UCC27323, UCC27324, UCC27325

UCC37323, UCC37324, UCC37325

SLUS492H – JUNE 2001 – REVISED MAY 2013

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APPLICATION INFORMATION

General Information

High-frequency power supplies often require high-speed, high-current drivers such as the UCC37323/4/5 family.

A leading application is the needed to provide a high-power buffer stage between the PWM output of the control

IC and the gates of the primary power MOSFET or IGBT switching devices. In other cases, the driver IC is used

to drive the power-device gates through a drive transformer. Synchronous rectification supplies are also needed

to simultaneously drive multiple devices which presents an extremely large load to the control circuitry.

Driver ICs are used when having the primary PWM regulator IC directly drive the switching devices for one or

more reasons is not feasible. The PWMIC does not have the brute drive capability required for the intended

switching MOSFET, limiting the switching performance in the application. In other cases there may be a desire to

minimize the effect of high-frequency switching noise by placing the high current driver physically close to the

load. Also, newer ICs that target the highest operating frequencies do not incorporate onboard gate drivers at all.

Their PWM outputs are only intended to drive the high impedance input to a driver such as the UCC37323/4/5.

Finally, the control IC is under thermal stress due to power dissipation, and an external driver helps by moving

the heat from the controller to an external package.

Input Stage

The inputs of UCC37323/4/5 family of drivers are designed to withstand 500-mA reverse current without either

damage to the IC for logic upset. The input stage of each driver must be driven by a signal with a short rise or fall

time. This condition is satisfied in typical power-supply applications, where the input signals are provided by a

PWM controller or logic gates with fast transition times (<200 ns). The input stages to the drivers function as a

digital gate, and are not intended for applications where a slow-changing input voltage is used to generate a

switching output when the logic threshold of the input section is reached. While this may not be harmful to the

driver, the output of the driver may switch repeatedly at a high frequency.

Users must not attempt to shape the input signals to the driver in an attempt to slow down (or delay) the signal at

the output. If limiting the rise or fall times to the power device is desired, limit the rise or fall times to the power

device, then add an external resistance between the output of the driver and the load device, which is generally a

power MOSFET gate. The external resistor also helps remove power dissipation from the IC package, as

discussed in the section on Thermal Considerations (see THERMAL INFORMATION).

Output Stage

Inverting outputs of the UCC37323 and OUTA of the UCC37325 are intended to drive external P-channel

MOSFETs. Noninverting outputs of the UCC37324 and OUTB of the UCC37325 are intended to drive external N-

channel MOSFETs.

Each output stage is capable of supplying ±4-A peak current pulses and swings to both VDD and GND. The

pullup and pulldown circuits of the driver are constructed of bipolar and MOSFET transistors in parallel. The peak

output current rating is the combined current from the bipolar and MOSFET transistors. The output resistance is

the bipolar transistor. Each output stage also provides a very low impedance to overshoot and undershoot due to

the body diode of the internal MOSFET. In many cases, external-schottky-clamp diodes are not required.

The UCC37323 family delivers 4-A of gate drive where it is most needed during the MOSFET switching

transition — at the Miller plateau region — providing improved efficiency gains. A unique BiPolar and MOSFET

hybrid output stage in parallel also allows efficient current sourcing at low supply voltages.

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