Datasheet
SLUS486B − AUGUST 2001 − REVISED JULY 2003
12
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APPLICATION INFORMATION
selecting D
2
, D
3
, and D
4
Selection of suitable diodes is based upon the conducted peak and average currents. D2 simply provides a path
to charge C2 at converter power-up. Virtually any one of the common BAT54 series of Schottky diodes can be
used. To select D3 and D4, the peak currents of these two diodes need to be taken into account. First, the
average current flowing in both D3 and D4 is the same as the regulator current described in equation (3). The
peak currents in D3 and D4 are described as:
I
D3PK
+
I
REG
1 * D
I
D4PK
+
I
REG
D
For most UCC27222 applications, the duty cycle is much less than 50%, and the peak current in D3 is quite
reasonable. However, the peak current in D4 is quite high. This high peak current requires using a diode with
a higher current rating for D4.
To maintain a reasonable charge pump efficiency, BAT54-type diodes can be used for applications where the
peak currents are below approximately 40 mA. For applications where the peak current is greater than 40 mA,
a 350-mA or 500-mA diode should be used. A typical 350-mA diode is SD103CW, SOD−123 package,
manufactured by Diodes Inc. A typical 500-mA diode is the ZHCS500, SOT−23 package, available from Zetex
Inc.
selection of the flying capacitor C3
The flying capacitor is subjected to large peak currents, and to keep the peak-to-peak ripple voltage low, this
capacitor has to be larger than C1 and C2. Selection of C3 should be done based on allowable peak-to-peak
ripple on C3:
C3
MIN
+
I
REG
F
SW
k
ǒ
VIN * V
FD3
Ǔ
where I
REG
is the regulator output current, F
SW
is the switching frequency, k is the percent ripple on C3, and
V
FD2
is the forward drop of D3.
selection of bypass capacitor C4
The bypass capacitor C4 needs to be sized to take the peak current from the charge pump diode D4. The
capacitor is sized based on allowable ripple voltage:
C
MIN
+
I
REG
(
1 * D)
F
SW
k
ǒ
2 VIN * V
FD3
* V
FD4
Ǔ
where V
FD3
and V
FD4
are the forward voltages of D3 and D4 and k is the percent ripple allowed on C4.
(5)
(6)
(7)
(8)