Rev. 1.00

1�

���i� ��� �011

Rev. 1.00

13

���i� ��� �011

HT7L4091

Universal Step-Down PWM Control

For High Brightness LED Lighting Control

Application Description

This section shows how to design a buck circuit LED application using a simple example. For other

application conditions, such as high efficiency solutions, refer to the HT7L4091 application notes

for more details.

For example:

Output Voltage: LED string ×LED Voltage =8×(3~3.3)=24V~26.4V, typical 25.2

Expected efficiency: η=90%

Refer to the typical application circuit.

Input Bulk Capacitor – C1

The input Bulk Capacitor determines the ripple amplitude of input voltage after rectification. A

large capacitance generates a smaller input voltage ripple amplitude. The first design criterion to

meet is that the maximum LED string voltage should be less than 80% of the minimum AC input

frequency variation which leads to increase in circuit power losses. Assume that the input voltage

Above formula means 30% input voltage ripple is approved that exceed output voltage.

Finally, a useful rule can find the valley voltage of the input voltage. Using the figure below, it is

necessary to calculate the charge time and discharge time of the input bulk capacitor.

t

�.333ms

1

1�0Hz

=

Δ

V

The waveform of input voltage in the C1

AC

1

T

8.333ms

2F

=

=

×

−

∆



−



×−









1

CP

IN

cha�ge

V

sin (1

)

T

V

T=

1

�

90

� whe�e

IN

V

V

∆