9

FN6264.2

April 10, 2007

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, shown in Figure 18, can be considered

for such application. A single cell Li-Ion battery operating a

cellular phone 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 for 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 where the input voltage is level

shifted downward at the same magnitude and the lowest

The SEPIC works as follows; assume the circuit in Figure 18

operates normally, when the ISL97634 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

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

boost regulator operation, except the lowest reference point is

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 19

shows the efficiency measurement of a single LED application

as the input varies between 2.7V and 4.2V.

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 19.

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

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

of the package. Maximizing the copper area connected to this

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

helpful for the EMI performance.

FIGURE 17. CONCEPTUAL 8 LEDs HIGH VOLTAGE DRIVER

VOUT

LX

ISL97634

PWM/EN

VIN

FBSW

GND

FB

R1

6.3

Ω

D3

D4

D5

D6

D7

D8

L1

2.2µ

12

C3 4.7µ

D0

10BQ100

M1

FQT13N06L

D1

C1

1µ

SK011C226KAR

C2

0.1µ

D2

FIGURE 18. SEPIC LED DRIVER

C1

1µ

L1

22µ

1

2

22µ

L2

C3

1µ

C4

0.22µ

D1

R1

1

Ω

C2

0.1µ

VIN

SDIN

LX

VOUT

FBSW

FB

GND

ISL97634

D0

V

OUT

V

IN

D

1D

–

()

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

=

(EQ. 9)

0

510

15

20

ILED (mA)

76

72

68

64

60

1 LED

L1 = L2 = 22µH

C3 = 1µF

R1 = 4.7

Ω

FIGURE 19. EFFICIENCY MEASUREMENT OF A SINGLE LED

SEPIC DRIVER

ISL97634