ZLED7025 datasheet(13/26 Pages) IDT

13 / 26 page

ZSLS7025 Datasheet

April 20, 2016

OUT

AVG

2.6.3

PWM Dimming with a Dimming Control MOSFET (Q2)

Figure 2.4 shows the configuration for using an external PWM signal with a dimming control MOSFET Q2. When the

PWM input is high (VH>2.4V), Q2 is on and the ZSLS7025 operates normally to regulate the output current. When the

PWM signal is low (VL<0.5V), Q2 is off; the input voltage on the FB pin will be below VFB and the ZSLS7025 is

shutdown. Using a fixed frequency PWM signal and changing the duty cycle adjusts the average LED current. The

recommended 5V PWM frequency is between 200Hz and 1KHz.

Figure 2.4

PWM Dimming Circuit Using a Dimming Control MOSFET (Q2)

TOFF

GATE

ZSLS7025

LED

String

VIN =5 to 100 VDC

COUT

CIN

OVP

GND

RFB

VDD

CVDD

RVDD

RTOFF

ADJ

RCS

PWM Control Signal

2.7

Peak Input Current Control

The ZSLS7025 limits the peak inductor current and therefore the peak input current through the feedback path of RCS

connected from the source of the external MOSFET (Q1) to ground. The required average input current is based on

the boost ratio VOUT/VIN and the designed value for average LED current. The required average input current can be

calculated with equation (5):

(5)

Where

η = Assumed power conversion efficiency (recommended value for calculation: 0.9)

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