ZSLS7025 Datasheet

© 2016 Integrated Device Technology, Inc.

12

April 20, 2016

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)

FB

5

4

FB

PWM

PWM

3

FB

OUT

R

R

R

V

D

V

R

V

I













+

−

×

×

−

=

2.6.2

Dimming Control Using an RC Filter to Convert a PWM Signal to a DC Voltage

As shown in Figure 2.3, a filtered PWM signal can be used as an adjustable DC voltage for LED dimming control, and

it functions the same as the DC control signal described in section 2.6.1. An external RC filter converts the PWM

signal to a DC voltage, which is summed with the FB voltage to regulate the output current. Using a fixed frequency

PWM signal and changing the duty cycle adjusts the average LED current. The LED current can be calculated with

equation (4):

(4)

Where

= Output current through the LED(s) with a PWM control voltage

=

Internal feedback reference voltage (see section 1.3, parameter 1.3.11)

The LED current is inversely proportional to the PWM duty cycle; i.e., when the PWM signal has a 100% duty cycle,

the output current is minimum, ideally zero, and when the PWM signal has a 0% duty cycle, the output current is at its

maximum. See the example in section 4.4 for more details.

Note: Care must be taken to ensure that the minimum required current is not already exceeded when the LEDs are

Figure 2.3

RC Filter PWM Dimming Circuit

TOFF

GATE

ZSLS7025

LED

String

OVP

GND

VDD

ADJ

FB

CS

PWM Control Signal

-