SLUS545D – NOVEMBER 2002 – REVISED MAY 2013
APPLICATION INFORMATION
General Information
High frequency power supplies often require high-speed, high-current drivers such as the UCC27423/4/5 family.
A leading application is the need 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 utilized
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.
Driver ICs are utilized when it is not feasible to have the primary PWM regulator IC directly drive the switching
devices for one or more reasons. The PWM IC 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 ICs 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
UCC27423/4/5. Finally, the control IC 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 3.3V logic sensitivity over the full range of V
DD
voltages; yet it is equally compatible
with 0 to V
DD
signals. The inputs of UCC27423/4/5 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 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 may also help remove power dissipation from the devoce
package, as discussed in the section on Thermal Considerations.
Output Stage
Inverting outputs of the UCC27423 and OUTA of the UCC27425 are intended to drive external P-channel
MOSFETs. Noninverting outputs of the UCC27424 and OUTB of the UCC27425 are intended to drive external N-
channel MOSFETs.
Each output stage is capable of supplying ±4A peak current pulses and swings to both VDD 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 UCC27423 family delivers 4A 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.
Source/Sink Capabilities During Miller Plateau
Large power MOSFETs present a large load to the control circuitry. Proper drive is required for efficient, reliable
operation. The UCC27423/4/5 drivers have been optimized to provide maximum drive to a power MOSFET
during the Miller plateau region of the switching transition. This interval occurs while the drain voltage is swinging
between the voltage levels dictated by the power topology, requiring the charging/discharging of the drain-gate
capacitance with current supplied or removed by the driver device.
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