ISL97634 Просмотр технического описания (PDF) - Renesas Electronics

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ISL97634

White LED Driver with Wide PWM Dimming Range

Renesas Electronics 

ISL97634

Typical Performance Curves (Continued)

VOUT

LX

PWM/EN

ILED

VIN = 4V

R1 = 4

L1 = 22µH

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

VOUT

LX

PWM/EN

ILED

VIN = 4V

R1 = 4

L1 = 22µH

FIGURE 9. PWM DIMMING AT 1kHz, D = 99%

VOUT

RV1IN==44V

L1 = 22µH

PWM/EN

LX

ILED

FIGURE 10. PWM DIMMING AT 20kHz, D = 50%

Detailed Description

The ISL97634 uses a constant frequency, current mode

control scheme to provide excellent line and load

regulation. There are three OVP thresholds 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 using the RSET resistor.

In the steady state mode, the LED peak current is given by

Equation 1:

ILED = R---V--S--F--E--B--T-

(EQ. 1)

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 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 shown in Equation 2:

ILED-AVG = R---V--S--F--E-B---T-  D

(EQ. 2)

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 7 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;

FN6264 Rev 4.00

August 27, 2013

Page 6 of 12

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