Rev. 1.00

1�

���i� ��� �011

Rev. 1.00

13

���i� ��� �011

HT7L4091

Universal Step-Down PWM Control

For High Brightness LED Lighting Control

1

�.333ms

sin (1 30%)

1

�

90

−



−

×−





= 6.��3�ms

Then, the minimum capacitor value can be calculated as:

(

)

LEDmax

LED

Discha�ge

1

�

�

�Cmin

DCmin

(2 n V

I ) T

C

2 V

V

××

×

×

≥





η×

×

−









=

(

) (

)

��

(2 16 3.3V 200mA) 6.2232ms

0.9

� �5

� �5 0.7

××

×

×





×

×

−

×

×









= 14.3uF

Choose C1=��uF

Considering ±20% capacitance variation, the worst case lower value of the capacitance is 17.6uF,

which is much larger than 14.3uF. It can be calculated that the input DC ripple is 24.5% when the

input Buck capacitor is 17.6uF.

Therefore, if the real capacitor value is less than the calculated value, the voltage ripple will exceed

the maximum range of 30% which is the specified assumption in the calculation.

Switching Frequency and Duty Cycle

Frequency interference should be taken into account to minimise interference with other electrical

appliances. Here set the minimum switching frequency to a value of 40kHz for safety. If EMI

suppression is good, the switching frequency can be decreased to 30kHz to obtain better efficiency.

Since the HT7L4091 operates in constant off time, the switching frequency would be changed by

the input and output voltage. The slowest switching frequency occurs when the duty cycle is at a

maximum value.

The maximum duty cycle can be calculated as,

⋅+

=

=

⋅

×−

o_ max

LED_ max

F�D1

max

DC_ min

�C_ min

�i���e

V

nV

V

D

V

2 V

× +

×

×

3.3 � 1.3

� 95 (1-�4.5%)

=0.�731

Turn-off Time

−

=

max

off

PWM_ min

1D

T

F

=

−

1 0.�731

40k

=1�.173us