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SI9122 датащи(PDF) 11 Page - Vishay Siliconix |
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SI9122 датащи(HTML) 11 Page - Vishay Siliconix |
11 / 20 page Document Number: 71815 S-80038-Rev. J, 14-Jan-08 www.vishay.com 11 Vishay Siliconix Si9122 conditions the oscillator frequency is reduced by the current overload protection to enable a constant current to be maintained into a low impedance circuit. Current Limit Current mode control providing constant current operation is achieved by monitoring the differential voltage VCS between the CS1 and CS2 pins, which are connected to a current sense resistor on the primary low-side MOSFET. In the absence of an overcurrent condition, VCS is less than lower current limit threshold VTLCL (typical 100 mV); CL_CONT is pulled up linearly via the 120 µA current source (IPU) and both DL and DH switch at half the oscillator set frequency. When a moderate overcurrent condition occurs (VTLCL < VCS < VTHCL), the CL_CONT capacitor will be discharged at a rate that is proportional to VCS - 100 mV by the IPD current source. Both driver outputs are in frequency fold-back mode and the switching frequency becomes roughly 20 % of normal switching frequency. When a severe overcurrent condition occurs (VTHCL < VCS), the NMOS discharges CL_CONT capacitor immediately at 2 mA rate and the CL_CONT voltage will be clamped to 1.2 V disabling both DL and DH outputs. Before VCS reaches severe overcurrent condition, a lowering of the CL_CONT voltage results in PWM control of the output drive being taken over by the current limit control loop through CL_CONT. Current control initially reduces the switching duty cycle toward the minimum the chip can reach (DMIN). If this duty cycle reduction still cannot lower the load current, then the switching frequency will start to fold back to minimum 1/5 of the nominal frequency. This prevents the on-time of the primary drivers from being reduced to below 100 ns and avoids current tails. If VCS > VTHCL, the switching will then stop. With constant current mode control and frequency foldback, protection of the MOSFET switches is increased. The converter reverts to voltage mode operation immediately when the primary current falls below the limit level, and CL_CONT capacitor is charged up and clamped to 6.5 V. The soft-start function does not apply during current limit period, as this would constitute hiccup mode operation. VIN Voltage Monitor - VINDET The chip provides a means of sensing the voltage of VIN, and withholding operation of the output drivers until a minimum voltage of VREF (3.3 V, 300 mV hysteresis), is achieved. This is achieved by choosing an appropriate resistive tap between the ground and VIN, and comparing this voltage with the reference voltage. When the applied voltage is greater than VREF, the output drivers are activated as normal. VINDET also provides the input to the voltage feed forward function. However, if the divided voltage applied to the VINDET pin is greater than VCC - 0.3 V, the high-side driver, DH, will stop switching until the voltage drops below VCC - 0.3 V. Thus, the resistive tap on the VIN divider must be set to accommodate the normal VCC operating voltage to avoid this condition. Alternatively, a zener clamp diode from VINDET to GND may also be used. Shutdown Mode If VINDET is forced below the lower threshold, a minimum of 350 mV (VSD), the device will enter SHUTDOWN mode. This powers down all unnecessary functions of the controller, ensures that the primary switches are off and results in a low level current demand from the VIN or VCC supplies. Figure 5. High-Voltage Pre-Regulator Circuit CVCC 0.5 µF CEXT 2 nF GND VCC PNP Ext REG_COMP VREF VIN VINEXT HVDMOS 14.5 V REXT (Si9122) Auxillary VCC Figure 6 . Current Limit Circuit - + GM CEXT + - AV AV 150 mV - + GM Peak Detect Blank CS1 CS2 IPD 0 to 240 µA (nom) IPU 120 µA (nom) AV VOFFSET OSC CL_CLAMP REXT CL_CONT AV 100 mV VCC |
Аналогичный номер детали - SI9122_08 |
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Аналогичное описание - SI9122_08 |
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