ISL97634(2007) データシートの表示（PDF） - Intersil

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ISL97634
(Rev.:2007)

White LED Driver with Wide PWM Dimming Range

Intersil 

ISL97634

The analog dimming circuit can be tailored to a desired

relative brightness for different VDim ranges using

Equation 8.

R2

=

-[--(--V-----D----i-m-----_---m----a---x-----–----V----F----B----)---•-----R----1----]

[VFB • (1 – Fmin)]

(EQ. 8)

Where VDim_max is the maximum VDim voltage and Fmin is

the minimum relative brightness (i.e., the brightness with

VDim_max applied).

i.e., VDim_max = 5V, Fmin = 10% (i.e., 0.1), R2 = 189k

i.e., VDim_max = 1V, Fmin = 10% (i.e., 0.1), R2 = 35k

Efficiency Improvement

Figure 1 shows the efficiency measurements during PWM

operation. 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 effect by lowering the peak current is greater

than the resistance increases with larger value of inductor.

Efficiency can also be improved for systems that have high

supply voltages. Since the ISL97634 can only supply from

2.4V to 5.5V, VIN must be separated from the high supply

voltage for the boost circuit as shown in Figure 15 and the

efficiency improvement is shown in Figure 16.

Vs = 12V

C1 1µ

L1

1

2

22µ

C3 0.22µ

D1

D2

D3

VIN = 2.7V to 5.5V

VIN

C2 0.1µ

LX

VOUT

ISL97634

FBSW

FB

PWM/EN

GND

D4

D5

D6

R1 4Ω

FIGURE 15. SEPARATE HIGH INPUT VOLTAGE FOR HIGHER

EFFICIENCY OPERATION

.

90

85

VS = 12V

VS = 9V

80

75

70

0

VIN = 4V

6 LEDs

L1 = 22µH

R1 = 4Ω

fPWM

5

10

15

20

25

30

ILED (mA)

FIGURE 16. EFFICIENCY IMPROVEMENT WITH 9 AND 12V

INPUTS

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 ISL97634 is not sufficient. However, the ISL97634 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 17. 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 ISL97634 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 ISL97634 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.

8

FN6264.2

April 10, 2007

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