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MIC9131BM Datasheet(PDF) 9 Page - Micrel Semiconductor |
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MIC9131BM Datasheet(HTML) 9 Page - Micrel Semiconductor |
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9 / 19 page  August 2006 9 M9999-080206 MIC9131 Micrel, Inc. 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. Ablock 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 • 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 MOSFETis 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- Transformer Bias Winding VIN 180V DEPLETION FET Internal Circuitry MIC9131 VCC 2 Line 1 THERMAL SHUTDOWN 1.21V VCC UVLO VPR VPR(OFF) Depletion FET Pre-Regultor turn-off threshold Depletion FET turn-on threshold VGLO VCC Gate Lockout Hysteresis VCC voltage when powered from VLINE VGLO(ON) VCC gate lockout turn on threshold Figure 2 |
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