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PIN ASSIGNMENTS

UCC29411, UCC29412

UCC29413, UCC39411

UCC39412, UCC39413

SLUS245E – MARCH 2000 – REVISED JULY 2005

VIN: Input voltage to supply the IC during start-up. After the output is running the IC draws power from VOUT or

VGD.

SW: An inductor is connected between this pin and VIN. The VGD (gate drive supply) flyback diode is also

connected to this pin. When servicing the main output supply this pin pulls low, charging the inductor, then shuts

off dumping the energy through the synchronous rectifier to the output. When servicing the VGD supply, the

internal synchronous rectifier stays off and the energy is diverted to VGD through the flyback diode. During

discontinuous portions of the inductor current, a MOSFET resistively connects VIN to SW damping excess

circulating energy to eliminate undesired high-frequency ringing.

VGD: The VGD pin, which is coarsely regulated around 7 V (8.5 V for the UCC39413), is primarily used for the

gate drive supply for the power switches in the IC. This pin can be loaded with up to 10 mA as long as it does

not present a load at voltages below 2 V (this ensures proper start-up of the IC). The VGD supply can go as low

as 6.3 V without interfering with the servicing of the main output. When below 6.3 V, VGD has the highest

priority.

VOUT: Main output voltage (3.3 V, 5 V, or adjustable), has highest priority in the multiplexing scheme, as long as

VGD is above the critical level of 6.3 V. Startup at full load is achievable at input voltages down to 1 V.

CT: This pin provides the timer for determining the reset period. The period is controlled by placing a capacitor to

× t where t is the desired reset period.

RESB: This pin provides an active low signal to alert the user when the main output voltage falls below 10% of

its targeted value. The open-drain output can be used to reset a microcontroller that may be powered off of the

main output voltage.

SD/FB: For the UCC39411, this pin is used to adjust the output voltage via a resistive divider from VOUT. It also

serves as the shutdown pin for all three versions. Pulling this pin low provides a shutdown signal to the IC.

GND: Ground of the IC.

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