UCC27321, UCC27322

UCC37321, UCC37322

SLUS504C − SEPTEMBER 2002 − REVISED NOVEMBER 2004

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APPLICATION INFORMATION

general information

The UCC37321 and UCC37322 drivers serve as an interface between low-power controllers and power

MOSFETs. They can also be used as an interface between DSPs and power MOSFETs. High-frequency power

supplies often require high-speed, high-current drivers such as the UCC37321/2 family. A leading application

is the need to provide a high power buffer stage between the PWM output of the control device and the gates

of the primary power MOSFET or IGBT switching devices. In other cases, the device drives the power device

gates through a drive transformer. Synchronous rectification supplies also have the need to simultaneously

drive multiple devices which can present an extremely large load to the control circuitry.

The inverting driver (UCC37321) is useful for generating inverted gate drive signals from controllers that have

N-channel synchronous rectifier MOSFETs in buck derived converters. This driver can also be used for

generating a gate drive signal for a P-channel MOSFET from a controller that is designed for N-channel

applications.

MOSFET gate drivers are generally used when it is not feasible to have the primary PWM regulator device

directly drive the switching devices for one or more reasons. The PWM device may 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 devices that target the highest operating frequencies may

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 UCC37321/2. Finally, the control device may be under thermal stress due to power

dissipation, and an external driver can help by moving the heat from the controller to an external package.

input stage

current without either damage to the device or logic upset. In addition, the input threshold turn-off of the

UCC37321/2 has been slightly raised for improved noise immunity. The input stage of each driver should 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

IN input of the driver functions as a digital gate, and it is 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 should 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, then an external resistance can be

added between the output of the driver and the load device, which is generally a power MOSFET gate. The

external resistor may also help remove power dissipation from the device package, as discussed in the section

on Thermal Considerations.