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SI9123 数据表(PDF) 9 Page - Vishay Siliconix |
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SI9123 数据表(HTML) 9 Page - Vishay Siliconix |
9 / 16 page Si9123 Vishay Siliconix New Product Document Number: 72098 S-03639—Rev. B, 20-Mar-03 www.vishay.com 9 The pre-regulator will remain on until VCC equals VREG but between VUVLO and VREG, excessive current may result in VCC falling below VUVLOand stopping soft-start operation. This situation is avoided by the hysteresis between VREG and VUVLO and correct sizing of the VCC capacitor, bootstrap capacitor, the soft-start capacitor, the primary MOSFET gate driving charge, and load on the SEC_SYNC output. The value of the VCC capacitor should be chosen to be capable of maintaining soft-start operation with VCC above VUVLO until the VCC current can be supplied from the external circuit (e.g., via an auxiliary winding on the secondary inductor). CVCC 4.7 mF GND VCC VREF VIN VINEXT Figure 6. High-Voltage Pre-Regulator Circuit HVDMOS 14.5 V REXT = 1.4 kW Auxillary VCC The feedback voltage from the output of the auxiliary winding must sustain VCC above VREG to fully disconnect the pre-regulator, isolating VCC from VINEXT. VCC is then maintained above VREG for the duration of operation. In the event of an over voltage condition on VCC, an internal voltage clamp turns on at 14.5 V to shunt excessive current to GND. In systems where operation is directly from a 12 V supply, VINEXT and VCC can be connected to the 12 V bus. The soft-start circuit is designed for the dc-dc converter to start up in an orderly manner and reduce component stress. Soft-start is achieved by ramping the maximum attainable duty cycle during the soft-start time. The duty cycle is increased from zero to the final value at the rate set by an external capacitor, CSS as shown in Figure 7. The hiccup time is set by an internal 20 µA current source charging CSS from 0 V to 2 Vbe, at which point switching begins. Then a 100 µA charging current is applied to CSS to charge from 2 Vbe to the final value controlling the duty cycle as it rises. In the event of UVLO, shutdown or over current, the SS pin will be held low (<1 V) disabling driver switching. A longer soft-start time may be needed for highly capacitive loads and high peak-output current applications. In the event of an over current condition being detected, the soft-start pin will be pulled low and the cycle will start again performing a hiccup as shown in Figure 4. The hiccup off-time, t1, is given by: t 1 [ CSS 1.2 V 20 mA The soft-start time t2 is can be estimated as: t 2 [ C SS V OUT n (K 100 mA) where VOUT is the output of the converter, and n is the turns ratio of the primary to each secondary winding, and K is the ratio of the resistive divider from VINEXT to VINDET (typically 10/1). - + GM CSS + - AV AV 150 mV Peak Detect Blank CS1 CS2 SS Control Figure 7. Current-Sense and Soft-Start Circuit Block Diagram AV 100 mV I 4I VCC SS SS Enable |
类似零件编号 - SI9123 |
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类似说明 - SI9123 |
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