UCC28019P datasheet(13/48 Pages) TI

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Power Supply

VCC

ON 10.5V

VCC

OFF 9.5V

VCC

VCC(start) <200µA

VCC(stby) <2.9mA

VCC(ON)

UVLO

Soft-Start

UVLO

Run

Fault/Standby

Controller

State

PWM

State

OFF

Ramp

Regulated

OFF

Regulated

OFF

Soft-

Start

Ramp

UCC28019

SLUS755 – APRIL 2007

APPLICATION INFORMATION (continued)

The UCC28019 operates from an external bias supply. It is recommended that the device be powered from a

regulated auxiliary supply. This device is not intended to be used from a bootstrap bias supply. A bootstrap bias

supply is fed from the input high voltage through a resistor with sufficient capacitance on VCC to hold up the

voltage on VCC until current can be supplied from a bias winding on the boost inductor. The minimal hysteresis

on VCC would require an unreasonable value of hold-up capacitance.

During normal operation, when the output is regulated, current drawn by the device includes the nominal run

current plus the current supplied to the gate of the external boost switch. Decoupling of the bias supply must

take switching current into account in order to keep ripple voltage on VCC to a minimum. A ceramic capacitor

with a minimum value of 0.1

µF is recommended from VCC to GND with short, wide traces.

Figure 19. Device Supply States

The device bias operates in several states. During startup, VCC Under-Voltage LockOut (UVLO) sets the

minimum operational dc input voltage of the PFC controller. There are two UVLO thresholds. When the UVLO

turn-on threshold is exceeded, the controller turns ON. If VCC falls below the UVLO lower turn-off threshold, the

controller turns OFF. During UVLO, current drawn by the device is minimal. After the device turns on, Soft Start

(SS) is initiated and the output is ramped up in a controlled manner to reduce the stress on the external

components and prevents output voltage overshoot. During soft start and after the output is in regulation, the

device draws its normal run current. If any of several fault conditions is encountered or if the device is put in

Standby with an external signal, the device draws a reduced standby current.

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