HT7L4091 datasheet(15/26 Pages) HOLTEK

15 / 26 page

Rev. 1.00

��i� �� 011

Rev. 1.00

��i� �� 011

HT7L4091

Universal Step-Down PWM Control

For High Brightness LED Lighting Control

Input Supply Current

Assume that the input current drawn by the internal circuit from the VIN pin is the sum of the

current with a value of 1.0mA and the current drawn by the gate driver of the external MOSFET

(which in turn depends upon the switching frequency and the gate charge of the external FET).

Assume that the gate charge Qgate is equal to 12nC.

= +⋅

INSP

gate

PWM

F = 1m� + 1�nC × 50kHz = 1.6m�

, where IINSP is the input current taken from the VIN pin, FPWM is the switching frequency, Qgate is the

gate charge of the external FET and IIN is the current taken by the internal circuit. FPWM is considered

about the minimum input voltage and CT has a minimum value.

Input Limit Resistor (RIN)

In this design, VAC_min = 95Vrms, VUVLO(H)_max = 17V

⋅

× − ∆

−

AC_ min

DCripple%

UVLO(H)_max

INSP

2 V

) V

= � 95 (1 30%) 17

1.6m�

× −

−

=62.4kΩ

Choose Rin=60kΩ

The input limit resistor consider the high input voltage from the rectified clamp voltage of the

internal Zener diode and operating current. Two 30KΩ / 1W resistors are used for Rin.

Output Capacitor – CO

The capacitor, CO, filters the current through the LEDs thus limiting the peak current of the LED

string. Increasing the inductor ripple current corresponds to decreasing the inductor value and

inductor size. In order to reduce the inductor value and size and obtain a smaller LED current ripple,

the addition of a capacitor CO is a good way to do this. Usually, a several μF output capacitor is

added in practical application circuits.

Adding CO connected across the LED strings can reduce the LED current ripple and while increasing

the inductor current ripple variation can decrease the inductor value and size.

To assume inductor current ripple is 80%, a smaller inductor value could be calculated.

××

off

LED _ max

LED

Ri��e

××

17.7us � 3.15

400m� 0.�

=1.393mH

Choose L=1.4mH and CO=1uF

The actual values of CO and RCS may need to be adjusted to reduce the current ripple and obtain

the target average LED current. A 1uF capacitor and an RCS as shown in the above calculation are a

good start point to obtain an acceptable result. Since it takes some effort, it can reduce the inductor

size/cost significantly.

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