UCC2880-4/-5/-6
UCC3880-4/-5/-6
PIN DESCRIPTIONS (cont.)
bit is set high by floating it, or connecting it to a 5V circuit board trace close to the IC. Decouple VIN to
source. Each control pin is pulled up to approximately 5V
PGND with a low ESR capacitor ≥ 0.10µF.
by an internal 70µA current source.
PWRGOOD (Undervoltage/Lower Overvoltage Out-
ENBL (Chip Enable Pin): This input is used to disable put): This pin is an open drain output which is driven low
the GATE and PWRGOOD outputs. Grounding this pin to reset the microprocessor when VSENSE rises above
causes the GATE output to be held low; floating the pin or or falls below its nominal value by 7.5%. The on resis-
pulling it up to 5V ensures normal operation. ENBL is tance of the open drain switch will be no higher than
pulled up to 5V internally.
470Ω. The OV and UV comparators’ hysteresis is fixed at
20mV independent of the COMMAND voltage.
GATE (PWM Output, MOSFET Driver): This output pro-
vides a 2Ω totem pole driver. Use a series resistor of at 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.
least 5Ω 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.
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.
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.
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 0.01µF.
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.
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.
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
APPLICATION INFORMATION
Current Limit
1.4V
16A • 20
RSENSE =
= 4.4mΩ
The short circuit current limit, ISC, is set according to:
A lower resistance value may be needed if the AC ripple
current in the inductor is more than 20% of the full load
current.
1.4V
ISC =
RSENSE • GCSA
where RSENSE is the average current sense resistor and
GCSA is the current sense amplifier gain, where GCSA
Related Publications
equals 20. Example: Choose RSENSE to set the short cir- U-156 and U-157 are Unitrode Application Notes describ-
cuit current limit at 16A using the UCC3880-5
ing the operation of the UC3886 and the UC3886/
UC3910 together in a Pentium® Pro application.
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