Document Number: 002-08393 Rev.*A

Page 12 of 37

MB39C601

8. Function Explanation

8.1

LED Current Control Function

MB39C601 is a flyback type switching regulator controller. The LED current is regulated by controlling the switching on-time or

controlling the switching frequency depending on the LED load.

The LED current is converted into detecting voltage (Vs) by sense resistor (Rs) connected in series with LED. Vs is compared by

an external error amplifier (Err AMP).When Vs falls below a reference voltage, Err AMP output rises and the current that flows into

the Opto-Coupler is decreased.

The OTM pin current is controlled via the Opto-Coupler in the on-time control block. In on-time control, it controls on-time at OTM

pin current. So, on-time increases when the current of the OTM pin decreases. And the average current supplied to LED is

regulated, because on-time is regulated at the constant switching frequency.

The FB pin current is controlled via the Opto-Coupler in the switching frequency control block. In switching frequency control, it

controls switching frequency at FB pin current. So, switching frequency becomes high when the current of the FB pin decreases.

And the average current supplied to LED is regulated, because switching frequency is regulated at the constant on-time.

8.2

Cascode Switching

The switch in Primary Winding is a cascode connection.The gate of external MOSFET is connected with the VCG pin, and the

source is connected with the drain of internal Driver MOSFET.

When the swich is on-state, internal Driver MOSFET is turned on, HS Driver MOSFET is turned off, and the source voltage of

external MOSFET goes down to GND. For this period the DC bias is supplied to the gate of external MOSFET from the VCG pin.

Therefore external MOSFET is turned on.

When the switch is off-state, internal Driver MOSFET is turned off, HS Driver MOSFET is turned on, and the source voltage of

external MOSFET goes up to VCG voltage. For this period the DC bias is supplied to the gate of external MOSFET from VCG pin.

Therefore external MOSFET is turned off.

Moreover, the current flowing into internal Driver MOSFET is equal to the current of Primary Winding. Therefore, the peak current

into Primary Winding can be detected without the sense resistor.

8.3

Natural PFC (Power Factor Correction) Function

In the AC voltage input, when the input current waveform is brought close to the sine-wave, and the phase difference is brought

close to Zero, Power Factor is improved.

In the flyback method operating in discontinuous conduction mode, when the input capacitance is set small, the input current

almost becomes equal with peak current of Primary Winding.

: Supply voltage of Primary Winding

: Inductance of Primary Winding

: On-time

In on-time control, if loop response of Error Amp. is set to lower than the AC frequency (below 1/10 of the AC frequency), on-time

can be constant. Therefore, input current is proportional to input voltage, so Power Factor is regulated.

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