Datasheet

UCC28089
SLUS623A -- SEPTEMBER 2004 -- REVISED AUGUST 2006
8
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APPLICATION INFORMATION
Using the UCC28089 as the Primary-Side start-up Controller in a Cascaded Push-Pull Buck
Two-Stage Converter
The cascaded push-pull topology is ideal for converting from moderate bus voltages, such as 48-V telecom
buses, to sub 2-V output voltages. The general topology is shown in Figure 2 using the UCC28089 as the
primary-side start-up controller and the UCC2540 as the secondary-side regulator [3].
UDG-04100
CT
RB
RA
10 V
C
R1
C
R2
V
IN
=48V
VO
1.2 V
2
1
4
3
7
8
5
6
UCC28089
SYNC
DIS
CT
CS
VDD
OUTA
OUTB
GND
+
--
IN OUT
COM
L.REG
2
1
4
3
19
20
17
18
UCC2540
ISET/SD
REF
G2C
SYNCIN
SWS
BST
G1
SW
6
5
8
7
RAMP
GND
VEA--
CEA--
10
9COMP
TR
15
16
13
14
VDD
PGND
G2
VDRV
11
12G2S
SS
+
--
Figure 2. Cascaded Push-Pull Buck Two-Stage Converter