ZLED7025 datasheet(10/26 Pages) IDT

10 / 26 page

ZSLS7025 Datasheet

April 20, 2016

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OUT

When the input voltage on the FB pin does not exceed the internal feedback reference voltage VFB, such as during

start-up, Q1 will remain off for the configured minimum tOFF time (see sections 2.8 and 2.9), and then it is switched on

again.

2.3

Internal 5V Regulator

The ZSLS7025 includes an internal 5V (typical) shunt regulator connected to the VDD pin which maintains a 5V

power supply for the gate driver and control circuitry. Connect VIN to the VDD pin via the current limiting series

resistor RVDD (see section 4.2.1 for required values). Consideration should be given to the tolerances on the VDD pin

operating conditions (see section 1.2, parameter 1.2.3) and VIN.

2.4

Input Current

The current required by the ZSLS7025 is 0.25mA (typical) plus the switching current of the external MOSFET, Q1. The

switching frequency of Q1 affects the amount of current required, as does Q1's gate charge requirement (found in the

MOSFET manufacturer's data sheet).

(1)

Where

fS = switching frequency of Q1

QG = gate charge of Q1

2.5

LED Current Control

The ZSLS7025 regulates the LED current by sensing the voltage across the external feedback resistor RFB in series

with the LEDs. The voltage is sensed via the FB pin where the internal feedback reference voltage VFB is 0.3V

(typical; see section 1.3, parameter 1.3.11). The LED current can easily be set according to equation (2).

(2)

Where

IOUT = Average output current through the LED(s) in amperes

VFB = Internal feedback reference voltage

Note: For an accurate LED current, a precision resistor is required for RFB (1% is recommended).

2.6

Dimming Control

There are three options for LED dimming:

•

DC voltage dimming control

•

RC-filtered PWM signal dimming control

•

PWM signal with a dimming control MOSFET(Q2)

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