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SI9121DB-5 Datasheet(PDF) 7 Page - Vishay Siliconix |
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SI9121DB-5 Datasheet(HTML) 7 Page - Vishay Siliconix |
7 / 9 page Si9121 Vishay Siliconix Document Number: 71112 S-40708—Rev. C, 19-Apr-04 www.vishay.com 7 DETAILED DESCRIPTION Bypass The bypass voltage of the Si9121 is set at a particular positive reference relative to the VNEG pin. The bypass voltage is used to set an accurate voltage and bias current for the on-chip oscillator and soft-start circuits. The 0.1-mF ceramic decoupling capacitor is recommended between the bypass and VNEG. No other components should be connected to this pin. Error Amplifier With a −48-V bus voltage, the converter is referenced to the −48-V (VNEG) node as its system ground. However, the +5-V or +3.3-V output is referenced to the GND pin, which is normally connected to 0 V. To regulate this output voltage, the Si9121 has an on-chip error amplifier which continuously monitors the output voltage and compares it to a reference voltage. This difference signal is level-shifted to the low side circuit to control the power switch duty-cycle and hence the regulation of the output voltage. Frequency compensation for the error amplifier is achieved by connecting an external network between the COMP pin and the VNEG pin. VCC Regulator VCC is an internally generated bias supply voltage which should be externally bypassed with a 0.1-mF capacitor connected to the negative supply voltage, VNEG. No load current should be drawn from the VCC pin. VCC may be supplied from an external source of 9.5-V to 12-V referenced to VNEG. In this configuration, the internal VCC regulator is disabled when the VCC receives 9.5-V, or greater, above VNEG. Thermal Shutdown The Si9121 also includes thermal shutdown which shuts down the device when junction temperature exceeds 170_C due to over heating. In thermal shutdown once the die temperature cools to below 145_C the regulator is enabled. If the die temperature is excessive due to high package power dissipation the regulator’s thermal circuit will continue to pulse the regulator on and off. This is called thermal cycling. Hiccup Mode Short Circuit Protection In addition to the thermal shutdown and the cycle-by-cycle current limiting features already described, the Si9121 has a built-in hiccup mode timer to handle a continuous output short-circuit and to automatically restart the device when the short-circuit is removed. If an output short-circuit occurs, the device immediately enters a cycle-by-cycle current limiting mode. As long as the thermal shutdown is not activated then the Si9121 automatically determines whether the internal hiccup timer should be started by monitoring the COMP pin. If VCOMP exceeds an upper threshold voltage (approximately 3.5 V) the timer is started and the external network at the COMP pin is discharged by an internal 2-mA current sink until VCOMP reaches approximately 1.5 V. (Note: all voltages are with respect to VNEG). At this point, the circuit reverts to the normal soft-start mode, whereby the COMP network is charged by its internal soft-start 10-mA current source and the circuit will attempt to start up in the normal manner. However, if the output short circuit is still present, the converter will again enter the cycle-by-cycle current limiting mode until the COMP pin voltage reaches 3.5 V, whereupon this cycle repeats until the short circuit is removed. The duty cycle imposed by the hiccup timer allows the Si9121 to handle continuous short-circuit without damage as long as the recommended component values shown in the Typical Application Circuit are used. MOSFET Switch The low-side n-channel MOSFET switch is integrated to provide optimum performance and to minimize the overall converter size. The typical 1.5-W rDS(on) of the MOSFET allows the converter to deliver up to 2 W of output power. |
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