7

FN9239.2

April 10, 2007

Applications

Efficiency Improvement

Figure 2 shows the efficiency measurements. The choice of

the inductor has a significant impact on the power efficiency.

As shown in Equation 4, the higher the inductance, the lower

the peak current therefore the lower the conduction and

switching losses. On the other hand, it has also a higher

series resistance. Nevertheless, the efficiency improvement

from lowering the peak current is greater than the impact of

the resistance increase with larger value of inductor.

Efficiency can also be improved for systems that have high

supply voltages. Since the ISL97632 can only supply from

voltage for the boost circuit as shown in Figure 7 and the

efficiency improvement is shown in Figure 8.

8 LEDs Operation

For medium size LCDs that need more than 6 low power

LEDs for backlighting, such as a Portable Media Player or

Automotive Navigation Panel displays, the voltage range of

the ISL97632 is not sufficient. However, the ISL97632 can

be used as an LED controller with an external protection

MOSFET connected in cascode fashion to achieve higher

output voltage. A conceptual 8 LEDs driver circuit is shown

in Figure 9. A 60V logic level N-Channel MOSFET is

configured such that its drain ties between the inductor and

the anode of schottky diode, its gate ties to the input, and its

source ties to the ISL97632 LX node connecting to the drain

of the internal switch. When the internal switch turns on, it

pulls the source of M1 down to ground, and LX conducts as

normal. When the internal switch turns off, the source of M1

will be pulled up by the follower action of M1, limiting the

maximum voltage on the ISL97632 LX pin to below Vin, but

allowing the output voltage to go much higher than the

breakdown limit on the LX pin. The switch current limit and

maximum duty cycle will not be changed by this setup, so

input voltage will need to be carefully considered to make

sure that the required output voltage and current levels are

achievable. Because the source of M1 is effectively floating

when the internal LX switch is off, the drain-to-source

capacitance of M1 may be sufficient to capacitively pull the

node high enough to breaks down the gate oxide of M1. To

prevent this, VOUT should be connected to VIN, allowing the

internal schottky to limit the peak voltage. This will also hold

the VOUT pin at a known low voltage, preventing the built in

OVP function from causing problems. This OVP function is

effectively useless in this mode as the real output voltage is

outside its intended range. If the user wants to implement

their own OVP protection (to prevent damage to the output

capacitor, they should insert a zener from vout to the FB pin.

In this setup, it would be wise not to use the FBSW to FB

switch as otherwise the zener will have to be a high power

one capable of dissipating the entire LED load power. Then

the LED stack can then be connected directly to the sense

resistor and via a 10k resistor to FB. A zener can be placed

from Vout to the FB pin allowing an over voltage event to pull

up on FB with a low breakdown current (and thus low power

zener) as a result of the 10k resistor.

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

Vs = 12V

L1

22µ

1

2

R1

4

Ω

C3

0.22µ

D2

D3

D1

D5

D6

D4

ISL97632

FBSW

LX

VOUT

SDIN

VIN

FB

GND

C1

1µ

25mA

C2

0.1µ

EN

FIGURE 7. SEPERATE HIGH INPUT VOLTAGE FOR HIGHER

EFFICIENCY OPERATION

510

15

20

90

85

70

0

75

80

25

30

ILED (mA)

6 LEDs

L1 = 22µH

R1 = 4

Ω

FIGURE 8. EFFICIENCY IMPROVEMENT WITH 9V AND 12V

INPUTS

VOUT

LX

ISL97632

SDIN

VIN

FBSW

GND

FB

EN

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 9. CONCEPTUAL 8 LEDS HIGH VOLTAGE DRIVER

ISL97632