HT7L4091 datasheet(8/26 Pages) HOLTEK

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Rev. 1.00

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HT7L4091

Universal Step-Down PWM Control

For High Brightness LED Lighting Control

Functional Description

The HT7L4091 is a universal AC/DC constant current LED driver designed for peak current mode

control. The device provides both LED Linear and PWM dimming current functions. The high input

voltage from a rectified 85V to 260V AC power is clamped to under 25V by an external circuit and

an internal Zener diode. The device also contains an input Under-Voltage-Lockout (UVLO) circuit.

When the voltage supplied on the VIN pin exceeds the UVLO high threshold, the gate driver is

enabled. If the input voltage falls below the UVLO low threshold, the gate driver is turned off.

LED Current Control

The HT7L4091 device is a constant off-time peak current mode controller. With reference to the

Application Circuit, the LED peak current is programmed by an external current sense resistor (RCS)

connected between the CS and the ground pins. The CS pin is connected to a non-inverting terminal

of an internal comparator of which an internal 250mV reference is tied to the inverting terminal. The

LED peak current through the RCS resistor will generate a voltage which is applied on the comparator

non-inverting terminal and compare with the internal 250mV reference voltage. If the voltage on

the CS pin is less than the internal reference voltage 250mV, the LED gate driving circuitry will be

turned on. While the voltage on the CS pin is larger than the internal reference voltage, the LED

gate driving circuitry will be turned off for a constant Toff time. After the Toff time, the gate driving

circuitry will be turn on if the voltage on the CS pin is less than the internal reference voltage. Good

line regulation is a feature of constant off-time operation and the LED current is independent of the

input voltage. Since the inductor current ripple is dependent on the LED string voltage, the LED

string voltage variation will result in LED current variation. This is typically not a problem since the

LED voltage variation for a given load is fairly small.

RCS can be calculated using the following equation:

(

)

⋅

�eak

LED

0.�5

Ri��e I

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Where Ipeak is the Maximum LED Current, Ripple is the Peak to Peak LED Current, and ILED is the

Average LED Current. Ripple can be controlled by the inductor.

off

out

LED

Ripple

Ripple=I

×=

Refer to “Inductor Design” for the inductor calculation. Refer to “Programmable Off Time” for Toff

calculation.

Programmable Off Time

The device operates in a constant off-time mode. A resistor connected between the RT pin and the

ground pin generates a constant current source which is used to charge an internal capacitor and

determine the off-time. Increasing the resistance reduces the amplitude of the current source and

increases the off-time. The relationship between the resistor RT and the off-time is given by the

following formula:

Toff = CT × RT CT=36pF~44pF, CT_typ=40pF.

For a given Toff and duty cycle, the switching frequency (fs) can be decided. The duty cycle is

determined by the input and output voltages.

HT7L4091 数据表(PDF) 8 Page - Holtek Semiconductor Inc

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