UCC28950_11 Datasheet, PDF (11/46 Pages) Texas Instruments – Green Phase-Shifted Full-Bridge Controller With Synchronous Rectification

UCC28950
www.ti.com
SLUSA16A – MARCH 2010 – REVISED JULY 2010
DETAILED PIN DESCRIPTION AND PARAMETER SETTINGS
Start-Up Protection Logic
Before the UCC28950 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.
Voltage Reference (VREF)
The accurate (±1.5%) 5-V reference voltage regulator with the short circuit protection circuit supplies internal
circuitry and provides up to 20-mA external output current for setting DC/DC converter parameters. Place low
ESR and ESL, preferably ceramic decoupling capacitor C_REF 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 under voltage lockout.
Error Amplifier (EA+, EA-, COMP)
The error amplifier has two uncommitted inputs, EA+ and EA-, with a 3-MHz unity bandwidth, which allows
flexibility in closing the feedback loop. The EA+ is a non-inverting 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 error
amplifier are guaranteed, is from 0.5 V to 3.6 V. The output of the error amplifier is connected internally to the
non-inverting 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 non-inverting input of the error amplifier. The lower of the two non-inverting 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.
Copyright © 2010, Texas Instruments Incorporated
Product Folder Link(s): UCC28950
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