Datasheet
SLUS486B − AUGUST 2001 − REVISED JULY 2003
8
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APPLICATION INFORMATION
selection of bypass capacitor C1
Bypass capacitors should be selected based upon allowable ripple voltage, usually expressed as a percent of
the regulated power supply rail to be bypassed. In all of the UCC27222 application circuits shown herein, C1
provides the bypass for the main (high-side) gate driver. Every time Q1 is switched on, a packet of charge is
removed from C1 to charge Q1’s gate to approximately 6.0 V. The charge delivered to the gate of Q1 can be
found in the manufacturer’s datasheet curves. An example of a gate charge curve is shown in Figure 4.
020304010
0
2
4
6
8
31 nC
GATE-TO-SOURCE VOLTAGE
vs
TOTAL GATE CHARGE
V
GS
− Gate-to-Source Voltage − V
Q6 − Total Gate Charge − nC
Figure 4.
As shown in Figure 4, 31 nC of gate charge is required in order for Q1’s gate to be charged to 6.0 V, relative
to its source. The minimum bypass capacitor value can be found using the following calculation:
C1
MIN
+
Q
G
k
ǒ
VHI * V
SW
Ǔ
where k is the percent ripple on C1, Q
G
is the total gate charge required to drive the gate of Q1 from zero to
the final value of (VHI−VSW). In this example gate charge curve, the value of the quantity (VHI−VSW) is taken
to be 6.0 V. This value represents the nominal VLO regulator output voltage minus the forward voltage drop of
the external Schottky diode, D1. For the MOSFET with the gate charge described in Figure 4, the minimum
capacitance required to maintain a 3% peak-to-peak ripple voltage can be calculated to be 172 nF, so a 180-nF
or a 220-nF capacitor could be used. The maximum peak-to-peak C1 ripple must be kept below 0.4 V for proper
operation.
(1)