Datasheet

UCC27323-Q1, UCC27324-Q1, UCC27325-Q1
SLUS678A –MARCH 2008–REVISED APRIL 2012 www.ti.com
APPLICATION INFORMATION
General Information
High-frequency power supplies often require high-speed high-current drivers such as those available in the
UCC2732x 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 driver is used to drive 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.
Drivers are used when it is not feasible to have the primary PWM regulator 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 intended to drive only the high-impedance input to a driver such as the UCC2732x.
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
The input thresholds have a 3.3-V logic sensitivity over the full range of V
DD
voltage, yet it is equally compatable
with 0 V to V
DD
signals.
The inputs of UCC2732x family of drivers are designed to withstand 500-mA reverse current without damage to
the device or logic upset. 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 input stages to the drivers function as a digital
gate, and they 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 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, limit the rise or fall times to the power
device, 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 also may help remove power dissipation from the device
package, as discussed in the Thermal Considerations section.
Output Stage
Inverting outputs of the UCC27323 and OUTA of the UCC27325 are intended to drive external P-channel
MOSFETs. Noninverting outputs of the UCC27324 and OUTB of the UCC27325 are intended to drive external N-
channel MOSFETs.
Each output stage is capable of supplying ±4-A peak current pulses and swings to both V
DD
and GND. The
pullup/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 R
DS(ON)
of the MOSFET transistor when the voltage on the driver output is less than the saturation voltage of
the bipolar transistor. Each output stage also provides a very low impedance to overshoot and undershoot, due
to the body diode of the external MOSFET. This means that, in many cases, external Schottky-clamp diodes are
not required.
The UCC27323 family delivers a 4-A gate drive when 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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