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NCP1421 600 mA Sync-Rect PFM Step-Up DC-DC Converter with True-Cutoff and Ring-Killer
NCP1421 is a monolithic micropower high-frequency step-up switching converter IC specially designed for battery-operated hand-held electronic products up to 600 mA loading. It integrates Sync-Rect to improve efficiency and to eliminate the external Schottky Diode. High switching frequency (up to 1.2 MHz) allows for a low profile, small-sized inductor and output capacitor to be used. When the device is disabled, the internal conduction path from LX or BAT to OUT is fully blocked and the OUT pin is isolated from the battery. This True-Cutoff function reduces the shutdown current to typically only 50 nA. Ring-Killer is also integrated to eliminate the high-frequency ringing in discontinuous conduction mode. In addition to the above, Low-Battery Detector, Logic-Controlled Shutdown, Cycle-by-Cycle Current Limit and Thermal Shutdown provide value-added features for various battery-operated applications. With all these functions on, the quiescent supply current is typically only 8.5 mA. This device is available in the compact and low profile Micro8t package.
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MARKING DIAGRAM
Micro8 DM SUFFIX CASE 846A
8 1
1421 AYW
1421 A Y W
= Device Code = Assembly Location = Year = Work Week
* Pb-Free Package is Available * High Efficiency: 94% for 3.3 V Output at 200 mA from 2.5 V Input * * * * * * * * * * * * * * * * * * *
88% for 3.3 V Output at 500 mA from 2.5 V Input High Switching Frequency, up to 1.2 MHz (not hitting current limit) Output Current up to 600 mA at VIN = 2.5 V and VOUT = 3.3 V True-Cutoff Function Reduces Device Shutdown Current to typically 50 nA Anti-Ringing Ring-Killer for Discontinuous Conduction Mode High Accuracy Reference Output, 1.20 V $1.5%, can Supply 2.5 mA Loading Current when VOUT > 3.3 V Low Quiescent Current of 8.5 mA Integrated Low-Battery Detector Open Drain Low-Battery Detector Output 1.0 V Startup at No Load Guaranteed Output Voltage from 1.5 V to 5.0 V Adjustable 1.5 A Cycle-by-Cycle Current Limit Multi-function Logic-Controlled Shutdown Pin On Chip Thermal Shutdown with Hysteresis Personal Digital Assistants (PDA) Handheld Digital Audio Products Camcorders and Digital Still Cameras Hand-held Instruments Conversion from one to two Alkaline, NiMH, NiCd Battery Cells to 3.0-5.0 V or one Lithium-ion cells to 5.0 V White LED Flash for Digital Cameras
1 FB LBI/EN LBO REF
PIN CONNECTIONS
1 2 3 4 (Top View) 8 7 6 5 OUT LX GND BAT
ORDERING INFORMATION
Device NCP1421DMR2 NCP1421DMR2G Package Micro8 Micro8 (Pb-Free) Shipping 4000 Tape & Reel 4000 Tape & Reel
Typical Applications
For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging Specifications Brochure, BRD8011/D.
(c) Semiconductor Components Industries, LLC, 2004
October, 2004 - Rev. 6
Publication Order Number: NCP1421/D
NCP1421
VBAT M3 BAT 5
ZLC 0.5 V Chip Enable
+ -
+ 20 mV TRUE CUTOFF CONTROL VDD
VDD
LX 7 OUT
VOUT
CONTROL LOGIC _ZCUR _MSON _MAINSW2ON _TSDON
M2
SENSEFETt
8
M1 6 GND VDD
GND FB 1 _CEN + - _PFM PFM _MAINSWOFD _SYNSW2ON
REF 4
Voltage Reference
_SYNSWOFD _VREFOK _ILIM + -
GND
+ GND
RSENSE LBO 3
1.20 V LBI/EN 2
+ -
GND
Figure 1. Detailed Block Diagram
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NCP1421
PIN FUNCTION DESCRIPTIONS
Pin 1 2 3 4 5 6 7 8 Symbol FB LBI/EN LBO REF BAT GND LX OUT Output Voltage Feedback Input. Low-Battery Detector Input and IC Enable. With this pin pulled down below 0.5 V, the device is disabled and enters the shutdown mode. Open-Drain Low-Battery Detector Output. Output is LOW when VLBI is < 1.20 V. LBO is high impedance in shutdown mode. 1.20 V Reference Voltage Output, bypass with 1.0 mF capacitor. If this pin is not loaded, bypass with 300 nF capacitor; this pin can be loaded up to 2.5 mA @ VOUT = 3.3 V. Battery input connection for internal ring-killer. Ground. N-Channel and P-Channel Power MOSFET drain connection. Power Output. OUT also provides bootstrap power to the device. Description
MAXIMUM RATINGS (TC = 25C unless otherwise noted.)
Rating Power Supply (Pin 8) Input/Output Pins (Pin 1-5, Pin 7) Thermal Characteristics Micro8 Plastic Package Thermal Resistance Junction-to-Air Operating Junction Temperature Range Operating Ambient Temperature Range Storage Temperature Range PD RqJA TJ TA Tstg 520 240 -40 to +150 -40 to +85 -55 to +150 mW _C/W _C _C _C Symbol VOUT VIO Value -0.3, 5.5 -0.3, 5.5 Unit V V
Maximum ratings are those values beyond which device damage can occur. Maximum ratings applied to the device are individual stress limit values (not normal operating conditions) and are not valid simultaneously. If these limits are exceeded, device functional operation is not implied, damage may occur and reliability may be affected. 1. This device contains ESD protection and exceeds the following tests: Human Body Model (HBM) 2.0 kV per JEDEC standard: JESD22-A114. *Except OUT pin, which is 1k V. Machine Model (MM) 200 V per JEDEC standard: JESD22-A115. *Except OUT pin, which is 100 V. 2. The maximum package power dissipation limit must not be exceeded. TJ(max) * TA PD + RqJA 3. Latchup Current Maximum Rating: 150 mA per JEDEC standard: JESD78. 4. Moisture Sensitivity Level: MSL 1 per IPC/JEDEC standard: J-STD-020A.
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NCP1421
ELECTRICAL CHARACTERISTICS (VOUT = 3.3 V, TA = 25C for typical value, -40C v TA v 85C for min/max values unless
otherwise noted.) Characteristic Operating Voltage Output Voltage Range Reference Voltage (VOUT = 3.3 V, ILOAD = 0 mA, CREF = 200 nF, TA = 25C) Reference Voltage (VOUT = 3.3 V, ILOAD = 0 mA, CREF = 200 nF, TA = -40C to 85C) Reference Voltage Temperature Coefficient Reference Voltage Load Current (VOUT = 3.3 V, VREF = VREF_NL "1.5% CREF = 1.0 mF) (Note 5) Reference Voltage Load Regulation (VOUT = 3.3 V, ILOAD = 0 to 100 mA, CREF = 1.0 mF) Reference Voltage Line Regulation (VOUT from 1.5 V to 5.0 V, CREF = 1.0 mF) FB Input Threshold (ILOAD = 0 mA, TA = 25C) FB Input Threshold (ILOAD = 0 mA, TA = -40C to 85C) LBI Input Threshold (ILOAD = 0 mA, TA= -40_C to 85_C) LBI Input Threshold (TA = 25_C) Internal NFET ON-Resistance Internal PFET ON-Resistance LX Switch Current Limit (N-FET) (Note 7) Operating Current into BAT (VBAT = 1.8 V, VFB = 1.8 V, VLX = 1.8 V, VOUT = 3.3 V) Operating Current into OUT (VFB = 1.4 V, VOUT = 3.3 V) LX Switch MAX. ON-Time (VFB = 1.0 V, VOUT = 3.3 V, TA = 25_C) LX Switch MIN. OFF-Time (VFB = 1.0 V, VOUT = 3.3 V, TA = 25_C) FB Input Current True-Cutoff Current into BAT (LBI/EN = GND, VOUT = 0, VIN = 3.3 V, LX = 3.3 V) BAT-to-LX Resistance (VFB = 1.4 V, VOUT = 3.3 V) (Note 7) LBI/EN Input Current LBO Low Output Voltage (VLBI = 0, ISINK = 1.0 mA) Soft-Start Time (VIN = 2.5 V, VOUT = 5.0 V, CREF = 200 nF) (Note 6) EN Pin Shutdown Threshold (TA = 25C) Thermal Shutdown Temperature (Note 7) Thermal Shutdown Hysteresis (Note 7) 5. Loading capability increases with VOUT. 6. Design guarantee, value depends on voltage at VOUT. 7. Values are design guaranteed. Symbol VIN VOUT VREF_NL VREF_NL TCVREF IREF VREF_LOAD VREF_LINE VFB VFB VLBI VLBI RDS(ON)_N RDS(ON)_P ILIM IQBAT IQ tON tOFF IFB IBAT RBAT_LX ILBI VLBO_L TSS VSHDN TSHDN TSDHYS Min 1.0 1.5 1.183 1.174 - - Typ - - 1.200 - 0.03 2.5 Max 5.0 5.0 1.217 1.220 - - Unit V V V V mV/C mA
- - 1.192 1.184 1.162 1.182 - - - - - 0.46 - - - - - - - 0.35 - -
0.05 0.05 1.200 -
1.0 1.0 1.208 1.210 1.230
mV mV/V V V V V W W A mA mA ms ms nA nA W nA V ms V C C
1.200 0.3 0.3 1.5 1.3 8.5 0.72 0.12 1.0 50 100 1.5 - 1.5 0.5 - 30
1.218 - - - 3 14 1.15 0.22 50 - - 50 0.2 20 0.67 145 -
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NCP1421
TYPICAL OPERATING CHARACTERISTICS
1.220 REFERENCE VOLTAGE, VREF/V REFERENCE VOLTAGE, VREF/V VOUT = 3.3 V L = 10 mH CIN = 22 mF COUT = 22 mF CREF = 1.0 mF TA = 25_C VIN = 1.5 V VIN = 2.0 V
1.220 CREF = 200 nF IREF = 0 mA TA = 25C
1.210
1.210
1.200
VIN = 2.5 V
1.200
1.190
1.190
1.180
1
10
100
1000
1.180 1.5
2
2.5
3
3.5
4
4.5
5
OUTPUT CURRENT, ILOAD/mA
VOLTAGE AT OUT PIN, VOUT/V
Figure 2. Reference Voltage vs. Output Current
SWITCH ON RESISTANCE, RDS(ON)/W 1.205 REFERENCE VOLTAGE, VREF/V
Figure 3. Reference Voltage vs. Voltage at OUT Pin
0.6 VOUT = 3.3 V 0.5 0.4 P-FET (M2) 0.3 0.2 0.1 0.0 -40 N-FET (M1)
1.200
1.195
1.190 VOUT = 3.3 V CREF = 200 nF IREF = 0 mA -20 0 20 40 60 80 100
1.185
1.180 -40
-20
0
20
40
60
80
100
AMBIENT TEMPERATURE, TA/C
AMBIENT TEMPERATURE, TA/C
Figure 4. Reference Voltage vs. Temperature
LX SWITCH MAXIMUM, ON TIME, tON/mS 1.0 MINIMUM STARTUP BATTERY VOLTAGE, VBATT/V 1.6
Figure 5. Switch ON Resistance vs. Temperature
0.9
1.4
0.8
1.1
0.7
0.6
0.9 TA = 25C 0.6 0 50 100 150 200 250
0.5 -40
-20
0
20
40
60
80
100
AMBIENT TEMPERATURE, TA/C
OUTPUT LOADING CURRENT, ILOAD/mA
Figure 6. LX Switch Max. ON Time vs. Temperature
Figure 7. Minimum Startup Battery Voltage vs. Loading Current
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NCP1421
TYPICAL OPERATING CHARACTERISTICS
100
100
90 EFFICIENCY/% EFFICIENCY/%
90
80 VIN = 3.3 V VOUT = 5.0 V L = 12 mH CIN = 22 mF COUT = 22 mF TA = 25_C 10 100 1000
80 VIN = 2.5 V VOUT = 3.3 V L = 10 mH CIN = 22 mF COUT = 22 mF TA = 25_C 10 100 1000
70
70
60
60
50 1
50 1
OUTPUT LOADING CURRENT, ILOAD/mA
OUTPUT LOADING CURRENT, ILOAD/mA
Figure 8. Efficiency vs. Load Current
100 100
Figure 9. Efficiency vs. Load Current
90 EFFICIENCY/% EFFICIENCY/%
90
80 VIN = 2.5 V VOUT = 5.0 V L = 6.8 mH CIN = 22 mF COUT = 22 mF TA = 25_C 10 100 1000
80 VIN = 2.0 V VOUT = 3.3 V L = 10 mH CIN = 22 mF COUT = 22 mF TA = 25_C 10 100 1000
70
70
60
60
50 1
50 1
OUTPUT LOADING CURRENT, ILOAD/mA
OUTPUT LOADING CURRENT, ILOAD/mA
Figure 10. Efficiency vs. Load Current
100 100
Figure 11. Efficiency vs. Load Current
90 EFFICIENCY/% EFFICIENCY/%
90
80 VIN = 1.5 V VOUT = 5.0 V L = 2.2 mH CIN = 22 mF COUT = 22 mF TA = 25_C 10 100 1000
80 VIN = 1.5 V VOUT = 1.8 V L = 2.2 mH CIN = 22 mF COUT = 22 mF TA = 25_C
70
70
60
60
50 1
50 1
10
100
1000
OUTPUT LOADING CURRENT, ILOAD/mA
OUTPUT LOADING CURRENT, ILOAD/mA
Figure 12. Efficiency vs. Load Current
Figure 13. Efficiency vs. Load Current
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NCP1421
TYPICAL OPERATING CHARACTERISTICS
10 OUTPUT VOLTAGE CHANGE/% OUTPUT VOLTAGE CHANGE/% 10
5
5
0 VOUT = 3.3 V L = 5.6 mH CIN = 22 mF COUT = 22 mF TA = 25_C 100
VIN = 2.5 V
0 VOUT = 5.0 V L = 5.6 mH CIN = 22 mF COUT = 22 mF TA = 25_C
VIN = 3.3 V
-5
VIN = 2.0 V
-5
VIN = 1.5 V
VIN = 2.5 V
-10 10
1000
-10 10
100 OUTPUT LOADING CURRENT, ILOAD/mA
1000
OUTPUT LOADING CURRENT, ILOAD/mA
Figure 14. Output Voltage Change vs. Load Current
50 RIPPLE VOLTAGE, VRIPPLE/mVp-p VIN = 2.5 V VOUT = 3.3 V L = 6.8 mH CIN = 22 mF COUT = 22 mF TA = 25_C
Figure 15. Output Voltage Change vs. Load Current
40
30
500 mA
20 300 mA 10 100 mA 0 1.5 1.7 1.9 2.1 2.3 2.5 Upper Trace: Voltage at LBI Pin, 1.0 V/Division Lower Trace: Voltage at LBO Pin, 1.0 V/Division
BATTERY INPUT VOLTAGE, VBATT/V
Figure 16. Battery Input Voltage vs. Output Ripple Voltage
NO LOAD OPERATING CURRENT, IBATT/mA 15
Figure 17. Low Battery Detect
12.5
10
7.5 VIN = 2.5 V VOUT = 5.0 V ILOAD = 10 mA 2.0 2.5 3.0 3.5 4.0 4.5 5.0
5.0
2.5 1.5
INPUT VOLTAGE AT OUT PIN, VOUT/V
Upper Trace: Input Voltage Waveform, 1.0 V/Division Lower Trace: Output Voltage Waveform, 2.0 V/Division
Figure 18. No Load Operating Current vs. Input Voltage at OUT Pin http://onsemi.com
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Figure 19. Startup Transient Response
NCP1421
TYPICAL OPERATING CHARACTERISTICS
(VIN = 2.5 V, VOUT = 3.3 V, ILOAD = 50 mA; L = 5.6 mH, COUT = 22 mF) Upper Trace: Output Voltage Ripple, 20 mV/Division Lower Trace: Voltage at Lx pin, 1.0 V/Division
(VIN = 2.5 V, VOUT = 3.3 V, ILOAD = 500 mA; L = 5.6 mH, COUT = 22mF) Upper Trace: Output Voltage Ripple, 20 mV/Division Lower Trace: Voltage at LX pin, 1.0 V/Division
Figure 20. Discontinuous Conduction Mode Switching Waveform
Figure 21. Continuous Conduction Mode Switching Waveform
(VIN = 1.5 V to 2.5 V; L = 5.6 mH, COUT = 22mF, ILOAD = 100 mA) Upper Trace: Output Voltage Ripple, 100 mV/Division Lower Trace: Battery Voltage, VIN, 1.0 V/Division
(VIN = 1.5 V to 2.5 V; L = 5.6 mH, COUT = 22mF, ILOAD = 100 mA) Upper Trace: Output Voltage Ripple, 100 mV/Division Lower Trace: Battery Voltage, VIN, 1.0 V/Division
Figure 22. Line Transient Response for VOUT = 3.3 V
Figure 23. Line Transient Response For VOUT = 5.0 V
(VOUT = 3.3 V, ILOAD = 50 mA to 500 mA; L = 5.6 mH, COUT = 22 mF) Upper Trace: Output Voltage Ripple, 50 mV/Division Lower Trace: Load Current, ILOAD, 500 mA/Division
(VOUT = 5.0 V, ILOAD = 50 mA to 500 mA; L = 5.6 mH, COUT = 22 mF) Upper Trace: Output Voltage Ripple, 100 mV/Division Lower Trace: Load Current, ILOAD, 500 mA/Division
Figure 24. Load Transient Response For VIN = 2.5 V
Figure 25. Load Transient Response For VIN = 3.0 V
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NCP1421
DETAILED OPERATION DESCRIPTION NCP1421 is a monolithic micropower high-frequency step-up voltage switching converter IC specially designed for battery operated hand-held electronic products up to 600 mA loading. It integrates a Synchronous Rectifier to improve efficiency as well as to eliminate the external Schottky diode. High switching frequency (up to 1.2 MHz) allows for a low profile inductor and output capacitor to be used. Low-Battery Detector, Logic-Controlled Shutdown, and Cycle-by-Cycle Current Limit provide value-added features for various battery-operated applications. With all these functions ON, the quiescent supply current is typically only 8.5 mA. This device is available in a compact Micro8 package.
PFM Regulation Scheme
From the simplified functional diagram (Figure 1), the output voltage is divided down and fed back to pin 1 (FB). This voltage goes to the non-inverting input of the PFM comparator whereas the comparator's inverting input is connected to the internal voltage reference, REF. A switching cycle is initiated by the falling edge of the comparator, at the moment the main switch (M1) is turned ON. After the maximum ON-time (typically 0.72 mS) elapses or the current limit is reached, M1 is turned OFF and the synchronous switch (M2) is turned ON. The M1 OFF time is not less than the minimum OFF-time (typically 0.12 mS), which ensures complete energy transfer from the inductor to the output capacitor. If the regulator is operating in Continuous Conduction Mode (CCM), M2 is turned OFF just before M1 is supposed to be ON again. If the regulator is operating in Discontinuous Conduction Mode (DCM), which means the coil current will decrease to zero before the new cycle starts, M1 is turned OFF as the coil current is almost reaching zero. The comparator (ZLC) with fixed offset is dedicated to sense the voltage drop across M2 as it is conducting; when the voltage drop is below the offset, the ZLC comparator output goes HIGH and M2 is turned OFF. Negative feedback of closed-loop operation regulates voltage at pin 1 (FB) equal to the internal reference voltage (1.20 V).
Synchronous Rectification
of dead time is introduced to make sure M1 is completely turned OFF before M2 is being turned ON. The previously mentioned situation occurs when the regulator is operating in CCM, M2 is being turned OFF, M1 is just turned ON, and M2 is not being completely turned OFF. A dead time is also needed to make sure M2 is completely turned OFF before M1 is being turned ON. As coil current is dropped to zero when the regulator is operating in DCM, M2 should be OFF. If this does not occur, the reverse current flows from the output bulk capacitor through M2 and the inductor to the battery input, causing damage to the battery. The ZLC comparator comes with fixed offset voltage to switch M2 OFF before any reverse current builds up. However, if M2 is switched OFF too early, large residue coil current flows through the body diode of M2 and increases conduction loss. Therefore, determination of the offset voltage is essential for optimum performance. With the implementation of the synchronous rectification scheme, efficiency can be as high as 94% with this device.
Cycle-by-Cycle Current Limit
In Figure 1, a SENSEFET is used to sample the coil current as M1 is ON. With that sample current flowing through a sense resistor, a sense-voltage is developed. The threshold detector (ILIM) detects whether the sense-voltage is higher than the preset level. If the sense voltage is higher than the present level, the detector output notifies the Control Logic to switch OFF M1, and M1 can only be switched ON when the next cycle starts after the minimum OFF-time (typically 0.12 mS). With proper sizing of the SENSEFET and sense resistor, the peak coil current limit is typically set at 1.5 A.
Voltage Reference
The voltage at REF is typically set at 1.20 V and can output up to 2.5 mA with load regulation 2% at VOUT equal to 3.3 V. If VOUT is increased, the REF load capability can also be increased. A bypass capacitor of 200 nF is required for proper operation when REF is not loaded. If REF is loaded, a 1.0 mF capacitor at the REF pin is needed.
True-Cutoff
The Synchronous Rectifier is used to replace the Schottky Diode to reduce the conduction loss contributed by the forward voltage of the Schottky Diode. The Synchronous Rectifier is normally realized by powerFET with gate control circuitry that incorporates relatively complicated timing concerns. As the main switch (M1) is being turned OFF and the synchronous switch M2 is just turned ON with M1 not being completely turned OFF, current is shunt from the output bulk capacitor through M2 and M1 to ground. This power loss lowers overall efficiency and possibly damages the switching FETs. As a general practice, a certain amount
The NCP1421 has a True-Cutoff function controlled by the multi-function pin LBI/EN (pin 2). Internal circuitry can isolate the current through the body diode of switch M2 to load. Thus, it can eliminate leakage current from the battery to load in shutdown mode and significantly reduce battery current consumption during shutdown. The shutdown function is controlled by the voltage at pin 2 (LBI/EN). When pin 2 is pulled to lower than 0.3 V, the controller enters shutdown mode. In shutdown mode, when switches M1 and M2 are both switched OFF, the internal
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NCP1421
reference voltage of the controller is disabled and the controller typically consumes only 50 nA of current. If the pin 2 voltage is raised to higher than 0.5 V (for example, by a resistor connected to VIN), the IC is enabled again, and the internal circuit typically consumes 8.5 mA of current from the OUT pin during normal operation.
Low-Battery Detection
A comparator with 30 mV hysteresis is applied to perform the low-battery detection function. When pin 2
(LBI/EN) is at a voltage (defined by a resistor divider from the battery voltage) lower than the internal reference voltage of 1.20 V, the comparator output turns on a 50 W low side switch. It pulls down the voltage at pin 3 (LBO) which has hundreds of kW of pull-high resistance. If the pin 2 voltage is higher than 1.20 V + 30 mV, the comparator output turns off the 50 W low side switch. When this occurs, pin 3 becomes high impedance and its voltage is pulled high again.
APPLICATIONS INFORMATION
Output Voltage Setting
A typical application circuit is shown in Figure 26. The output voltage of the converter is determined by the external feedback network comprised of R1 and R2. The relationship is given by:
VOUT + 1.20 V 1 ) R1 R2
where R1 and R2 are the upper and lower feedback resistors, respectively.
Low Battery Detect Level Setting
The Low Battery Detect Voltage of the converter is determined by the external divider network that is comprised of R3 and R4. The relationship is given by:
VLB + 1.20 V 1 ) R3 R4
voltage/current waveforms. The currents flowing into and out of the capacitors multiply with the Equivalent Series Resistance (ESR) of the capacitor to produce ripple voltage at the terminals. During the Syn-Rect switch-off cycle, the charges stored in the output capacitor are used to sustain the output load current. Load current at this period and the ESR combine and reflect as ripple at the output terminals. For all cases, the lower the capacitor ESR, the lower the ripple voltage at output. As a general guideline, low ESR capacitors should be used. Ceramic capacitors have the lowest ESR, but low ESR tantalum capacitors can also be used as an alternative.
PCB Layout Recommendations
where R3 and R4 are the upper and lower divider resistors respectively.
Inductor Selection
Good PCB layout plays an important role in switching mode power conversion. Careful PCB layout can help to minimize ground bounce, EMI noise, and unwanted feedback that can affect the performance of the converter. Hints suggested below can be used as a guideline in most situations.
Grounding
The NCP1421 is tested to produce optimum performance with a 5.6 mH inductor at VIN = 2.5 V and VOUT = 3.3 V, supplying an output current up to 600 mA. For other input/output requirements, inductance in the range 3 mH to 10 mH can be used according to end application specifications. Selecting an inductor is a compromise between output current capability, inductor saturation limit, and tolerable output voltage ripple. Low inductance values can supply higher output current but also increase the ripple at output and reduce efficiency. On the other hand, high inductance values can improve output ripple and efficiency; however, it is also limited to the output current capability at the same time. Another parameter of the inductor is its DC resistance. This resistance can introduce unwanted power loss and reduce overall efficiency. The basic rule is to select an inductor with the lowest DC resistance within the board space limitation of the end application. In order to help with the inductor selection, reference charts are shown in Figure 27 and 28.
Capacitors Selection
A star-ground connection should be used to connect the output power return ground, the input power return ground, and the device power ground together at one point. All high-current paths must be as short as possible and thick enough to allow current to flow through and produce insignificant voltage drop along the path. The feedback signal path must be separated from the main current path and sense directly at the anode of the output capacitor.
Components Placement
Power components (i.e., input capacitor, inductor and output capacitor) must be placed as close together as possible. All connecting traces must be short, direct, and thick. High current flowing and switching paths must be kept away from the feedback (FB, pin 1) terminal to avoid unwanted injection of noise into the feedback path.
Feedback Network
In all switching mode boost converter applications, both the input and output terminals see impulsive
Feedback of the output voltage must be a separate trace detached from the power path. The external feedback network must be placed very close to the feedback (FB, pin 1) pin and sense the output voltage directly at the anode of the output capacitor.
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NCP1421
TYPICAL APPLICATION CIRCUIT
VIN C1 22 mF L 6.5 mH
R2 200 k Shutdown Open Drain Input
C4 10 p*
R3 220 k
R1 350 k 1 2 3 FB
NCP1421 OUT LX GND BAT 8 7 6 5 C2 22 mF + VOUT =3.3 V 500 mA
LBI/EN LBO REF
R4 330 k Low Battery Open Drain Output *Optional
4 C3 200 nF
Figure 26. Typical Application Schematic for 2 Alkaline Cells Supply
GENERAL DESIGN PROCEDURES Switching mode converter design is considered a complicated process. Selecting the right inductor and capacitor values can allow the converter to provide optimum performance. The following is a simple method based on the basic first-order equations to estimate the inductor and capacitor values for NCP1421 to operate in Continuous Conduction Mode (CCM). The set component values can be used as a starting point to fine tune the application circuit performance. Detailed bench testing is still necessary to get the best performance out of the circuit. Design Parameters: VIN = 1.8 V to 3.0 V, Typical 2.4 V VOUT = 3.3 V IOUT = 500 mA (600 mA max) VLB = 2.0 V VOUT-RIPPLE = 45 mVp-p at IOUT = 500 mA Calculate the feedback network: Select R2 = 200 k
R1 + R2 VOUT *1 VREF 3.3 V * 1 + 350 k 1.20 V
Determine the Steady State Duty Ratio, D, for typical VIN. The operation is optimized around this point:
VOUT +1 1*D VIN D+1* VIN + 1 * 2.4 V + 0.273 VOUT 3.3 V
Determine the average inductor current, ILAVG, at maximum IOUT:
I ILAVG + OUT + 500 mA + 688 mA 1 * 0.273 1*D
Determine the peak inductor ripple current, IRIPPLE-P, and calculate the inductor value: Assume IRIPPLE-P is 20% of ILAVG. The inductance of the power inductor can be calculated as follows:
L+ VIN tON 2.4 V 0.75 mS + + 6.5 mH 2 IRIPPLE*P 2 (137.6 mA)
A standard value of 6.5 mH is selected for initial trial. Determine the output voltage ripple, VOUT-RIPPLE, and calculate the output capacitor value: VOUT-RIPPLE = 40 mVP-P at IOUT = 500 mA
IOUT tON COUT u VOUT*RIPPLE * IOUT ESRCOUT
R1 + 200 k
Calculate the Low Battery Detect divider: VLB = 2.0 V Select R4 = 330 k
R3 + R4 VLB *1 VREF 2.0 V * 1 + 220 k 1.20 V
where tON = 0.75 uS and ESRCOUT = 0.05 W,
500 mA 0.75 mS COUT u + 18.75 mF 45 mV * 500 mA 0.05 W
R3 + 300 k
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NCP1421
From the previous calculations, you need at least 18.75 mF in order to achieve the specified ripple level at the conditions stated. Practically, a capacitor that is one level larger is used to accommodate factors not taken into account in the calculations. Therefore, a capacitor value of 22 mF is selected. The NCP1421 is internally compensated for most applications, but in case additional compensation
16 14 INDUCTOR VALUE (mH) 12 10 8 6 4 2 0 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 IOUT = 500 mA INDUCTOR VALUE (mH)
is required, the capacitor C4 can be used as external compensation adjustment to improve system dynamics. In order to provide an easy way for customers to select external parts for NCP1421 in different input voltage and output current conditions, values of inductance and capacitance are suggested in Figure 27, 28 and 29.
21 18 15 12 9 6 3 0 1.6 1.9 2.2 2.5 2.8 3.1 IOUT = 500 mA 3.4 3.7 4.0
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
Figure 27. Suggested Inductance of VOUT = 3.3 V
Figure 28. Suggested Inductance of VOUT = 5.0 V
40 35 CAPACITOR VALUE (mF) 30 25 20 15 10 5 0 200 250 300 350 400 450 500 550 VOUT-RIPPLE = 50 mV VOUT-RIPPLE = 45 mV VOUT-RIPPLE = 40 mV
25
CAPACITOR ESR (mW)
33
50
100
600
OUTPUT CURRENT (mA)
Figure 29. Suggested Capacitance for Output Capacitor
Table 1. Suggestions for Passive Components
Output Current 500 mA 250 mA Inductors Sumida CR43, CR54,CDRH6D28 series Sumida CR32 series Capacitors Panasonic ECJ series Kemet TL494 series Panasonic ECJ series Kemet TL494 series
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NCP1421
PACKAGE DIMENSIONS
Micro8 DM SUFFIX CASE 846A-02 ISSUE F
-A-
K
-B-
PIN 1 ID
G D 8 PL 0.08 (0.003)
M
NOTES: 1. DIMENSIONING AND TOLERANCING PER ANSI Y14.5M, 1982. 2. CONTROLLING DIMENSION: MILLIMETER. 3. DIMENSION A DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.15 (0.006) PER SIDE. 4. DIMENSION B DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSION. INTERLEAD FLASH OR PROTRUSION SHALL NOT EXCEED 0.25 (0.010) PER SIDE. 5. 846A-01 OBSOLETE, NEW STANDARD 846A-02. DIM A B C D G H J K L MILLIMETERS MIN MAX 2.90 3.10 2.90 3.10 --- 1.10 0.25 0.40 0.65 BSC 0.05 0.15 0.13 0.23 4.75 5.05 0.40 0.70 INCHES MIN MAX 0.114 0.122 0.114 0.122 --- 0.043 0.010 0.016 0.026 BSC 0.002 0.006 0.005 0.009 0.187 0.199 0.016 0.028
TB
S
A
S
-T- PLANE 0.038 (0.0015) H
SEATING
C J L
SOLDERING FOOTPRINT*
8X
1.04 0.041
0.38 0.015
8X
3.20 0.126
4.24 0.167
5.28 0.208
6X
0.65 0.0256
SCALE 8:1
mm inches
*For additional information on our Pb-Free strategy and soldering details, please download the ON Semiconductor Soldering and Mounting Techniques Reference Manual, SOLDERRM/D.
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NCP1421
Micro8 is a trademark of International Rectifier. SENSEFET is a trademark of Semiconductor Components Industries, LLC.
ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. "Typical" parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including "Typicals" must be validated for each customer application by customer's technical experts. SCILLC does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.
PUBLICATION ORDERING INFORMATION
LITERATURE FULFILLMENT: N. American Technical Support: 800-282-9855 Toll Free Literature Distribution Center for ON Semiconductor USA/Canada P.O. Box 61312, Phoenix, Arizona 85082-1312 USA Phone: 480-829-7710 or 800-344-3860 Toll Free USA/Canada Japan: ON Semiconductor, Japan Customer Focus Center 2-9-1 Kamimeguro, Meguro-ku, Tokyo, Japan 153-0051 Fax: 480-829-7709 or 800-344-3867 Toll Free USA/Canada Phone: 81-3-5773-3850 Email: orderlit@onsemi.com ON Semiconductor Website: http://onsemi.com Order Literature: http://www.onsemi.com/litorder For additional information, please contact your local Sales Representative.
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NCP1421/D

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