(

)

S1

VSR

S2

AS

OCV

F

VSR

R

V

R

N

V

V

V

´

=

´

+

-

IN(run)

S1

PA

VSL(run)

V

2

R

N

I

´

=

´

UCC28722

www.ti.com

SLUSBL7B – DECEMBER 2013 – REVISED OCTOBER 2015

7.3 Feature Description

7.3.1 Device Bias Voltage Supply (VDD)

The VDD pin is connected to a bypass capacitor to ground. The VDD turnon UVLO threshold is 21 V and turnoff

UVLO threshold is 7.7 V, with an available operating range up to 35 V on VDD. The USB charging specification

requires the output current to operate in constant-current mode from 5 V to a minimum of 2 V, which is easily

achieved with a nominal VDD of approximately 22 V. The additional VDD headroom (up to 35 V) allows for VDD

to rise due to the leakage energy delivered to the VDD capacitor in high-load conditions.

NOTE

It is possible for the start-up resistor to supply more current to the VDD node than the IC

will consume at higher bulk input voltages. A Zener diode clamp is required on the VDD

pin to keep the VDD pin voltage within limits if this is the case.

7.3.2 Ground (GND)

There is one ground reference external to the device for the base drive current and analog signal reference. TI

recommends placing the VDD bypass capacitor close to GND and VDD with short traces to minimize noise on

the VS and CS signal pins.

7.3.3 Voltage-Sense (VS)

The VS pin is connected to a resistor divider from the auxiliary winding to ground. The output-voltage feedback

information is sampled at the end of the transformer secondary current demagnetization time to provide an

accurate representation of the output voltage. Timing information for achieving valley-switching and to control the

duty cycle of the secondary transformer current is determined by the waveform on the VS pin. Avoid placing a

filter capacitor on this input because it would interfere with accurate sensing of this waveform.

The VS pin also senses the bulk capacitor voltage to provide for AC-input run and stop thresholds, and to

compensate the current-sense threshold across the AC-input range. During the transistor on-time, the VS pin is

clamped to approximately 250 mV below GND and the current out of the VS pin is sensed. For the AC-input run

and stop function, the run threshold on VS is 225 µA and the stop threshold is 80 µA. The values for the auxiliary

where

•

•

•

Characteristics).

(1)

where

•

•

•

•

•

(2)

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