mic9131 Micrel Semiconductor, mic9131 Datasheet - Page 9

• Manufacturer Part Number: mic9131
• Description: Mic9131 High-voltage, High-speed Telecom Dc-to-dc Controller
• Manufacturer: Micrel Semiconductor

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
• Enable and Undervoltage monitoring circuits
• Oscillator and sync circuitry
• Soft-start and soft-start reset circuits
• MOSFET gate drive circuits
• Control loop operation
August 2006
MIC9131
• High voltage start-up circuit
• V
• V
• Enable
CC
CC
and Bias supplies
and V
IN
Transformer
Winding
UVLO
Bias
V
IN
VCC
Line
2
1
DEPLETION
180V
FET
V
V
CC
GLO
voltage when powered from V
V
CC
Gate Lockout Hysteresis
Figure 2
Internal
Circuitry
9
SHUTDOWN
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. V
THERMAL
• Current sensing & overcurrent protection
• Slope compensation
• Error amplifier
MIC9131
LINE
1.21V
V
PR
Depletion FET turn-on threshold
V
V
GLO(ON)
PR(OFF)
Depletion FET Pre-Regultor turn-off threshold
V
CC
gate lockout turn on threshold
VCC
UVLO
M9999-080206
Micrel, Inc.
IN
is ap-