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UCC3880-6 Datasheet(PDF) 5 Page - Texas Instruments |
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UCC3880-6 Datasheet(HTML) 5 Page - Texas Instruments |
5 / 8 page UCC2880-4/-5/-6 UCC3880-4/-5/-6 PIN DESCRIPTIONS (cont.) bit is set high by floating it, or connecting it to a 5V source. Each control pin is pulled up to approximately 5V by an internal 70 µA current source. ENBL (Chip Enable Pin): This input is used to disable the GATE and PWRGOOD outputs. Grounding this pin causes the GATE output to be held low; floating the pin or pulling it up to 5V ensures normal operation. ENBL is pulled up to 5V internally. GATE (PWM Output, MOSFET Driver): This output pro- vides a 2 Ω totem pole driver. Use a series resistor of at least 5 Ω between this pin and the gate of the external MOSFET to prevent excessive overshoot. GND (Signal Ground): All voltages are measured with respect to GND. Bypass capacitors on the VCC and VREF pins should be connected directly to the ground plane near the GND pin. IS– (Current Sense Amplifier Inverting Input): This pin is the inverting input to the current sense amplifier and is connected to the low side of the average current sense resistor. IS+ (Current Sense Amplifier Noninverting Input): This pin is the noninverting input to the current sense am- plifier and is connected to the high side of the average current sense resistor. ISOUT (Current Sense Amplifier Output): This pin is the output of the current sense amplifier. The voltage on this pin is (COMMAND + GCSA • I • RSENSE), where COMMAND is the voltage on the COMMAND pin, GCSA is the fixed gain of the current sense amplifier, equal to 20, I is the current through the sense resistor, and RSENSE is the value of the average current sensing resis- tor. PGND (Power Ground): This pin provides a dedicated ground for the output gate driver. The GND and PGND pins should be connected externally using a short printed circuit board trace close to the IC. Decouple VIN to PGND with a low ESR capacitor ≥ 0.10µF. PWRGOOD (Undervoltage/Lower Overvoltage Out- put): This pin is an open drain output which is driven low to reset the microprocessor when VSENSE rises above or falls below its nominal value by 7.5%. The on resis- tance of the open drain switch will be no higher than 470 Ω. The OV and UV comparators’ hysteresis is fixed at 20mV independent of the COMMAND voltage. VIN (Positive Supply Voltage): This pin supplies power to the chip. Connect VIN to a stable voltage source of at least 10.8V. The GATE and PWRGOOD outputs will be held low until VCC exceeds the upper undervoltage lock- out threshold. This pin should be bypassed directly to the GND pin. VFB (Voltage Amplifier Inverting Input): This input is connected to COMP through a feedback network and to the power supply output through a resistor or a divider network. VREF (Voltage Reference Output): This pin provides an accurate 5V reference and is internally short circuit cur- rent limited. VREF powers the D/A converter and also provides a threshold voltage for the UVLO comparator. For best reference stability, bypass VREF directly to GND with a low ESR, low ESL capacitor of at least 0.01 µF. VSENSE (Output Voltage Sensing Input): This pin is connected to the system output voltage through a low pass filter. When the voltage on VSENSE rises above or falls below the COMMAND voltage by 7.5%, the PWRGOOD output is driven low to reset the microproc- essor. When the voltage on VSENSE rises above the COMMAND voltage by 15%, the OVP comparator pulls the current amplifier output voltage below the oscillator valley voltage to force zero duty cycle at the GATE out- put. This pin is also used by the foldback current limiting circuitry. Current Limit The short circuit current limit, ISC, is set according to: ISC = 1.4V RSENSE • GCSA where RSENSE is the average current sense resistor and GCSA is the current sense amplifier gain, where GCSA equals 20. Example: Choose RSENSE to set the short cir- cuit current limit at 16A using the UCC3880-5 RSENSE = 1.4V 16A • 20 = 4.4m Ω A lower resistance value may be needed if the AC ripple current in the inductor is more than 20% of the full load current. Related Publications U-156 and U-157 are Unitrode Application Notes describ- ing the operation of the UC3886 and the UC3886/ UC3910 together in a Pentium® Pro application. APPLICATION INFORMATION 5 |
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