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UC2910 Datasheet(PDF) 4 Page - Texas Instruments |
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UC2910 Datasheet(HTML) 4 Page - Texas Instruments |
4 / 7 page pins. OVP (Overvoltage Comparator Output): This output pin drives an external SCR circuit with up to 65mA when the voltage on VSENSE rises above its nominal value by a percentage set by the voltage on the OVTH/UVTH pin (see below). The OVP comparator hys- teresis is a function of both the DACBUF voltage and the OV/UV percentage programmed. OVPB (Overvoltage Comparator Complementary Output): This output is a complement to the OVP out- put (see above) and provides an open collector capable of sinking 1mA when the voltage on VSENSE rises above its nominal value by a percentage set by the volt- age on the OVTH/UVTH pin (see below). OVTH/UVTH (Undervoltage and Lower Overvoltage Threshold Input): This pin is used to program the win- dow thresholds for the OV and UV comparators. The OV-UV window is centered around the DACBUF voltage and can be programmed from ±5% to ±15% about DACBUF. Connect a resistor divider between DACBUF and GND to set the percentage. The threshold for the OVP comparator is internally set to a percentage 2 times larger than the programmed OV percentage; therefore, its range extends from 10% to 30% above DACBUF. PWRGOOD (Undervoltage/Lower Overvoltage Out- put): This pin is an open collector output which is driven low to reset the microprocessor when VSENSE rises above or falls below its nominal value by a per- centage programmed by OVTH/UVTH. The OV and UV comparators’ hysteresis is a function of the DACBUF voltage and the OV/UV programmed percentage. VCC (Positive Supply Voltage): This pin supplies power to the chip. Connect VCC to a stable voltage source of at least 9V and capable of sourcing at least 15mA. The OVP and PWRGOOD outputs are held low, the OVPB output is in a high impedance state, and the VSENSE pin is pulled low until VCC exceeds the upper undervoltage lockout threshold. This pin should be by- passed directly to the GND pin with a 0.1 µF low ESR, low ESL capacitor. VREF (Voltage Reference Output): This pin provides an accurate 5V reference, capable of delivering up to 10mA to external circuitry, and is internally short circuit current limited. For best reference stability, bypass VREF directly to the GND pin with a 0.1 µF, low ESR, low ESL capacitor. VSENSE (Output Voltage Sensing Input): This pin is the input to the OVP and PWRGOOD comparators and is connected to the system output voltage through a lowpass filter. When choosing the resistor value for this filter, make sure that no more than 500 µA will flow PIN DESCRIPTIONS (cont.) UC1910 UC2910 UC3910 APPLICATION INFORMATION The Overvoltage (OV), Undervoltage (UV) and Overvol- tage Protection Voltage (OVP) threshold detection volt- ages are programmed as a percentage about the nominal DAC output voltage, DACOUT. Figure 1 illus- trates how to program the UC3910 by setting a voltage divider, RDIV, at the OVTH/UVTH pin. The voltage di- vider ratio is defined as RDIV = RS1 RS1 + RS2 The UC3910 allows a ratio RDIV at the OVTH/UVTH pin from 0.3 to 0.9, which corresponds to overvoltage and undervoltage percentage thresholds from 5% to 15% and an OVP percentage threshold from 10% to 30%. These thresholds are shown in Figure 2. The OV, UV and OVP percentage thresholds are given by %VOV = RDIV • 16.7 %VUV = –(RDIV • 16.7) %VOVP = %VOV • 2.0 = RDIV • 33.4 An R-C filter is added to the VSENSE pin to filter noise and ripple at the comparator inputs. An R-C filter fre- quency of FSWITCH/10 is recommended. Choose the Figure 1. Setting the OV/UV/OVP Threshold Percentages UDG-96020 4 |
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