LSP7503 Step Up Converter for White LED Back Lighting FEATURES GENERAL DESCRIPTION 1.6A, 0.23, Internal Switch The LSP7503 is a step-up DC/DC converter specifically designed to drive LEDs with a constant current. The device can drive two or more LEDs in series from a Li-Ion battery. Series connection of the LEDs provides identical LED currents resulting in uniform brightness. The 0.2V feedback voltage minimizes power loss in the current setting resistor for better efficiency. Input Range: +2.6V to +5.5V Low Shutdown Current: 0.1uA Adjustable Frequency: 640kHz or 1.3MHz 0.2V Feedback Voltage Small 8-Pin MSOP Green Package The LSP7503 can be operated at 640KHz or 1.3MHz allowing for small filter solution and low noise. An external compensation pin gives the user flexibility in setting loop compensation, which allows the use a low-ESR ceramic output capacitors. Internal Soft-start function results in small inrush current and can be programmed with an external capacitor. TYPICAL APPLICATIONS Cellular Phones, PDA. Handheld Computers, UMPC Digital Cameras, Photo Frames. MP3 Players Released 1.0 The LSP7503 device includes under-voltage lockout, and current limiting protection preventing damage in the event of an output overload. GPS Receivers PIN ASSIGNMENT COMP 1 8 FB 2 7 FREQ SHDN 3 6 VDD GND 4 5 SW SS MSOP-8 (Top View) PIN DESCRIPTION Pin 1 2 Name COMP FB Function Compensation pin for error amplifier Feedback pin with a typical reference voltage of 0.2V for setting LED driving 0 . 2V current. I LED = R SET 3 SHDN Shut Down pin. When SHDN is low, the LSP7503 will turn off. 4 GND Ground pin. 5 SW Switch Pin. 6 VDD Power Supply pin. 7 FS Frequency select pin. The oscillator frequency is set to 640kHz when FREQ is low, and 1.3MHz when FREQ is high 8 SS Soft-Start control pin. 1/9 20090720_1.0 LSP7503 Step Up Converter for White LED Back Lighting TYPICAL APPLICATION CIRCUIT Released 1.0 ABSOLUTE MAXIMUM RATINGS Parameters SW to GND __________ Input Voltage: SHDN / VDD / FREQ to GND SS to GND SW pin maximum current Operating temperature Maximum Operating Junction Temperature, TJ Storage Temperature Range Lead Temperature (Soldering, 10 seconds) Rating Unit 18 V 6 V -0.3 ~ VDD + 0.3 V 2.3 A -20 ~ +85 C 150 C -45 to 125 C 260 C Note: Exceeding these ratings could cause damage to the device. All voltages are with respect to Ground. Currents are positive into, negative out of the specified terminal. 2/9 20090720_1.0 LSP7503 Step Up Converter for White LED Back Lighting THERMAL IMPEDIENCE Thermal Resistance from Junction to Ambient, JA 180C /W Junction Temperature Calculation: TJ = TA + (PD x JA). The JA numbers are guidelines for the thermal performance of the device/pc-board system. Connect the ground pin to ground using a large pad or ground plane for better heat dissipation. All of the above assume no ambient airflow. Maximum Power Calculation: T - TA(MAX) PD(MAX)= J(MAX) JA Maximum recommended junction temperature TJ (C): Ambient temperature of the application TA (C): Junction-to-Ambient thermal resistance of the package, and other heat dissipating JA (OC /W): materials. ELECTRICAL CHARACTERISTICS __________ Parameter Input Voltage Range VDD Under voltage Lockout Symbol VDD UVLO Quiescent Current IDD Shutdown Current ISC FB Reference Voltage VFB FB Input Current Ibias FB Voltage Line Regulation Error Amp Transconductance Error Amp Gain Conditions When VDD is rising, typical hysteresis is 40mV; SW remains off below this level VFB =0.25V, Not switching VFB =0.15V, switching Fosc Maximum Duty Cycle DMAX 2.25 EN = GND Max Unit 5.5 V 2.38 2.52 V 0.21 1.2 0.35 5.0 mA mA 0.1 10 uA 0.2 FB=VREF ICOMP=5uA Av Oscillated Frequency Typ 2.6 2.6VVDD5.5V Gm Min FREQ=GND FREQ=VDD FREQ=GND FREQ=VDD V 1 40 nA - 0.1 0.15 %/V 70 105 240 uA/V - 1500 - V/V 540 1100 79 640 1320 85 85 740 1600 92 1.2 1.6 2.3 A kHz % Current Limit ILIM VDD=1V, D=0.65 ON-Resistance RON Isw=1.2A 0.23 0.5 ISWOFF VSW=12V 0.01 20 uA 300 7 uA 0.3VDD V Leakage Current Reset Switch Resistance Soft Start Charge current Iss Vss=1.2V Input Low Voltage VIL SHDN, FREQ; VDD=2.6V to 5.5V. Input High Voltage VIH SHDN, FREQ; VDD=2.6V to 5.5V. Hysteresis 1.5 SHDN, FREQ; 3/9 4 0.7VDD V 0.1VDD V 20090720_1.0 Released 1.0 VDD=SHDN=3V, FREQ=GND; TA=25C, (Unless otherwise noted) LSP7503 Step Up Converter for White LED Back Lighting ELECTRICAL CHARACTERISTICS (Continued) __________ VDD=SHDN=3V, FREQ=GND; TA=25C, (Unless otherwise noted) Parameter Symbol FREQ Pull-Down Current IFREQ SHDN Input Current ISHDN Conditions Min Typ Max Unit 1.8 5.0 9.0 uA 0.001 1 uA Note: Guaranteed by design, not 100% tested in production. FUNCTIONAL BLOCK DIAGRAM 5 SW FREQ 7 LOGIC S CONTROL Oscillator Sync Ramp Summer SHDN Slope Compensation 3 0.2V Reference Error Amp Current Sense 4 GND 63m PWM Comparator 6 VDD 8 SS COMP 1 APPLICATION INFORMATION 1. Setting the LED Driving Current FB pin is used to set the LED driving current, ILED, of the LSP7503 driver. A resistor RSET connected to FB pin, the LED driving current is determined by: I LED = V FB 0.2V = R SET R SET Where, the feedback pin voltage, VFB, is fixed at 0.2V. 2. Selection of Output Capacitor It is recommended to select output capacitors that the capacitance is high enough and the ESR (Effective Series Resistance) is low enough. These (high capacitance & low ESR) are very important for the Boost Converter to be able to meet the VOUT ripple specification. Ceramic capacitors often have low ESR and can meet the Boost Converter requirements as long as the capacitance values are enough. Note that the capacitance values of all kinds of ceramic capacitor drop when there are DC voltages bias on them. The higher the DC bias, the lower the effective capacitance. The Zxx series capacitors (ex: Z5U) often drop more capacitance than what Yxx series capacitors (ex: Y5V) will drop. And Yxx series are usually worse than Xxx series (ex: X5R). Therefore, it is better to use X5R/X7R type of ceramic capacitors and don't use Yxx series (ex: Y5V) or Zxx series (ex: Z5U) types of capacitor. Although they could be cheaper than X5R or X7R, the permanence of Yxx series and Zxx series are not as good as X5R/X7R and are easier to have problems like audio noise problem. 4/9 20090720_1.0 Released 1.0 FB 2 Switch Driver R LSP7503 Step Up Converter for White LED Back Lighting The lifetime of a ceramic capacitor is shorter if the DC-bias is close to its maximum DC rating. For example, to a VOUT=13.4V application (ex: 4pcs LED in series), a 25VDC capacitor should have a longer lifetime than a 16VDC capacitor does, even when other characteristics of these two capacitors are similar. Electrolytic capacitors have higher ESR than what ceramic capacitors do. If electrolytic capacitors are used as output capacitors, the ESR should be low enough to meet the VOUT ripple voltage requirement: ESR << VOUT Ripple ( Peak to Peak Voltage ) I OUT Ripple For example, if the VOUT ripple voltage of a 5V output DC/DC converter should be smaller than 250mVPeak-to-Peak and if the ripple current is 0.5A, an capacitor whose ESR is << (0.25V/0.5A)500m should be chosen. A 680uF of ESR<500 m capacitor can be used in this case. Note that the ESR of electrolytic capacitor is highly dependent on the temperature - the lower the temperature the higher the ESR and vice versa. This temperature dependence causes VOUT ripple problem and system stability problem sometimes. It may need to use tantalum capacitors or other capacitors that the ESR are temperature independent for applications that temperature ranges are wide. 3. Selection of Input Capacitor It is recommended to put a ceramic capacitor(s) of several uF to 10uF as input capacitor of the Boost Converter. The capacitor(s) is better to be X7R/X5R type. 4. Selection of Inductor It is recommended to use ferrite core as the chock material. Don't use iron powder core because the core loss will be too high for applications that the operation frequency is larger than 300KHz, although the cost of an iron powder core could be cheaper. The DC-R of the chock wire should be as low as possible to reduce the power loss. Below is an equation about the inductor value: L=( V IN , MIN VOUT )2 ( VOUT - V IN , MIN I OUT , MAX f SW ) (3 ) Where, L : Inductor Value ( H ) V IN , MIN : Minimum Input Voltage (V ) VOUT : Typical Output Voltage (V ) I OUT , MAX : Maximum Output Current ( A) f SW : Switching Frequency ( Hz ) : Typical Efficiency Using a higher value inductor can reduce the power loss of the Boost converter. Anyway, a higher value inductor often is bigger in size or has higher DC-R, and the higher DC-R may increase the inductor power loss. Shielding inductor has better EMI performance but the DC-R is often higher than non-shielding inductors of the same size. It is recommended to adopt an inductor value that the DC/DC converter will not transfer from Discontinue-Current-Mode (DCM) to Continue-Current-Mode (CCM) or vise versa when VIN or IOUT change. Such mode changing will cause the duty cycle of the Boost DC/DC converter becomes unstable. 5/9 20090720_1.0 Released 1.0 It is important to ensure the ripple current rating of the output capacitor is enough or the capacitor might burn out during operation. To most electrolytic capacitors, the body temperatures should not be higher than environment temperature plus 10C. If the body temperature of the capacitor is too high, the ripple current could be higher than the rating of the capacitor. For example, if the air temperature that close to the input capacitor is 45C, it is better that the body temperature is << (45C + 10C) = 55C. LSP7503 Step Up Converter for White LED Back Lighting 5. Selection of Flywheel Diode An Schottky diode that the voltage rating larger than 20V and the current rating larger than IOUT is recommended. Adopt a Schottky diode of lower dropout voltage can improve the system efficiency. Please also check the leakage current specification of the Schottky diode at the same time. Note that the maximum working temperatures of many Schottky diodes are only 120C. Please double check the working temperature of the Schottky diode to ensure it is within the specification. 6. Minimum & Maximum Duty Cycle Limitation PWM ICs often have trouble to convert a VOUT from a VIN if the duty cycle is too small (close to 0%) or too big (close to 100%). Small duty cycle happens when VOUT/VIN is low and big duty cycle happens when VOUT/VIN is High. DC/DC converter designers need to carefully examine whether the DC/DC converter under design has such duty cycle limit problem, especially when the nominal VOUT/VIN is already <1.2 or >5. Note that factors like VIN deviation, component value deviation, temperature change, switching frequency deviation...etc, can push the duty cycle to be much higher/lower than what we expect from the nominal VOUT/VIN values. For example, the duty of a 3.3V input, 12V output DC/DC converter seems to be around 72.5%, but the actual duty cycle could be up to 80% if we include the voltage drops of output Schottky, VIN trace drop, VIN ripple voltage drop, inductor line drop ...etc. 7. Open Circuit Protection. LAYOUT GUIDELINES PCB layout is an important stage for power circuit, especially the switching type DC/DC converter that providing high current/voltage and using high switching frequency. If PCB layout is not carefully done, the Boost converter may be unstable or cause serious EMI problems. Use wide, short, and straightforward traces for high current paths. About the input capacitors, two or more ceramic capacitors of several uF or bigger are recommended to be used. Place one of them very close to the VIN pin of IC and ground, and at least one another very close to the inductor. It is very important to keep the loop of the SW pin, Schottky diode, output capacitor, and the GND pin of LSP7503 as small as possible, and also minimize the length of the traces between these components, as shown in the following Fig. 1. This is because the di/dt at these traces is very high and according to the formula of v = L di dt the related voltage spikes will be very high if the trace inductance is high. Such voltage spikes not just cause EMC problems, but may interfere or even damage the IC sometimes. The most important is, it is better NOT to use via holes in the loop described above, because via holes have high inductance. 6/9 20090720_1.0 Released 1.0 The Zener diode DOVP and Resistor R5 are used to protect the converter while any LED failure results in the open circuit of output. LSP7503 Step Up Converter for White LED Back Lighting The loop should be as small as possible. L DF VOUT VIN CIN COUT VDD SW FREQ GND A710 SHDN ON OFF COMP LSP7503 FB SS CP1 CP2 RSET CSS RP Fig. 1 Noisy area. L DF VOUT VIN CIN COUT VDD SW FREQ SHDN ON OFF GND A710 LSP7503 COMP CP2 CP1 RP FB SS RSET CSS Analog components & traces should be far away from noisy area. Fig. 2 A big ground plane (form input to out put) can help almost all the performance of the chip. Beside the ground trace on the top layer, please use another layer as the ground layer. 7/9 20090720_1.0 Released 1.0 Second, keep all the analog components and signal traces, for example the FB sense trace, far away from the noisy areas, that is, the areas near inductor, LSP7503 switch pin, and Schottky diode. If the FB sense trace is close to the noisy area, large noise may be coupled into FB pin and cause ILED value not accurate or unstable. Please refer to Fig. 2. LSP7503 Step Up Converter for White LED Back Lighting ORDERING INFORMATION LSP7503X X X X Package: MS: MSOP8L Output Voltage: Blank: Adj Packing: A: Tape & Real Temperature Grade: E: -40125 MARKING INFORMATION MSOP - 8 Pin 8 7 6 5 LSC 7503 YWX Released 1.0 Logo Part Number Internal Code 1 2 3 4 Date Code Y: Year(9=2009) W: Week Code# 1 2 3 Week 1 2 3 Code# E F G Week 14 15 16 Code# T U V Week 27 28 29 Code# g h I Week 40 41 42 8/9 4 5 6 7 8 9 A 4 5 6 7 8 9 10 H J K L M N P 17 18 19 20 21 22 23 W X Y Z a b c 30 31 32 33 34 35 36 m n p q s t u 43 44 45 46 47 48 49 B 11 O 24 d 37 v 50 C 12 R 25 e 38 w 51 D 13 S 26 f 39 x 52 20090720_1.0 LSP7503 Step Up Converter for White LED Back Lighting PACKAGE INFORMATION MSOP - 8 Pin A J INCHES B P MIN TYP MAX MIN TYP MAX A 0.114 0.118 0.122 2.90 3.00 3.10 B 0.114 2.90 0.118 0.122 C 0.040 0.044 D 0.012 G 0.30 0.016 0.021 0.031 0.40 0.53 0.80 - 0.026 - - 0.65 - 0.006 0.00 0.006 0.000 - 0.15 - 0.15 M 0 - 8 0 - 8 P 0.185 0.193 0.201 4.70 4.90 5.10 C K 9/9 20090720_1.0 Released 1.0 D 3.10 1.12 F J M 3.00 1.02 G K F MILLIMETERS