(

)

S1

VSR

S2

AS

OCV

F

VSR

R

V

R

N

V

V

V

´

=

´

+

-

UCC28710, UCC28711

UCC28712, UCC28713

UCC28714, UCC28715

SLUSB86 – NOVEMBER 2012

www.ti.com

Detailed Pin Description

VDD (Device Bias Voltage Supply): The VDD pin is connected to a bypass capacitor to ground. The VDD turn-

on UVLO threshold is 21 V and turn-off UVLO threshold is 8.1 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; this is easily achieved with a nominal VDD of approximately 25 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.

GND (Ground): There is a single ground reference external to the device for the gate drive current and analog

signal reference. Place the VDD bypass capacitor close to GND and VDD with short traces to minimize noise on

the VS and CS signal pins.

HV (High Voltage Startup): The HV pin is connected directly to the bulk capacitor to provide startup current to

the VDD capacitor. The typical startup current is ~300 µA which provides fast charging of the VDD capacitor. The

internal HV start-up device is active until VDD exceeds the turn-on UVLO threshold of 21 V at which time the HV

start-up device is turned off. In the off state the leakage current is very low to minimize standby losses of the

controller. When VDD falls below the 8.1-V UVLO turn-off threshold the HV start-up device is turned on.

VS (Voltage-Sense): 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 to achieve 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 which 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. This information is sensed during the

MOSFET on-time. For the AC-input run/stop function, the run threshold on VS is 225 µA and the stop threshold

determined by the equations below.

where

•

•

•

ELECTRICAL CHARACTERISTICS)

(1)

where

•

•

•

•

•

(2)

DRV (Gate Drive): The DRV pin is connected to the MOSFET gate pin, usually through a series resistor. The

gate driver provides a gate-drive signal limited to 14 V. The turn-on characteristic of the driver is a 25-mA current

source which limits the turn-on dv/dt of the MOSFET drain and reduces the leading-edge current spike, but still

provides gate-drive current to overcome the Miller plateau. The gate-drive turn-off current is determined by the

dv/dt by adding external gate resistance.

8

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