ISL97631 ® Data Sheet December 21, 2005 White LED Boost Regulator With Integrated Schottky Diode FN7370.1 Features • Up to 6 LEDs in series The ISL97631 represents a high efficiency, constant frequency PWM regulator with integrated boost diode and FET. Designed for use in white LED driving applications, the ISL97631 features efficiencies up to 85%. It operates at 1.35MHz switching frequency and operates from an input voltage of between 2.7V and 5.5V. The maximum output voltage of 27V enables the ISL97631 to drive up to 6 LEDs in series. It is also possible to use the ISL97631 to drive LEDs in series/parallel combination for applications requiring up to 15 LEDs. • 27V maximum output • 2.7V to 5.5V input • Up to 85% efficient • 1.35MHz constant frequency • Enable/PWM dimming control • Pb-free plus anneal available (RoHS compliant) Applications Available in the 6 Ld TSOT package, the ISL97631 offers high efficiency, constant frequency operation. It is specified for operation over the -40°C to +85°C ambient temperature range. • LED backlighting Pinout • Handheld games • Cell phones • PDAs • MP3 players ISL97631 (6 LD TSOT) TOP VIEW • GPS • Other handheld devices VOUT 1 6 VIN GND 2 5 FB LX 3 4 ENAB Ordering Information PART MARKING PART NUMBER PCS. TAPE & REEL PACKAGE (TAPE AND REEL) PKG. DWG. # ISL97631IHTZ-T7 (See Note) 631Z 3,000 7” 6 Ld TSOT (Pb-free) MDP0049 ISL97631IHTZ-T7A (See Note) 631Z 270 7” 6 Ld TSOT (Pb-free) MDP0049 NOTE: Intersil Pb-free plus anneal products employ special Pb-free material sets; molding compounds/die attach materials and 100% matte tin plate termination finish, which are 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. 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. 2005. All Rights Reserved All other trademarks mentioned are the property of their respective owners. ISL97631 Absolute Maximum Ratings (TA = 25°C) Thermal Information Input Voltage (VIN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +6V LX, Vout Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +27V FB Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +6V ENAB Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +6V Operating Temperature . . . . . . . . . . . . . . . . . . . . . . .-40°C to +85°C Thermal Resistance (Typical, Note 1) θJA (°C/W) 6 Ld TSOT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 Maximum Junction Temperature (Plastic Package . . . . . . . . . 150°C Maximum Storage Temperature Range . . . . . . . . . .-65°C to +150°C Maximum Lead Temperature (Soldering, 10s). . . . . . . . . . . . +300°C (TSOT - Lead Tips Only) CAUTION: Stresses above those listed in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only rating and operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied. IMPORTANT NOTE: All parameters having Min/Max specifications are guaranteed over the specified temperature range. All parameters are based on pulsed tests, therefore: TJ = TC = TA NOTE: 1. θJA is measured with the component mounted on a high effective thermal conductivity test board in free air. See Tech Brief TB379 for details. Electrical Specifications PARAMETER VIN = 3V, VENAB = 3V, TA = -40°C to 85°C unless otherwise specified. DESCRIPTION CONDITION VIN-MIN Minimum Operating Voltage VOUT = 16V, ILED = 20mA VIN-MAX Maximum Operating Voltage VOUT = 25V, ILED = 20mA VFB Feedback Voltage IFB FB Pin Bias Current IIN Supply Current MIN TYP 2.7 80 ENAB = 3V, output not switching Switching Frequency DMAX Maximum Duty Cycle ILIM rDS(ON) Switch Current Limit TA = 25°C TA = 25°C UNIT V 100 0.6 ENAB = 0V FOSC MAX 5.5 V 120 mV 100 nA 1.0 mA 1 µA 1.8 MHz 0.8 1.35 85 90 % 82 90 % 280 350 mA 250 350 mA Switch On Resistance ILX = 100mA 750 ILEAK(LX) Switch Leakage Current VLX = 27V, Vout = 27V 0.01 1 µA ILEAK(VOUT) Diode Leakage Current VOUT = 27V 0.01 1 µA LX-VOUT Diode Forward Voltage IDIODE = 100mA, TA = 25°C 0.75 0.9 V VDIODE VENAB-HI ENAB Voltage High VENAB-LO ENAB Voltage Low IENAB ∆ILED/∆VIN Eff mΩ 2.5 V ENAB Pin Bias Current 0.6 V 1 µA Line Regulation VIN = 2.7V to 5V 0.2 %/V Efficiency ILED = 20mA, 3 LEDs 85 % 2 FN7370.1 December 21, 2005 ISL97631 Typical Application 90 L1 22µH, VIN = 4V 85 VIN 2.7V~5.5V C1 LEDs LX VOUT ISL97631 1µF FB ENAB OFF/ON RSET 4.75Ω GND C2 0.22µF EFFICIENCY (%) 22µH VIN 80 75 70 65 60 55 50 0 5 10 15 20 25 30 ILED (mA) FIGURE 1. TYPICAL APPLICATION CIRCUIT FIGURE 2. EFFICIENCY vs LED CURRENT (VIN = 4V) Typical Performance Curves 18.244 700 18.242 600 18.240 IO (mA) Iq (µA) 500 400 300 18.238 18.236 18.234 200 18.232 100 18.230 18.228 0 0 2 4 0 6 5 VIN (V) FIGURE 3. QUIESCENT CURRENT vs VIN (ENAB = hi) 1.36 18.31 1.35 FREQUENCY (MHz) 1.37 18.32 18.30 IO (mA) 15 20 FIGURE 4. LOAD REGULATION (VIN = 4V) 18.33 18.29 18.28 18.27 18.26 18.25 18.24 18.23 2.5 10 VOUT (V) 1.34 1.33 1.32 1.31 1.30 1.29 1.28 3.5 4.5 VIN (V) FIGURE 5. LINE REGULATION 3 5.5 1.27 -60 -40 -20 0 20 40 60 TEMPERATURE (°C) 80 100 FIGURE 6. SWITCHING FREQUENCY vs TEMPERATURE FN7370.1 December 21, 2005 ISL97631 Block Diagram Vin Enable LX ISL97631 1.35MHz 1.2MHz Oscillator and Ramp Generator Vout PWM Comparator PWM Logic Controller FET Driver Current Sense GND GM Amp Compensation GM Amplifier FB 95mV Bandgap Reference Generator FIGURE 7. ISL97631 BLOCK DIAGRAM Pin Descriptions PIN PIN NUMBER NAME DESCRIPTION 1 VOUT Output Pin. Connect to the anode of the top LED and the output filter capacitor . 2 GND 3 LX 4 Ground Pin. Connect to local ground. Switching Pin. Connect to inductor. ENAB Enable Pin. Connect to enable signal to turn-on or off the device. 5 FB Feedback Pin. Connect to the cathode of bottom LED and the sense resistor. 6 VIN Input Supply Pin. Connect to the input supply voltage, the inductor and the input supply decoupling capacitor. Detailed Description The ISL97631 uses a constant frequency, current mode control scheme to provide excellent line and load regulation. It can drive up to 6 LEDs in series or 15 LEDs in parallel/series configuration, with efficiencies of up 85%. The ISL97631 operates from an input voltage of 2.7V to 5.5V and can boost up to 27V. Steady-State Operation The ISL97631 operates with constant frequency PWM. The switching frequency is around 1.35MHz. Depending on the input voltage, inductance, number of LEDs and the LED 4 current, the converter operates in either continuous conduction mode or discontinuous conduction mode. Both are normal. The forward current of the LED is set using the RSET resistor. In steady state mode, this current is given by the equation: V FB I LED = -------------R SET (EQ. 1) Shut-Down When taken low the ENAB pin places the ISL97631 into power down mode. When in power down, the supply current is reduced to less than 1µA. Dimming Control PWM DIMMING The ENAB pin also doubles as a brightness control. There are two different possible dimming control methods. The first dimming method is controlled through the duty-cycle of the ENAB input PWM waveform, which can operate at frequencies up to 1kHz. For frequencies greater than 1kHz, see Analog Dimming. The LEDs operate at either zero or full current. This is the PWM dimming control method. The relationship between the average LED current and the duty-cycle (D) of the ENAB pin’s waveform is as follows: V FB average I LED = --------------- ⋅ D R SET (EQ. 2) FN7370.1 December 21, 2005 ISL97631 The magnitude of the PWM signal should be higher than the minimum ENAB voltage high. The bench PWM dimming test results are shown in Figure 8. In the test, two PWM frequencies 400Hz and 1kHz are chosen to compare the linear dimming range. It is clear that there is a wider linear dimming range for the lower PWM frequency than for the higher one, due to the self discharge of the output capacitor through the LEDs during the low ENAB periods. To achieve a better linearity with high frequencies an NMOS FET can be placed between the FB pin and the LED stack, with its gate driven by the same signal as ENAB. This acts to prevent self discharge of the output capacitor during the off periods. In the PWM dimming test, the output capacitor is 0.22µF. L1 22µH VIN 2.7V~5.5V C1 1µF OFF/ON VIN LEDs LX VOUT C2 0.22µF ISL97631 ENAB FB GND R1 3.3k R2 RSET 4.75Ω VDim FIGURE 9. ANALOG DIMMING CONTROL APPLICATION CIRCUIT 20 18 The analog dimming circuit can be tailored to a desired relative brightness for different VDim ranges using Equation 5. 16 IO (mA) 14 12 10 [ ( V Dim_max – V FB ) • R 1 ] R 2 = ------------------------------------------------------------------[ V FB • ( 1 – F min ) ] 1kHz 8 400Hz 6 (EQ. 5) Where VDim_max is the maximum VDim voltage and Fmin is the minimum relative brightness (i.e., the brightness with VDim_max applied). 4 2 0 0 20 40 60 80 100 DUTY-CYCLE (%) FIGURE 8. PWM DIMMING LINEAR RANGE (FOR 400Hz AND 1kHz PWM FREQUENCIES CONDITION, COUT = 0.22µF) ANALOG DIMMING The second dimming method applies a variable DC voltage (VDim) at FB pin (see Figure 9) to adjust the LED current. As the DC dimming signal voltage increases above VFB, the voltages drop on R1 and R2 increase and the voltage drop on RSET decreases. Thus, the LED current decreases. V FB ⋅ ( R 1 + R 2 ) – V Dim ⋅ R 1 I LED = ------------------------------------------------------------------------R2 ⋅ R (EQ. 3) SET The DC dimming signal voltage can be a variable DC voltage or a DC voltage generated by filtering a high frequency PWM control signal. As brightness is directly proportional to LED currents, VDim may be calculated for any desired “relative brightness” (F) using Equation 4. R2 R V Dim = ------- ⋅ V FB ⋅ 1 + ------1- – F R1 R2 i.e., VDim_max = 5V, Fmin = 10% (i.e., 0.1), R2 = 189k i.e., VDim_max = 1V, Fmin = 10% (i.e., 0.1), R2 = 35k Open-Voltage Protection In some applications, it is possible that the output is opened, e.g. when the LEDs are disconnected from the circuit or the LEDs fail. In this case the feedback voltage will be zero. The ISL97631 will then switch to a high duty cycle resulting in a high output voltage, which may cause the LX pin voltage to exceed its maximum 27V rating. To implement overvoltage protection, a zener diode Dz and a resistor R1 can be used at the output and FB pin to limit the voltage on the LX pin as shown in Figure 10. It is clear that as the zener is turned on, due to the overvoltage, the zener diode’s current will set up a voltage on R1 and RSET and this voltage is applied on FB pin as the feedback node. This feedback will prevent the output from reaching the overvoltage condition. In the overvoltage protection circuit design, the zener voltage should be larger than the maximum forward voltage of the LED string. (EQ. 4) Where F = ILED (dimmed)/ILED (undimmed). These equations are valid for values of R1 and R2 such that both R1>>RSET and R2>>RSET. 5 FN7370.1 December 21, 2005 ISL97631 VIN 2.7V~5.5V C1 1µF OFF/ON 90% 22µH 85% LEDs LX VOUT ISL97631 ENAB Dz FB R1 C2 0.22µF RSET 4.75Ω GND EFFICIENCY (%) VIN L1 80% 22µH, VIN = 4V 3LED 2LED 75% 4LED 70% 5LED 65% 6LED 60% 55% 50% 0 5 10 15 20 25 30 ILED (mA) FIGURE 10. LED DRIVER WITH OVERVOLTAGE PROTECTION CIRCUIT FIGURE 11. EFFICIENCY CURVE WITH 2, 3, 4, 5 AND 6 LEDs LOAD Components Selection The input capacitance is typically 0.22µF. The output capacitor should be in the range of 0.22µF to 1µF. X5R or X7R type of ceramic capacitors of the appropriate voltage rating are recommended. The output capacitor value affects PWM dimming performance. Lower output capacitor values increase the range of PWM dimming. However, the ripple voltage will be greater for lower values. When choosing an inductor, make sure the average and peak current ratings are adequate by using the following formulas (80% efficiency assumed): I LED ⋅ V OUT I LAVG = -------------------------------0.8 ⋅ V IN (EQ. 6) 1 I LPK = I LAVG + --- ⋅ ∆I L 2 (EQ. 7) White LED Connections One leg of LEDs connected in series will ensure brightness uniformity. The 27V maximum output voltage specification enables up to 6 LEDs to be placed in series. In order to drive more LEDs, series/parallel connections are used. A current mirror circuit (as shown in Figure 12) should be used to balance LED currents. Parallel strings of LEDs may draw significantly different currents due to manufacturing and temperature differences. For correct operation of the current mirror circuit, the total voltage between FB pin and the top of the primary LED string must be equal to or greater than the other strings. To ensure this, a small value resistor may be inserted between FB pin and the primary LED string. D1 V IN ⋅ ( V OUT – V IN ) ∆I L = -------------------------------------------------L ⋅ V OUT ⋅ f OSC L1 (EQ. 8) VIN Where: • ∆IL is the peak-to-peak inductor current ripple in Amps 2.7V~5.5V C1 • L is the inductance in H. • fOSC is the switching frequency, typically 1.35MHz The ISL97631 supports a wide range of inductance values (22µH~82µH). For lower inductor values or lighter loads, the boost inductor current may become discontinuous. For high boost inductor values, the boost inductor current will be in continuous mode. OFF/ON VDD VIN LX ISL97631 EL7630 VOUT ENAB FB ENAB FB GND C2 LEDs RSET FIGURE 12. LEDs IN SERIES/PARALLEL WITH MIRROR CURRENT BALANCE The demo board efficiency bench test results are shown in Figure 11. The input voltage is 4V and curves are shown for 2, 3, 4, 5 and 6 LEDs (boost inductor L = 22µH). 6 FN7370.1 December 21, 2005 ISL97631 PCB Layout Considerations The layout is very important for the converter to function properly. RSET must be located as close as possible to the FB and GND pins. Longer traces to the LEDs are OK. Similarly, the supply decoupling cap and the output filter cap should be as close as possible to the VIN and VOUT pins. The heat of the IC is mainly dissipated through the GND pin. Maximizing the copper area connected to this pin is preferable. In addition, a solid ground plane is always helpful for the EMI performance. The demo board is a good example of layout based on the principle. Please refer to the ISL97631 Application Brief for the layout. 7 FN7370.1 December 21, 2005 ISL97631 Package Outline 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 8 FN7370.1 December 21, 2005