HT7938

Rev 1.10

5

March 19, 2010

The output rectifier diode conducts during the internal

MOSFET is turn off. The average and peak current

rating must be greater than the maximum output cur-

rent and peak inductor current. The reverse break-

down voltage must be greater than the maximum

output voltage. It is recommended to use a schottky

diode with low forward voltage to minimize the power

dissipation and therefore to maximize the efficiency of

the converter. A 1N5819 type diode is recommended

for HT7938 applications.

The LED current is controlled by the current sense

erence voltage is 200mV. In order to have accurate

LED currents, precision resistors are the preferred

type with a 1% tolerance. The LED current can be cal-

culated from the following formula.

The LED illumination level can be controlled using

both digital and analog methods.

The digital method uses a PWM signal applied to the

EN pin. This is shown in figure 8. The average LED

current increases proportionally with the PWM signal

duty cycle. A 0% duty cycle corresponds to zero LED

current. A 100% duty cycle corresponds to full LED

current. The PWM signal frequency should be set be-

low 1kHz due to the delay time of device startup.

There are two methods of analog LED brightness con-

trol. The first method uses a DC voltage to control the

feedback voltage. If the DC voltage range is from 0V

to 3.3V, the selection of resistors control the LED cur-

rent from 20mA to 0mA. The other way is to use a fil-

tered PWM signal, as shown in figure 10. The filtered

PWM signal application acts in the same way as the

DC voltage dimming control.

Layout Considerations

Circuit board layout is a very important consideration for

switching regulators if they are to function properly. Poor

circuit layout may result in related noise problems.

In order to minimize EMI and switching noise, please fol-

low the guidelines below:

be placed close to the VIN, VO and GND pins.

placed close to the SW pin.

FB and GND pins.

performance.

A recommended PCB layout with component loca-

tions is shown below.

f b

R

f b

R

F B

V

L E D

I

2 0 0 m V

=

=

Top Layer

Bottom Layer