UCC28250_15 Datasheet, PDF (11/54 Pages) Texas Instruments – Advanced PWM Controller With Pre-Bias Operation

UCC28250
www.ti.com
DETAILED PIN DESCRIPTIONS
SLUSA29C – APRIL 2010 – REVISED JULY 2011
VDD (5/12)
The UCC28250 can be powered up by a wide supply range from 4.3 V (UVLO rising typical) to 20 V (absolute
maximum), making it suitable for primary-side control or secondary-side control. When the voltage at the VDD
pin is lower than 4.1 V (typical), the controller is in stand-by mode and consumes 150 µA (typical) at 3.6 V VDD.
In stand-by mode, VREF continues to be regulated to 3.3 V or follows VDD if VDD is lower than 3.3 V. Please
refer to the VREF description for more detailed information. A minimum 1-µF bypass capacitor is required from
VDD to ground. Keep the bypass capacitor as close to the device as possible.
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 0.5 µF to 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).
EN (Enable Pin) (18/5)
The following conditions must be met before the controller allows start up:
1. VDD voltage is above the rising UVLO threshold 4.3 V (typical);
2. The 3.3-V reference voltage output at the VREF pin is available and above 2.4 V (typical);
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 2.25 V, the device is disabled (see Figure 3).
UVLO
EN
0.3V
SS
CLK
Figure 3. Level Enable at EN pin
Copyright © 2010–2011, Texas Instruments Incorporated
Product Folder Link(s): UCC28250
Submit Documentation Feedback
11