8

FN9239.3

February 22, 2008

Converter) topology, (shown in Figure ), 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

outweights the disadvantage of adding additional component

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

The simplest way to understand SEPIC topology is to think

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

shifted downward at the same magnitude and the lowest

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

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

boost regulator operation, except the lowest reference point

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.

standard boost regulator. The higher the input voltage, 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.

PCB Layout Considerations

The layout is very important for the converter to function

and GND pins. Longer traces to the LEDs are acceptable.

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 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.

C1

1µ

L1

22µ

1

2

VIN = 2.7V to 5.5V

22µ

L2

C3

1µ

C4

0.22µ

D1

R1

1

Ω

C2

0.1µ

VIN

EN

SDIN

LX

VOUT

FBSW

FB

GND

FIGURE 10. SEPIC LED DRIVER

V

OUT

V

IN

D

1D

–

()

------------------

=

(EQ. 5)

1 LED

L1 = L2 = 22µH

C3 = 1µF

R1 = 4.7

Ω

0

5

10

15

20

ILED (mA)

76

72

68

64

60

FIGURE 11. EFFICIENCY MEASUREMENT OF A SINGLE LED

SEPIC DRIVER

ISL97632