ISL97634

10

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FN6264.4

August 27, 2013

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may be beneficial enough that it outweighs the disadvantage of

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 reference level

The SEPIC works as follows; assume the circuit in Figure 19

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

standard boost regulator operation, except the lowest reference

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

contributes additional loss. Figure 20 shows the efficiency

measurement of a single LED application as the input varies

between 2.7V and 4.2V.

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 20.

PCB Layout Considerations

The layout is very important for the converter to function properly.

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. Maximizing the copper area connected to this pad if

possible and connect to ground plane on the PCB. Connect all vias to

the correct voltage potential (power plane) indicated in the

datasheet. In addition, a solid ground plane is always helpful for the

EMI performance.

FIGURE 19. SEPIC LED DRIVER

C1

1µF

L1

22µH

1

2

22µH

L2

C3

1µF

C4

0.22µF

D1

R1

1

Ω

C2

0.1µF

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

FIGURE 20. EFFICIENCY MEASUREMENT OF A SINGLE LED SEPIC

DRIVER