8

FN9239.3

March 22, 2010

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.

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

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

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

boost regulator operation, except the lowest reference point

Equation 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.

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 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.

C1

1µF

L1

22µH

1

2

VIN = 2.7V to 5.5V

22µH

L2

C3

1µF

C4

0.22µ

D1

R1

1

Ω

C2

0.1µF

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 1 LED SEPIC

DRIVER

ISL97632