Document Number: 002-08393 Rev.*A

Page 13 of 37

MB39C601

8.4

Power-Up Sequencing

When the voltage is input to VBULK, the electric charge is charged to capacitance of the VCG pin (CVCG) through starting resistor

(Rst). So, the voltage of the VCG pin rises. The voltage of the DRN pin rises by source follower when the voltage of the VCG pin

reaches the threshold voltage of the external HVMOSFET.

The DRN pin is connected with the VDD pin through the internal VDD Switch, and VDD capacitor (CVDD) is charged from the DRN

pin. When the voltage at the VDD pin reaches the threshold voltage of UVLO, the VDD Switch is turned off, and the internal Bias

circuit operates, and the switching is started.

After the switching begins, the voltage at the VDD pin is supplied from Auxiliary Winding through the external diode (DBIAS). The

voltage of an Auxiliary Winding is decided by rolling number ratio of Auxiliary Winding and Secondary Winding, and the voltage of

Secondary Winding. Therefore, the voltage at the VDD pin is not supplied, until the voltage of Auxiliary Winding rises more than the

voltage at the VDD pin. In this period, it is necessary to set the capacitor of the VDD pin to prevent the voltage of the VDD pin from

falling below the threshold voltage of UVLO.

The external Schottky diode (D1) is required between the DRN pin and VDD pin. This diode is used to prevent the current that

flows through the body diode of the VDD Switch.

Current Passing When Starting

10V/8V

VCG

Shunt

Enable

PWM

14V

2V

UVLO

DRN

6

VCG

5

Fault

8

VDD

VDD

Switch

HS

Drive

Driver

GND

7

PWM

Control

D1

DBIAS

CVDD

Rst

CVCG

VBULK

VDD Start-up Current

VDD Operating and LPM Current

HV-MOSFET

Primary

Winding

Auxiliary

Winding

Ist