Document Number: 002-08393 Rev.*A

Page 17 of 37

MB39C601

8.8

FB Part

The switching frequency is controlled by setting the current of the FB pin. In on-time control, the switching frequency is set by

pulling up the FB pin to VDD. Moreover, as shown in following figure, it is possible to control the switching frequency by connecting

the emitter of the Opto-Coupler from the FB pin through resistor. Resistor (RFB) is connected to bleed off the dark current of Opto-

Coupler.

FB pin Control

1. Frequency Modulation Mode (FM)

The peak current of HV-MOSFET is set to the maximum, and the LED current is regulated by adjusting the switching frequency

with IFB.The range of the switching frequency is from 30 kHz to 130 kHz. Maximum peak current IDRN (peak, max) of HV-

MOSFET is set by the resistance of the PCL pin.

2. Amplitude Modulation Mode (AM)

The LED current is regulated by adjusting the peak current of HV-MOSFET with IFB.The switching frequency is about 30 kHz.

And the range of HV-MOSFET of the peak current is from 33% to 100% of the maximum. Maximum peak current IDRN (peak,

max) of HV-MOSFET is set by the resistance of the PCL pin.

3. Low Power Mode (LPM)

MB39C601 becomes two states of LPM-ON and LPM-OFF at a light load. In the LPM-ON mode, it is operated at 30 kHz

switching frequency. And the current is supplied to LED. At this time, the feedback current from the Opto-Coupler increases, and

it changes to the LPM-OFF mode. In the LPM-OFF mode, it is not operated. And the current is supplied to LED from Co. When

the feedback current from the Opto-Coupler decreases, it changes to the LPM-ON mode. LED is lit by the power saving

repeating these two states.

Three modes of the FM, AM, and LPM change depending on the load of LED. At the light load, three modes change from FM to

AM to LPM.

FB

1

Modulators

Low Power Mode

Over Load

Feedback

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