UCC27423, UCC27424, UCC27425
SLUS545B − NOVEMBER 2002 − REVISED NOVEMBER 2004
APPLICATION INFORMATION
The switching load presented by a power MOSFET can be converted to an equivalent capacitance by examining
the gate charge required to switch the device. This gate charge includes the effects of the input capacitance
plus the added charge needed to swing the drain of the device between the ON and OFF states. Most
manufacturers provide specifications that provide the typical and maximum gate charge, in nC, to switch the
device under specified conditions. Using the gate charge Qg, one can determine the power that must be
dissipated when charging a capacitor. This is done by using the equivalence Qg = CeffV to provide the following
equation for power:
P
+
C
V2
f
+
Qg
f
This equation allows a power designer to calculate the bias power required to drive a specific MOSFET gate
at a specific bias voltage.
Enable
UCC27423/4/5 provides dual Enable inputs for improved control of each driver channel operation. The inputs
incorporate logic compatible thresholds with hysteresis. They are internally pulled up to V
DD
with 100-kΩ
resistor for active high operation. When ENBA and ENBB are driven high, the drivers are enabled and when
ENBA and ENBB are low, the drivers are disabled. The default state of the Enable pin is to enable the driver
and therefore can be left open for standard operation. The output states when the drivers are disabled is low
regardless of the input state. See the truth table of Table 1 for the operation using enable logic.
Enable input are compatible with both logic signals and slow changing analog signals. They can be directly
driven or a power−up delay can be programmed with a capacitor between ENBA, ENBB and AGND. ENBA and
ENBB control input A and input B respectively.
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