Datasheet

UCC27321-Q1, UCC27322-Q1
SLUSA13C –FEBRUARY 2010–REVISED MARCH 2012 www.ti.com
It should be noted that the current-sink capability is slightly stronger than the current source capability at lower
VDD. This is due to the differences in the structure of the bipolar-MOSFET power output section, where the
current source is a P-channel MOSFET and the current sink has an N-channel MOSFET.
In a large majority of applications, it is advantageous that the turnoff capability of a driver is stronger than the
turnon capability. This helps to ensure that the MOSFET is held off during common power-supply transients that
may turn the device back on.
Operational Circuit Layout
It can be a significant challenge to avoid the overshoot/undershoot and ringing issues that can arise from circuit
layout. The low impedance of these drivers and their high di/dt can induce ringing between parasitic inductances
and capacitances in the circuit. Utmost care must be used in the circuit layout.
In general, position the driver physically as close to its load as possible. Place a 1-µF bypass capacitor as close
to the output side of the driver as possible, connecting it to pins 1 and 8. Connect a single trace between the two
VDD pins (pin 1 and pin 8); connect a single trace between PGND and AGND (pin 5 and pin 4). If a ground
plane is used, it may be connected to AGND; do not extend the plane beneath the output side of the package
(pins 5–8). Connect the load to both OUT pins (pins 7 and 6) with a single trace on the adjacent layer to the
component layer; route the return current path for the output on the component side, directly over the output
path.
Extreme conditions may require decoupling the input power and ground connections from the output power and
ground connections. The UCCx7321/2 has a feature that allows the user to take these extreme measures, if
necessary. There is a small amount of internal impedance of about 15 Ω between the AGND and PGND pins;
there is also a small amount of impedance (approximately 30 Ω) between the two VDD pins. In order to take
advantage of this feature, connect a 1-µF bypass capacitor between VDD and PGND (pins 5 and 8) and connect
a 0.1-µF bypass capacitor between VDD and AGND (pins 1 and 4). Further decoupling can be achieved by
connecting between the two VDD pins with a jumper that passes through a 40-MHz ferrite bead and connects
bias power only to pin 8. Even more decoupling can be achieved by connecting between AGND and PGND with
a pair of anti-parallel diodes (anode connected to cathode and cathode connected to anode).
VDD
Although quiescent VDD current is very low, total supply current is higher, depending on the OUT current and the
programmed oscillator frequency. Total VDD current is the sum of quiescent VDD current and the average OUT
current. Knowing the operating frequency and the MOSFET gate charge (Q
g
), average OUT current can be
calculated from:
I
OUT
= Q
g
× f
where f is frequency.
For the best high-speed circuit performance, two VDD bypass capacitors are recommended to prevent noise
problems. The use of surface-mount components is highly recommended. A 0.1-µF ceramic capacitor should be
located closest to the VDD-to-ground connection. In addition, a larger capacitor (such as 1-µF) with relatively low
ESR should be connected in parallel, to help deliver the high-current peaks to the load. The parallel combination
of capacitors should present a low-impedance characteristic for the expected current levels in the driver
application.
Drive Current and Power Requirements
The UCC37321/2 family of drivers is capable of delivering 9 A of current to a MOSFET gate for a period of
several hundred nanoseconds. High peak current is required to turn an N-channel device ON quickly. Then, to
turn the device OFF, the driver is required to sink a similar amount of current to ground. This repeats at the
operating frequency of the power device. An N-channel MOSFET is used in this discussion because it is the
most common type of switching device used in high-frequency power-conversion equipment.
References 1 and 2 contain detailed discussions of the drive current required to drive a power MOSFET and
other capacitive-input switching devices. Much information is provided in tabular form to give a range of the
current required for various devices at various frequencies. The information pertinent to calculating gate-drive
current requirements is summarized here; the original document is available from the TI web site (www.ti.com).
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