5

FN6264.2

April 10, 2007

Detailed Description

The ISL97634 uses a constant frequency, current mode

control scheme to provide excellent line and load

regulation. There are three OVP models for driving 3, 4,

and 6 LEDs and their OVP thresholds are set at 14V, 18V,

and 26V respectively. The ISL97634 operates from an

input voltage of 2.4V to 5.5V and ambient temperature from

-40°C to +85°C. The switching frequency is around

1.45MHz and allows the driver circuit to employ small LC

components. The peak forward current of the LED is set

peak current is given by the following equation:

PWM Dimming

The ISL97634’s PWM/EN pin can be tied permanently to

high for a fixed current operation. On the other hand, the

ISL97634 can be applied with an external PWM signal to

pulse width modulated the output current. It is well

understood that the LED brightness is a linear function of the

LED current. In addition, the average LED current

corresponds to the duty cycle “D” of the PWM signal as:

As a result, PWM signal provides a means to dim the LED

brightness. PWM dimming offers the best LEDs matching

over DC dimming. It is because the LED peak current

operating point is far away from the knee of the diode I-V

curve where part to part variations are high. The PWM

dimming test results are shown in Figure 6 with two PWM

frequencies, 1kHz and 20kHz. The vertical scale parameter

FB is proportional to the current and therefore the

brightness.

For the ISL97634, PWM dimming provides linear dimming

adjustment with low frequency signal, such as 1kHz and

below. The applied PWM dimming signal can be up to 32kHz

FIGURE 7. PWM DIMMING AT 1kHz, D = 1% ZOOM IN

FIGURE 8. PWM DIMMING AT 1kHz, D = 99%

FIGURE 9. PWM DIMMING AT 20kHz, D = 50%

ILED

PWM/EN

LX

R1 = 4

Ω

L1 = 22µH

ILED

PWM/EN

LX

R1 = 4

Ω

L1 = 22µH

ILED

PWM/EN

LX

L1 = 22µH

R1 = 4

Ω

I

LED

V

FB

R

SET

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

=

(EQ. 1)

I

LED

AVG

–

V

FB

R

SET

--------------- D

⋅

=

(EQ. 2)

ISL97634