MIC9131YM データシートの表示（PDF） - Micrel

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MIC9131YM

High-Voltage, High-Speed Telecom DC-to-DC Controller

Micrel 

MIC9131

Functional Description

Micrel’s MIC9131 is a high voltage, high speed current mode

switching power supply controller. It uses a BiC/DMOS pro-

cess to achieve a high voltage input, low quiescent current

and very fast internal delay times. The MIC9131 is designed

to drive an external low side N-channel MOSFET, which

makes it suitable for controlling Boost, Flyback and Forward

converter topologies. The high voltage startup pin eliminates

the requirement for an external start up circuit. This makes

it ideal for use with Telecom converters.

A block diagram of the MIC9131 is shown in Figure 1. The de-

scription of the controller is divided into 6 basic functions:

• Power and bias circuitry

• High voltage start-up circuit

• VCC and Bias supplies

• Enable and Undervoltage monitoring circuits

• VCC and VIN UVLO

• Enable

• Oscillator and sync circuitry

• Soft-start and soft-start reset circuits

• MOSFET gate drive circuits

• Control loop operation

Transformer

Bias

Winding

Micrel, Inc.

• Current sensing & overcurrent protection

• Slope compensation

• Error amplifier

High Voltage Start Up Circuit

Many conventional Off-Line and Telecom power supplies

use an external bias resistor and zener diode to supply the

initial start-up voltage for the control IC. The control IC gets

its supply voltage from a bias winding once the power sup-

ply is running. This method has the disadvantages of extra

components (diode and power resistor), continuous power

dissipation in the resistor and a large bias capacitor, used to

supply the IC until the bias winding takes over.

The MIC9131 eliminates these problems by using an internal

depletion mode MOSFET as a pre-regulator to provide the

start-up bias voltage from the high voltage input of the power

supply. This approach eliminates the need for external start

up components and reduces the size of the controller’s bias

supply capacitor. The MOSFET is turned off once the external

bias winding takes over, which eliminates power dissipation

in the start-up circuit. In some cases, the MIC9131 may be

run directly from the input voltage rail, eliminating the need

for an external bias winding.

Start-up circuit operation is illustrated in Figure 2. VIN is ap-

MIC9131

VCC

2

Internal

Circuitry

1.21V

VIN

Line

1

180V

DEPLETION

FET

VCC

UVLO

THERMAL

SHUTDOWN

VPR(OFF) Depletion FET Pre-Regultor turn-off threshold

VPR

VGLO VCC Gate Lockout Hysteresis

Depletion FET turn-on threshold

VGLO(ON) VCC gate lockout turn on threshold

VCC voltage when powered from VLINE

August 2006

Figure 2

9

M9999-080206

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