +1.0 V % VOSC RT=20k, CT=1200pF Set by RT and CT. RT range = 20k to 100k, CT range = 470pF to 1200pF above RTCT natural frequency VCC varied from 9.0V to 75V 100 100 330 0.11*VFF 190 600 600 300 kHz kHz kHz VP-P ns VCC = VFF VCC = VFF 6.8 7.8 220 8.5 V mV PVCC VCC = 15V to 75V, Load = 3mA to 20mA 20 10 60 V mA mA
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ISL8107
Electrical Specifications
PARAMETER ENABLE/SS Soft-Start Current ISS VENSS = 0V VENSS = 1.3V Enable Threshold Maximum Disable Voltage ERROR AMPLIFIER Transconductance Gain-Bandwidth Product (Note 3) Slew Rate (Note 3) COMP Pin Drive(Note 3) POWER GOOD (OPEN DRAIN) Power-Good Lower Threshold Power-Good Higher Threshold PGOOD Leakage Current PGOOD Voltage Low GATE DRIVER Gate Drive Source Current (Note 3) Gate Drive Source Impedance Gate Drive Sink Current (Note 3) Gate Drive Sink Impedance OVERCURRENT PROTECTION Dynamic Current Limit OFF-time OCP (OCSET) Current Source SYNCHRONIZATION Input HIGH Level (Asserted) Input LOW Level (Unasserted) Input Current HIGH Input Current LOW THERMAL SHUTDOWN Thermal Shutdown Temperature Thermal Shutdown Hysteresis Rising Threshold 150 40 C C VSYNC_HIGH VSYNC_LOW ISYNCHIGH ISYNCLOW 3.0 0.8 1 1 V V A A tOCOFF IOCSET 89 4 104 119 SS cycle A IG_SOURCE RG_SOURCE IG_SINK RG_SINK VBOOT - PHASE = 10V VBOOT - PHASE = 10V 1.5 0.25 1.5 0.24 A Ohm A Ohm VPGVPG+ IPGLKG Percentage of Nominal VFB; ~ 3s noise filter Percentage of Nominal VFB; ~ 3s noise filter VPULLUP = 5.5V IPGOOD = 4mA 84 112 88 116 1 0.5 % % A V GBW SR ICOMP 4.2 5.7 15 6 300 7.2 mS MHz V/s A VEN VDISEN Voltage level where soft-start current changes from low to high 22 0.5 2 33 0.77 43 1.0 0.5 A A V V Recommended Operating Conditions, unless otherwise noted. (Continued) SYMBOL TEST CONDITIONS MIN TYP MAX UNITS
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ISL8107 Functional Pin Description
VFF (Pin 1)
The voltage at this pin is used for input voltage feed forward compensation and sets the internal oscillator ramp peak to peak amplitude at 0.11*VFF. The oscillator ramp amplitude varies from approximately 1V to 8.5V as VFF changes from 9V to 75V to maintain constant frequency and provide feed forward compensation. An external RC filter may be required at this pin in noisy input environments.
COMP (Pin 8)
This pin is connected to the output of the transconductance error amplifier and is used to compensate the feedback loop.
VBG (Pin 9)
Band gap reference output. A 0.01F capacitor to SGND is required for decoupling this signal. This pin should not be loaded.
PGOOD (Pin 10)
This pin provides a power good status. It is an open drain output that is asserted when the voltage at FB pin is within 14% of the reference voltage.
OCSET (Pin 2)
The current limit is set by placing a resistor, ROCSET and capacitor, COCSET, between this pin and the drain of the MOSFET. The maximum allowable ROCSET resistor is 50k. A 104A current source develops a voltage across ROCSET which is then compared with the voltage developed across the MOSFET when on. An initial ~200ns blanking period is used to eliminate the sampling error due to switching noise before the current is measured.
PGND (Pin 11)
This pin provides the power ground to the IC. Tie this pin to the ground plane through the lowest impedance connection.
EN/SS (Pin 12)
This pin provides enable/disable function and soft-start timing function for the PWM output. The IC is disabled when this pin is held below 0.5V.
VCC (Pin 3)
VCC is power connection for the ISL8107. The pin should be connected to a 9V to 75V bias supply and must be well decoupled to signal ground (SGND) with a ceramic capacitor. The voltage at this pin must always be equal to or less than the voltage at VFF pin.
PHASE (Pin 13)
Connect this pin to the source of MOSFET. This pin provides the return path for the gate drive current. During normal switching, this pin is also used for current limiting measurements.
UGATE (Pin 14)
This pin provides the drive for the MOSFET and should be connected to its gate.
SYNC (Pin 4)
The switching frequency can be synchronized to an external clock through this pin. When the sync function is not used, this pin must be tied to ground. If the sync function is used, the RT/CT natural frequency must be set to a frequency lower than the sync input frequency. The termination of the ramp is synchronized with the rising edge of the sync input signal.
BOOT (Pin 15)
This pin provides the bootstrap bias for the gate driver. A 2.2 resistor may be placed in series with the bootstrap diode to prevent over charging of the BOOT capacitor during normal operation.
PVCC (Pin 16)
This pin is the output of the internal series linear regulator. A minimum 1F capacitor is required for decoupling PVCC to PGND. For proper operation the PVCC capacitor must be within 150 mils of the PVCC and the PGND pins and must be connected to these pins with dedicated traces.
RT/CT (Pin 5)
A resistor to VFF and a capacitor to GND determines the frequency of the sawtooth oscillator. The resistor should be in the range of 20k to 100k, the capacitor range should be in the range of 470pF to 1.2nF. When the controller is disabled, the voltage at RT/CT pin rises up to VFF. Hence, the voltage rating of the CT capacitor must be sufficient to support the maximum VFF.
Exposed Thermal Pad
This pad is electrically isolated. Connect this pad to the signal ground plane using at least five vias for a robust thermal conduction path.
SGND (Pin 6)
Signal ground for the IC. All voltage levels are measured with respect to this pin.
FB (Pin 7)
This is the feedback pin. The feedback ratio is set by an external resistor divider connected to the output.
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ISL8107 Typical Performance Curves
11 10 9 PVCC (V) VPVCC 8 7 6 5 12 10 8 VIN = 12V 6 VIN = 75V 4 2 0 VIN = 9V VIN = 48V
VIN = 24V
1
10 VCC (V)
100
0
20
40 IPVCC (mA)
60
80
FIGURE 1. PVCC vs VCC (IPVCC = 20mA)
FIGURE 2. VI CHARACTERISTIC of PVCC
11.5 11.0 10.5 10.0 9.5 VPVCC 9.0 8.5 8.0 7.5 7.0 6.5 6.0 -40 -25 -15 -5 5 15 25 35 45 55 65 75 85 VBIAS = 9V VBIAS = 75V IPVCC = 10mA
1.194
1.193 VFB VOLTAGE VBIAS = 40V
1.192
VBIAS = 12V
1.191
1.190 -40 -25 0 25 50 75 85 100 TEMPERATURE (C)
TEMPERATURE (C)
FIGURE 3. PVCC vs TEMPERATURE
FIGURE 4. VFB vs TEMPERATURE
190 188 186 FREQUENCY (kHz) 184 182 180 178 176 174 172 170 0 10 20 30 40 50 60 70 80 FREQUENCY (kHz)
192 190 188 186 184 182 180 178 176 174 -40 -25 -15 -5 5 VCC = VFF = 75V
VCC = VFF = 9V
VCC = VFF = 12V
VCC = VFF = 48V
15 25 35 45 55 65 75 85
VCC = VFF (V)
TEMPERATURE (C)
FIGURE 5. RT/CT FREQUENCY vs INPUT VOLTAGE (RT = 40.2k, CT = 1000pF COG)
FIGURE 6. RT/CT FREQUENCY vs TEMPERATURE (RT = 40.2k, CT = 1000pF COG)
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ISL8107 Typical Performance Curves (Continued)
100 95 90 EFFICIENCY (%) 85 80 75 70 65 60 1 2 3 4 5 6 7 8 VIN = 48V VIN = 36V VIN = 60V VIN = 12V VIN = 24V EFFICIENCY (%) 100 95 90 85 80 75 70 65 60 1 2 3 4 5 6 7 8 VIN = 36V VIN = 48V VIN = 60V VIN = 24V VIN = 18V
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
FIGURE 7. EFFICIENCY VS LOAD (OUTPUT VOLTAGE = 5V, FSW = 200kHz (SEE APPLICATION NOTE FOR MORE DETAILS)
FIGURE 8. EFFICIENCY VS LOAD (OUTPUT VOLTAGE = 12V, FSW = 200kHz (SEE APPLICATION NOTE FOR MORE DETAILS)
Functional Description
Enable/Soft-Start
5V VOUT
T delay, switching = 3.712 x 10 C SS
5
(EQ. 1)
The output voltage soft-start time is determined by the rise time of ENSS voltage from 1.4V to 2.6V (T3 -T2). The output voltage ramp time can be calculated from:
ENSS
4V
1.2 T SS = --------------------- C SS -6 33 x10
(EQ. 2)
3V 2.6V 2V 1.4V 1V POR 0.7V
The soft-start capacitor CSS is continuously charged up linearly and clamped at 5V. Note that any leakage current on ENSS node will extend the start-up period.
RT/CT RTCT
ENSS ENSS T0 T1 T2 T3
FIGURE 9. TYPICAL SOFT-START DIAGRAM
VOUT
Figure 9 illustrates the start-up scheme of the ISL8107. The Power-On Reset (POR) function continually monitors the bias voltage at VCC and VFF. When the voltage at VCC and VFF exceed their rising POR thresholds (T0), the ISL8107 provides initially 2A to charge the soft-start capacitor, CSS, connected to the ENSS pin. If the voltage at this pin is allowed to rise, it will ramp-up with at a slope determined by the 2A current and the value of the soft-start capacitor. When the voltage at ENSS reaches 0.77V (Typ) at T1, the oscillator circuit is active, causing the voltage at RT/CT pin to drop from VIN and generate sawtooth waveform. At the same time, the soft-start current is increased to 33A; the ENSS voltage then ramps up at a faster rate. The UGATE starts switching when the ENSS voltage reaches 1.4V (Typ). The delay from POR (T0) to the time the IC starts switching (T2) can be approximated by using Equation 1: 9
PHASE PHASE
FIGURE 10. TYPICAL SOFT-START WAVEFORM
Oscillator and Synchronization
The ISL8107 provides adjustable frequency from 100kHz to 600kHz by changing external resistor RT and capacitor CT. The time constant of RT/CT determines the oscillator frequency, which can be calculated through Equation 3:.
1 F OSC = -----------------------------------------------------------------9 0.1215R T C T + 140 x10 (EQ. 3)
FN6605.0 December 11, 2007
ISL8107
Note that when the controller is disabled, the voltage at RT/CT pin rises up to the input voltage. Hence, the voltage rating of the CT capacitor must be sufficient to support the maximum input voltage. The SYNC pin provides the ISL8107 to synchronize its switching frequency to the fundamental frequency of the input waveform. The rising edge of the input synchronization signal is used to terminate the RT/CT ramp signal; therefore, when frequency synchronization is used, the time constant of RT/CT must be set longer than the period of the sync signal. Tie the sync pin to ground when the sync feature is not used.
Overcurrent Protection
The overcurrent protection function protects the converter from overcurrent conditions by monitoring the current flowing through the MOSFET. OCP is implemented via a resistor (ROCSET) and a capacitor (COCSET) connecting the OCSET pin and the drain of the MOSFET. An internal 104A current source develops a voltage across ROCSET which is then compared with the voltage developed across the MOSFET at turn on as measured at the PHASE pin. When the voltage drop across the MOSFET exceeds the voltage drop across the resistor, a OCP event occurs. COCSET is placed in parallel with ROCSET to smooth the voltage across ROCSET in the presence of switching noise on the input bus. A 200ns blanking period is used to reduce the current sampling error due to leading-edge switching noise.
PHASE
SYNC RT/CT
The OCP trip point varies mainly due to MOSFET rDS(ON) variations and layout noise concerns. To avoid overcurrent tripping in the normal operating load range, find the ROCSET resistor from the following equations with: 1. The maximum rDS(ON) at the highest junction temperature; 2. The minimum IOCSET from the specification table Determine the overcurrent trip point greater than the maximum output continuous current at maximum inductor ripple current.
FIGURE 11. SYNCHRONIZATION OPERATION
Simple OCP Equation
I OC * r DS ( ON ) R OCSET = -------------------------------------100A
MINIMUM ON-TIME The ISL8107 requires the MOSFET to be turned on to a minimum of 200ns (Typ). This minimum gate pulse width is required to ensure proper samplings of the overcurrent protection circuit. For low duty cycle applications, the switching frequency must be selected to satisfy the condition shown in Equation 4:.
V OUT 1 F OSC < ----------------- -------------------- V IN t on - min (EQ. 4)
Detailed OCP Equation
I I + ---- * r OC_SOU 2 DS ( ON ) ---------------------------------------------------------------------R OCSET = I OCSET (EQ. 6)
Where is converter efficiency. MINIMUM OFF-TIME At the termination of the oscillator's ramp, there is a 190ns time interval before the next ramp starts. This time interval creates the minimum-off time of the PWM. This period ensures that the boot capacitor is refreshed. Equation 5 can be used to calculate the switching frequency to meet the condition:
V OUT 1 F OSC < 1 - ----------------- ---------------------- V IN t off - max (EQ. 5)
V IN - V OUT V OUT I = --------------------------------- * --------------F SW * L OUT V IN F SW = Regulator Switching Frequency
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ISL8107
If overcurrent is detected, the output immediately shuts off, it cycles the soft-start function in a hiccup mode (4 dummy soft-start time-outs, then up to one real one) to provide fault protection. If the shorted condition is not removed, this cycle will continue indefinitely.
Component Selection Guidelines
Output Capacitor Selection
An output capacitor is required to filter the output and supply the load transient current. The filtering requirements are a function of the switching frequency and the ripple current. The load transient requirements are a function of the slew rate (di/dt) and the magnitude of the transient load current. These requirements are generally met with a mix of capacitors and careful layout. For applications that have transient load rates above 1A/ns, high frequency capacitors initially supply the transient and slow the current load rate seen by the bulk capacitors. The bulk filter capacitor values are generally determined by the ESR (effective series resistance) and voltage rating requirements rather than actual capacitance requirements. High frequency decoupling capacitors should be placed as close to the power pins of the load as physically possible. Be careful not to add inductance in the circuit board wiring that could cancel the usefulness of these low inductance components. Consult with the manufacturer of the load on specific decoupling requirements. Use only specialized low-ESR capacitors intended for switching-regulator applications for the bulk capacitors. The bulk capacitor's ESR will determine the output ripple voltage and the initial voltage drop after a high slew-rate transient. An aluminum electrolytic capacitor's ESR value is related to the case size with lower ESR available in larger case sizes. However, the equivalent series inductance (ESL) of these capacitors increases with case size and can reduce the usefulness of the capacitor to high slew-rate transient loading. Unfortunately, ESL is not a specified parameter. Work with your capacitor supplier and measure the capacitor's impedance with frequency to select a suitable component. In most cases, multiple electrolytic capacitors of small case size perform better than a single large case capacitor.
VOUT VOUT
ENSS ENSS
IL IL
FIGURE 12. TYPICAL OVERCURRENT PROTECTION
VOUT VOUT
ENSS
IL IL
FIGURE 13. TYPICAL HICCUP RECOVER
Thermal Protection
If the ISL8107 IC junction temperature reaches a nominal temperature of +150C, the controller will be disabled. The ISL8107 will not be re-enabled until the junction temperature drops below +110C.
Output Inductor Selection
The output inductor is selected to meet the output voltage ripple requirements and minimize the converter's response time to the load transient. The inductor value determines the converter's ripple current and the ripple voltage is a function of the ripple current. The ripple voltage and current are approximated by Equation 7:
V IN - V OUT V OUT I = ------------------------------- * --------------Fs x L V IN
Power-Good
The PGOOD comparator monitors the voltage on the FB pin. PGOOD is asserted (open drain) when the FB pin voltage is within 14% of the reference voltage. The turn-on response of the PGOOD circuit has a typical 3s delay. The PGOOD is deasserted under disable, overcurrent event, or over-temperature event.
VOUT = I x ESR
(EQ. 7)
Increasing the value of inductance reduces the ripple current and voltage. However, the large inductance values reduce the converter's response time to a load transient. One of the parameters limiting the converter's response to a load transient is the time required to change the inductor
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ISL8107
current. The response time is the time required to slew the inductor current from an initial current value to the transient current level. During this interval the difference between the inductor current and the transient current level must be supplied by the output capacitor. Minimizing the response time can minimize the output capacitance required.
KICM 0.6
0.5 0.5IO 0.4
The response time to a transient is different for the application of load and the removal of load. Equation 8 gives the approximate response time interval for application and removal of a transient load:
L O x I TRAN t RISE = ------------------------------V IN - V OUT L O x I TRAN t FALL = -----------------------------V OUT (EQ. 8)
0.3 0.25IO 0.2 I = 0IO 0.1
0.0
0
0.1
0.2
0.3
where: ITRAN is the transient load current step, tRISE is the response time to the application of load, and tFALL is the response time to the removal of load.
0.4 0.5 0.6 DUTY CYCLE (D)
0.7
0.8
0.9
1.0
FIGURE 14. INPUT-CAPACITOR CURRENT MULTIPLIER FOR SINGLE-PHASE BUCK CONVERTER
Input Capacitor Selection
Use a mix of input bypass capacitors to control the voltage overshoot across the MOSFET and the diode. Use small ceramic capacitors for high frequency decoupling and bulk capacitors to supply the current needed each time the MOSFET turns on. Place the small ceramic capacitors physically close to the MOSFET and the diode, and between the drain of the MOSFET and the anode of diode. The important parameters for the bulk input capacitor are the voltage rating and the RMS current rating. For reliable operation, select a bulk capacitor with voltage and current ratings above the maximum input voltage and largest RMS current required by the circuit. The capacitor voltage rating should be at least 1.25x greater than the maximum input voltage, a voltage rating of 1.5x greater is a conservative guideline. The RMS current rating requirement for the input capacitor of a buck regulator is approximately as shown in Equation 9.
I IN, RMS =
OR 2 I O ( D - D 2 ) + ------- D 12
For a through hole design, several electrolytic capacitors (Panasonic HFQ series or Nichicon PL series or Sanyo MV-GX or equivalent) may be needed. For surface mount designs, solid tantalum capacitors can be used, but caution must be exercised with regard to the capacitor surge current rating. These capacitors must be capable of handling the surge-current at power-up. The TPS series available from AVX, and the 593D series from Sprague are both surge current tested.
MOSFET Selection/Considerations
The ISL8107 requires a N-Channel power MOSFET. This should be selected based upon rDS(ON), gate supply requirements, and thermal management requirements. The power dissipation includes two loss components; conduction loss and switching loss.
2 1 P MOSFET = I O x r DS ( ON ) x D + -- x I O x V IN x T SW x F S 2
(EQ. 10) I 2 VO D = ---------VIN (EQ. 9)
where: D is the duty cycle = VO / VIN, TSW is the switching interval, and Fs is the switching frequency. The gate-charge losses are dissipated by the ISL8107 and don't heat the MOSFETs. However, large gate-charge increases the switching interval, tSW which increases the MOSFET switching losses. Ensure that the MOSFET is within its maximum junction temperature at high ambient temperature by calculating the temperature rise according to package thermal-resistance specifications. A separate heatsink may be necessary depending upon MOSFET power, package type, ambient temperature and air flow. Note that at 9V input voltage, the PVCC voltage can be as low as 6V. Low gate-voltage threshold MOSFET must be used in this condition.
I IN, RMS = K ICM * I O
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Rectifier Selection
Power Schottky diode is recommended for better converter efficiency. The rectifier's rated reverse breakdown voltage must be at least equal to the maximum input voltage, preferably with a 20% derating factor. The power dissipation is shown in Equation 11:
V OUT P D [ W ] = I OUT V D 1 - --------------- V IN
break frequency at FLC and a zero at FCE . The DC gain of the modulator is simply the input voltage (VIN) divided by the peak-to-peak oscillator voltage VOSC . The ISL8107 incorporates a feed forward loop that accounts for changes in the input voltage. This maintains a constant modulator gain. For the purpose of this analysis, L and DCR represent the output inductance and its DCR, while C and ESR represents the total output capacitance and its equivalent series resistance in Equation 12.
1 F LC = --------------------------2 L C 1 F CE = -------------------------------2 C ESR (EQ. 12)
(EQ. 11)
where VD is the voltage of the Schottky diode = 0.5V to 0.7V
Application Guidelines
Feedback Compensation
Figure 15 highlights the voltage-mode control loop for a buck converter with type-III compensator. The output voltage is regulated to the reference voltage level. The error amplifier output is compared with the oscillator ramp wave to provide a pulse-width modulated wave with an amplitude of VIN at the PHASE node. The PWM wave is smoothed by the output filter. The output filter capacitor bank's equivalent series resistance is represented by the series resistor ESR.
OSC PWM COMPARATOR VOSC DRIVER VIN LX DRIVER D LO CO VOUT
The compensation network consists of the transconductance amplifier (internal to the ISL8107) and the external R1 to R4, C1 to C3 components. The goal of the compensation network is to provide a closed loop transfer function with high 0dB crossing frequency (F0; typically 0.1 to 0.3 of FSW) and adequate phase margin (better than 45). Phase margin is the difference between the closed loop phase at F0dB and 180. The equations that follow relate the compensation network's poles, zeros and gain to the components (R1 , R2 , R3 , R4 , C1 , C2 , and C3) in Figures 4 and 5. Use the following guidelines for locating the poles and zeros of the compensation network: 1. Select a value for R2, (10k to 100k typically) 2. Calculate C1 such that FZ1 is placed at a fraction of the FLC, at 0.1 to 0.75 of FLC. The higher the quality factor of the output filter and/or the higher the ratio FCE/FLC, the lower the FZ1 frequency (to maximize phase boost at FLC).
1 C 1 = -----------------------------------2 x F Z1 x R 2 (EQ. 13)
-
+
ZFB VE/A +
ESR (PARASITIC)
-
ZIN REFERENCE
ERROR AMP
DETAILED COMPENSATION COMPONENTS C2 C1 R2 ZFB ZIN C3 R1 FB R4 VREF R3 VOUT
3. Calculate C3 such that FBW is placed at desired frequency (typically, 0.1x to 0.5x FSW). FSW represents the switching frequency of the regulator.
2 x F BW x L x C 0 x V OSC C 3 = ----------------------------------------------------------------------V IN x R 2 (EQ. 14)
COMP
ISL8107 has feed forward compensation that adjusts the amplitude of 0.11*VIN. Therefore, the Equation 14 can be simplified as Equation 15:
0.22 x F BW x L x C 0 C 3 = --------------------------------------------------------R2 (EQ. 15)
gm +
ISL8107
R 1 V OUT = V REF x 1 + ------ R 4 FIGURE 15. VOLTAGE-MODE BUCK CONVERTER COMPENSATION DESIGN AND OUTPUT VOLTAGE SELECTION
4. Calculate C2 such that the placement of FP2 is at a fraction of the FSW. The lowering of the frequency helps reduce the gain of the compensation network at high frequency, in turn reducing the HF ripple component at the COMP pin and minimizing resultant duty cycle jitter.
1 C 2 = ------------------------------------2 x F P2 x R 2 (EQ. 16)
5. Calculate R3 such that the placement of FP1 is at the FCE.
1 R 3 = ------------------------------------2 x C 3 x F CE (EQ. 17)
The modulator transfer function is the small-signal transfer function of VOUT /VCOMP. This function is dominated by a DC gain and shaped by the output filter, with a double pole 13
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ISL8107
6. Calculate R1 such that the placement of FZ2 is at the FLC.
(EQ. 18)
GAIN MODULATOR GAIN COMPENSATION GAIN LOOP GAIN OPEN LOOP E/A GAIN FP2
1 R 1 = ------------------------------------- - R 3 2 x C 3 x F LC
FZ1 FZ2
FP1
7. Calculate R4 based on target output voltage.
V REF R 4 = ------------------------------------ x R 1 V OUT - V REF (EQ. 19)
R2 20 log ------- R1 0
It is recommended that a mathematical model be used to plot the loop response. Check the loop gain against the error amplifier's open-loop gain. Verify phase margin results and adjust as necessary. Equations 20 and 21 describe the frequency response of the buck converter in continuous conduction mode (Gvd), feedback compensation (Gcomp) and loop response (GLP):
D MAX V IN 1 + s ( f ) ESR C G vd ( f ) = ------------------------------ ---------------------------------------------------------------------------------------------------------2 V OSC 1 + s ( f ) ( ESR + DCR ) C + s ( f ) L C 1 + s ( f ) R2 C1 G COMP ( f ) = --------------------------------------------------- s ( f ) R1 ( C1 + C2 ) 1 + s ( f ) ( R1 + R3 ) C3 ------------------------------------------------------------------------------------------------------------------C1 C2 ( 1 + s ( f ) R 3 C 3 ) 1 + s ( f ) R 2 -------------------- C 1 + C 2 G LP ( f ) = G vd ( f ) G COMP ( f )
D MAX V IN 20 log ---------------------------------V OSC GFB GCL
LOG
GMOD LOG FLC FCE F0 FREQUENCY
FIGURE 16. ASYMPTOTIC BODE PLOT OF CONVERTER GAIN
target crossover frequencies in the range of 10% to 30% of the switching frequency (FSW).
Layout Considerations
(EQ. 20)
where, s ( f ) = 2 f j
COMPENSATION BREAK FREQUENCY EQUATIONS
1 F Z1 = -----------------------------2 R 2 C 1 1 F Z2 = ------------------------------------------------2 ( R 1 + R 3 ) C 3 1 F P1 = -----------------------------2 R 3 C 3 1 F P2 = -------------------------------------------C1 C2 2 R 2 -------------------C1 + C2 (EQ. 21)
As in any high frequency switching converter, layout is very important. Switching current from one power device to another can generate voltage transients across the impedances of the interconnecting bond wires and circuit traces. These interconnecting impedances should be minimized by using wide, short printed circuit traces. The critical components should be located as close together as possible using ground plane construction or single point grounding. A multi-layer printed circuit board is recommended. Figure 17 shows the critical components of the converter. Note that capacitors CIN and COUT could each represent numerous physical capacitors. Dedicate one solid layer, (usually a middle layer of the PC board) for a ground plane and make all critical component ground connections with vias to this layer. Dedicate another solid layer as a power plane and break this plane into smaller islands of common voltage levels. Keep the metal runs from the PHASE terminals to the output inductor short. The power plane should support the input power and output power nodes. Use copper filled polygons on the top and bottom circuit layers for the PHASE nodes. Use the remaining printed circuit layers for small signal wiring.
Figure 16 shows an asymptotic plot of the DC/DC converter's gain vs frequency. The actual Modulator Gain has a high gain peak dependent on the quality factor (Q) of the output filter, which is not shown. Using the previous guidelines should yield a compensation gain similar to the curve plotted. The open loop error amplifier gain bounds the compensation gain. Check the compensation gain at FP2 against the capabilities of the error amplifier. The loop gain, GLP, is constructed on the log-log graph of Figure 16 by adding the modulator gain, Gvd (in dB), to the feedback compensation gain, GCOMP (in dB). This is equivalent to multiplying the modulator transfer function and the compensation transfer function and then plotting the resulting gain. A stable control loop has a gain crossing with close to a -20dB/decade slope and a phase margin greater than 45. Include worst case component variations when determining phase margin. The mathematical model presented makes a number of approximations and is generally not accurate at frequencies approaching or exceeding half the switching frequency. When designing compensation networks, select
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ISL8107
.
VCC
VCC CBP_VCC
PVCC CBP_PVCC
ISL8107
VIN CIN Q1
Locate the ISL8107 within 2 to 3 inches of the MOSFET, Q1, (1 inch or less for 500kHz or higher operation). The circuit traces for the MOSFETs' gate and source connections from the ISL8107 must be sized to handle up to 2A peak current. Minimize any leakage current paths on the ENSS pin and locate the capacitor, CENSS close to the ENSS pin as the internal current source is only 33A. Provide local VCC decoupling between VCC and GND pins. Locate the capacitor, CBOOT as close as practical to the BOOT pin and the phase node.
UGATE BOOT CIN PHASE
LOUT
VOUT LOAD
ENSS GND PGND CENSS
COUT
KEY TRACE SIZED FOR 2A PEAK CURRENT SHORT TRACE, MINIMUM IMPEDANCE ISLAND ON POWER PLANE LAYER ISLAND ON CIRCUIT AND/OR POWER PLANE LAYER VIA CONNECTION TO GROUND PLANE
FIGURE 17. PRINTED CIRCUIT BOARD POWER PLANES AND ISLANDS
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ISL8107 Quad Flat No-Lead Plastic Package (QFN) Micro Lead Frame Plastic Package (MLFP)
L16.5x5B
16 LEAD QUAD FLAT NO-LEAD PLASTIC PACKAGE (COMPLIANT TO JEDEC MO-220VHHB ISSUE C) MILLIMETERS SYMBOL A A1 A2 A3 b D D1 D2 E E1 E2 e k L L1 N Nd Ne P 0.25 0.35 2.95 2.95 0.28 MIN 0.80 NOMINAL 0.90 0.20 REF 0.33 5.00 BSC 4.75 BSC 3.10 5.00 BSC 4.75 BSC 3.10 0.80 BSC 0.60 16 4 4 0.60 12 0.75 0.15 3.25 3.25 0.40 MAX 1.00 0.05 1.00 NOTES 9 9 5, 8 9 7, 8 9 7, 8 8 10 2 3 3 9 9 Rev. 1 10/02 NOTES: 1. Dimensioning and tolerancing conform to ASME Y14.5-1994. 2. N is the number of terminals. 3. Nd and Ne refer to the number of terminals on each D and E. 4. All dimensions are in millimeters. Angles are in degrees. 5. Dimension b applies to the metallized terminal and is measured between 0.15mm and 0.30mm from the terminal tip. 6. The configuration of the pin #1 identifier is optional, but must be located within the zone indicated. The pin #1 identifier may be either a mold or mark feature. 7. Dimensions D2 and E2 are for the exposed pads which provide improved electrical and thermal performance. 8. Nominal dimensions are provided to assist with PCB Land Pattern Design efforts, see Intersil Technical Brief TB389. 9. Features and dimensions A2, A3, D1, E1, P & are present when Anvil singulation method is used and not present for saw singulation. 10. Depending on the method of lead termination at the edge of the package, a maximum 0.15mm pull back (L1) maybe present. L minus L1 to be equal to or greater than 0.3mm.
All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems. Intersil Corporation's quality certifications can be viewed 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 16
FN6605.0 December 11, 2007

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