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ISL97631
Data Sheet December 21, 2005 FN7370.1
White LED Boost Regulator With Integrated Schottky Diode
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. Available in the 6 Ld TSOT package, the ISL97631 offers high efficiency, constant frequency operation. It is specified for operation over the -40C to +85C ambient temperature range.
Features
* Up to 6 LEDs in series * 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
* LED backlighting * Cell phones * PDAs * Handheld games
Pinout
ISL97631 (6 LD TSOT) TOP VIEW
VOUT 1 GND 2 LX 3 6 VIN 5 FB 4 ENAB
* MP3 players * GPS * Other handheld devices
Ordering Information
PART NUMBER ISL97631IHTZ-T7 (See Note) ISL97631IHTZ-T7A (See Note) PART MARKING 631Z 631Z PCS. 3,000 270 TAPE & REEL 7" 7" PACKAGE (TAPE AND REEL) 6 Ld TSOT (Pb-free) 6 Ld TSOT (Pb-free) PKG. DWG. # MDP0049 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.
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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. 2005. All Rights Reserved All other trademarks mentioned are the property of their respective owners.
ISL97631
Absolute Maximum Ratings (TA = 25C)
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 . . . . . . . . . . . . . . . . . . . . . . .-40C to +85C
Thermal Information
Thermal Resistance (Typical, Note 1) JA (C/W) 6 Ld TSOT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 Maximum Junction Temperature (Plastic Package . . . . . . . . . 150C Maximum Storage Temperature Range . . . . . . . . . .-65C to +150C Maximum Lead Temperature (Soldering, 10s). . . . . . . . . . . . +300C (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. JA 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-MIN VIN-MAX VFB IFB IIN
VIN = 3V, VENAB = 3V, TA = -40C to 85C unless otherwise specified. CONDITION VOUT = 16V, ILED = 20mA VOUT = 25V, ILED = 20mA 80 100 MIN 2.7 5.5 120 100 ENAB = 3V, output not switching ENAB = 0V 0.6 1.0 1 0.8 TA = 25C 85 82 1.35 90 90 350 350 750 0.01 0.01 0.75 2.5 0.6 1 VIN = 2.7V to 5V ILED = 20mA, 3 LEDs 0.2 85 1 1 0.9 1.8 TYP MAX UNIT V V mV nA mA A MHz % % mA mA m A A V V V A %/V %
DESCRIPTION Minimum Operating Voltage Maximum Operating Voltage Feedback Voltage FB Pin Bias Current Supply Current
FOSC DMAX
Switching Frequency Maximum Duty Cycle
ILIM
Switch Current Limit
TA = 25C
280 250
rDS(ON) ILEAK(LX) ILEAK(VOUT) VDIODE VENAB-HI VENAB-LO IENAB ILED/VIN Eff
Switch On Resistance Switch Leakage Current Diode Leakage Current LX-VOUT Diode Forward Voltage ENAB Voltage High ENAB Voltage Low ENAB Pin Bias Current Line Regulation Efficiency
ILX = 100mA VLX = 27V, Vout = 27V VOUT = 27V IDIODE = 100mA, TA = 25C
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FN7370.1 December 21, 2005
ISL97631 Typical Application
L1 22H VIN 2.7V~5.5V C1 1F OFF/ON ENAB FB GND RSET 4.75 VIN LX VOUT LEDs EFFICIENCY (%) 90 85 80 75 70 65 60 55 50 0 5 10 15 ILED (mA) 20 25 30 22H, VIN = 4V
ISL97631 C2 0.22F
FIGURE 1. TYPICAL APPLICATION CIRCUIT
FIGURE 2. EFFICIENCY vs LED CURRENT (VIN = 4V)
Typical Performance Curves
700 600 500 400 300 200 100 0 0 2 VIN (V) 4 6 IO (mA) Iq (A) 18.244 18.242 18.240 18.238 18.236 18.234 18.232 18.230 18.228 0 5 10 VOUT (V) 15 20
FIGURE 3. QUIESCENT CURRENT vs VIN (ENAB = hi)
FIGURE 4. LOAD REGULATION (VIN = 4V)
18.33 18.32 18.31 18.30 IO (mA) 18.29 18.28 18.27 18.26 18.25 18.24 18.23 2.5 3.5 VIN (V) 4.5 5.5 FREQUENCY (MHz)
1.37 1.36 1.35 1.34 1.33 1.32 1.31 1.30 1.29 1.28 1.27 -60 -40 -20 0 20 40 60 TEMPERATURE (C) 80 100
FIGURE 5. LINE REGULATION
FIGURE 6. SWITCHING FREQUENCY vs TEMPERATURE
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FN7370.1 December 21, 2005
ISL97631 Block Diagram
Vin Enable LX
1.35MHz Oscillator and Ramp 1.2MHz Generator
ISL97631
Vout
PWM Comparator
PWM Logic Controller
FET Driver
Current Sense
GND GM Amplifier FB 95mV Bandgap Reference Generator GM Amp Compensation
FIGURE 7. ISL97631 BLOCK DIAGRAM
Pin Descriptions
PIN PIN NUMBER NAME 1 2 3 4 5 6 DESCRIPTION
VOUT Output Pin. Connect to the anode of the top LED and the output filter capacitor . GND LX Ground Pin. Connect to local ground. Switching Pin. Connect to inductor.
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)
ENAB Enable Pin. Connect to enable signal to turn-on or off the device. FB VIN Feedback Pin. Connect to the cathode of bottom LED and the sense resistor. Input Supply Pin. Connect to the input supply voltage, the inductor and the input supply decoupling capacitor.
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 1A.
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)
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
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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 0.22F.
20 18 16 14 IO (mA) 12 10 8 6 4 2 0 0 20 40 60 80 100 DUTY-CYCLE (%) 1kHz 400Hz L1 22H VIN 2.7V~5.5V C1 1F OFF/ON VIN LX VOUT ISL97631 ENAB GND FB R1 3.3k RSET 4.75 C2 0.22F LEDs
R2
VDim
FIGURE 9. ANALOG DIMMING CONTROL APPLICATION CIRCUIT
The analog dimming circuit can be tailored to a desired relative brightness for different VDim ranges using Equation 5.
[ ( V Dim_max - V FB ) * R 1 ] R 2 = ------------------------------------------------------------------[ V FB * ( 1 - F min ) ] (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). 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
FIGURE 8. PWM DIMMING LINEAR RANGE (FOR 400Hz AND 1kHz PWM FREQUENCIES CONDITION, COUT = 0.22F)
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.
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
SET
(EQ. 3)
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 R1 V Dim = ------ V FB 1 + ------ - F R1 R2 (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.
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FN7370.1 December 21, 2005
ISL97631
L1 22H VIN 2.7V~5.5V C1 1F OFF/ON EFFICIENCY (%) VIN LX VOUT Dz R1 RSET 4.75 C2 0.22F LEDs 90% 85% 80% 75% 70% 65% 60% 55% 50% 0 5 10 15 ILED (mA) 20 25 30 3LED 2LED 4LED 5LED 6LED 22H, VIN = 4V
ISL97631 ENAB FB
GND
FIGURE 10. LED DRIVER WITH OVERVOLTAGE PROTECTION CIRCUIT
Components Selection
The input capacitance is typically 0.22F. The output capacitor should be in the range of 0.22F to 1F. 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 1 I LPK = I LAVG + -- I L 2 V IN ( V OUT - V IN ) I L = -------------------------------------------------L V OUT f OSC (EQ. 6)
FIGURE 11. EFFICIENCY CURVE WITH 2, 3, 4, 5 AND 6 LEDs LOAD
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
(EQ. 7)
(EQ. 8)
VIN 2.7V~5.5V C1 OFF/ON
L1
Where: * IL is the peak-to-peak inductor current ripple in Amps * L is the inductance in H. * fOSC is the switching frequency, typically 1.35MHz The ISL97631 supports a wide range of inductance values (22H~82H). 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. 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 = 22H).
VDD VIN LX ISL97631 EL7630 VOUT ENAB FB ENAB FB GND LEDs RSET
C2
FIGURE 12. LEDs IN SERIES/PARALLEL WITH MIRROR CURRENT BALANCE
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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.
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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
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