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Fabricante

ISL97632

LED Driver with 1-Wire Dimming

Renesas Electronics 

ISL97632

SEPIC Operation

For applications where the output voltage is not always above

the input voltage, a buck or boost regulation is needed. A

SEPIC (Single Ended Primary Inductance Converter)

topology, (see Figure 10), can be considered for such an

application. A single cell Li-Ion battery operating a cellphone

backlight or flashlight is one example. The battery voltage is

between 2.5V and 4.2V depending on the state of charge. On

the other hand, the output may require only one 3V to 4V

medium power LED for illumination because the light guard of

the backlight assembly is optimized or it is a cost efficiency

trade off reason.

In fact, a SEPIC configured LED driver is flexible enough to

allow the output to be well above or below the input voltage,

unlike the previous example. Another example is when the

number of LEDs and input requirements are different from

platform to platform, a common circuit and PCB that fit all the

platforms, in some cases, may be beneficial enough that it

outweighs the disadvantage of adding additional component

cost. L1 and L2 can be a coupled inductor in one package.

VIN = 2.7V to 5.5V 1 L1 2

VA C3 VB

C1

1µF

22µH

1µF

L2 C4 0.22µ

22µH

D1

VIN

LX

C2

0.1µF

VOUT

EN

SDIN

FBSW

FB

GND

R1 1

FIGURE 10. SEPIC LED DRIVER

The simplest way to understand SEPIC topology is to think

about it as a boost regulator in which the input volute is level

shifted downward at the same magnitude and the lowest

reference level starts at -VIN rather than 0V.

The SEPIC works as follows: Assume the circuit in Figure 10

operates normally when the ISL97632 internal switch opens,

and it is in the PWM ‘OFF’ state. After a short duration where

few LC time constants elapsed, the circuit is considered in the

steady-state within the PWM ‘OFF’ period that L1 and L2 are

shorted. VB is therefore shorted to the ground and C3 is

charged to VIN with VA = VIN. When the ISL97632 internal

switch closes, and the circuit is in the PWM on state, VA is now

pulled to ground. Since the voltage in C3 cannot be changed

instantaneously, VB is shifted downward and becomes -VIN.

The next cycle, when the ISL97632 switch opens, VB boosts

up to the targeted output like the standard boost regulator

operation, except the lowest reference point is at -VIN. The

output is approximated as shown in Equation 5:

VOUT = VIN ---1-----D–----D-----

(EQ. 5)

where D is the on-time of the PWM duty cycle.

The convenience of SEPIC comes with some trade off in

addition to the additional L and C costs. The efficiency is

usually lowered because of the relatively large efficiency loss

through the Schottky diode if the output voltage is low. The L2

series resistance also contributes additional loss. Figure 11

shows the efficiency measurement of a single LED application

as the input varies between 2.7V and 4.2V.

Note, VB is considered the level-shifted LX node of a standard

boost regulator. The higher the input voltage, the lower the VB

voltage will be during PWM on period. The result is that the

efficiency will be lower at higher input voltages because the

SEPIC has to work harder to boost up to the required level.

This behavior is the opposite to the standard boost regulator’s

and the comparison is shown in Figure 11.

76

VIN = 2.7V

72

68

VIN = 4.2V

1 LED

64

L1 = L2 = 22µH

C3 = 1µF

R1 = 4.7

60

0

5

10

15

20

ILED (mA)

FIGURE 11. EFFICIENCY MEASUREMENT OF 1 LED SEPIC

DRIVER

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 acceptable.

Similarly, the supply decoupling capacitor and the output filter

capacitor should be as close as possible to the VIN and

VOUTpins.

The heat of the IC is mainly dissipated through the thermal pad

of the package. Maximize the copper area connected to this pad

if possible. In addition, a solid ground plane is always helpful for

the EMI performance.

FN9239 Rev 4.00

March 22, 2010

Page 8 of 10

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