UCC28951
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ZHCSIQ7A –AUGUST 2018 –REVISED DECEMBER 2021
7.3 Feature Description
7.3.1 Start-Up Protection Logic
Before the UCC28951 controller will start up, the following conditions must be met:
• VDD voltage exceeds rising UVLO threshold 7.3-V typical.
• The 5-V reference voltage is available.
• Junction temperature is below the thermal shutdown threshold of 140°C.
• The voltage on the soft-start capacitor is not below 0.55-V typical.
If all those conditions are met, an internal enable signal EN is generated that initiates the soft-start process. The
duty cycle during the soft start is defined by the voltage at the SS pin, and cannot be lower than the duty cycle
set by TMIN, or by cycle-by-cycle current limit circuit depending on load conditions.
7.3.2 Voltage Reference (VREF)
The accurate (±1.5%) 5-V reference voltage regulator with a short-circuit protection circuit supplies internal
circuitry and provides up to 20-mA external output current. Place a low ESR and ESL, preferably ceramic
decoupling capacitor CREF in 1-µF to 2.2-µF range from this pin to GND as close to the related pins as possible
for best performance. The only condition where the reference regulator is shut down internally is during
undervoltage lockout.
7.3.3 Error Amplifier (EA+, EA–, COMP)
The error amplifier has two uncommitted inputs, EA+ and EA–, with a 3-MHz unity gain bandwidth, which allows
flexibility in closing the feedback loop. The EA+ is a noninverting input, the EA– is an inverting input and the
COMP is the output of the error amplifier. The input voltage common-mode range, where the parameters of the
error amplifier are ensured, is from 0.5 V to 3.6 V. The output of the error amplifier is connected internally to the
noninverting input of the PWM comparator. The range of the error amplifier output of 0.25 V to 4.25 V far
exceeds the PWM comparator input ramp-signal range, which is from 0.8 V to 2.8 V. The soft-start signal serves
as an additional noninverting input of the error amplifier. The lower of the two noninverting inputs of the error
amplifier is the dominant input and sets the duty cycle where the output signal of the error amplifier is compared
with the internal ramp at the inputs of the PWM comparator.
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Product Folder Links: UCC28951
English Data Sheet: SLUSDB2
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