UCC28700, UCC28701

UCC28702, UCC28703

SLUSB41 – JULY 2012

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Detailed Pin Description

VDD (Device Bias Voltage Supply): The VDD pin is connected to a bypass capacitor to ground and a start-up

resistance to the input bulk capacitor (+) terminal. 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. 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. Also, the wide VDD range provides the

advantage of selecting a relatively small VDD capacitor and high-value start-up resistance to minimize no-load

stand-by power loss in the start-up resistor.

GND (Ground): This 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.

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 220 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.

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