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UCC28251 Datasheet(PDF) 15 Page - Texas Instruments

Part No. UCC28251
Description  UCC28251 Advanced PWM Controller With Prebias Operation
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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UCC28251 Datasheet(HTML) 15 Page - Texas Instruments

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0.3V
SS
EN
UVLO
CLK
UCC28251
www.ti.com
SLUSBD8E – FEBRUARY 2013 – REVISED DECEMBER 2014
Feature Description (continued)
8.3.2 VREF (Reference Generator) (20/7)
The VREF pin is regulated at 3.3 V. An external ceramic capacitor must be placed as close as possible to the
VREF and GND pins for noise filtering and to provide compensation to the regulator. The capacitance range
must be limited between 0.47 µF to 2.2 µF for stability. This reference is used to power the controller’s internal
circuits, and can also be used to bias an opto-coupler transistor, an external house-keeping microcontroller, or
other peripheral circuits. This reference can also be used to generate the reference for an external error
amplifier. This regulator output is internally current limited to 25 mA (typical).
8.3.3 EN (Enable Pin) (18/5)
The following conditions must be met before the controller allows start up:
1. VDD voltage has been sustained above the rising UVLO threshold 4.3 V (typical);
2. The 3.3-V reference voltage output at the VREF pin is 2.4 V (typical) for at least 20 us;
3. Junction temperature is below the thermal shutdown threshold 130°C (minimum);
4. The voltage at OVP is below 0.7 V (typical).
If all these conditions are met, the signal driving the EN pin is able to initiate the soft start process. Once the
device is enabled, the 27-µA internal charging current at the SS pin is turned on and begins to charge the soft-
start capacitor. The EN pin can accept both level-enable and pulse-enable signals.
For level-enable, the voltage level on the EN pin needs to be continuously higher than 2.25 V to allow continuous
operation. Once the EN pin falls below threshold, the device is disabled after 2 µs (see Figure 21).
Figure 21. Level Enable at EN Pin
Copyright © 2013–2014, Texas Instruments Incorporated
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