Datasheet
47 mF
Vout1
GND
22 mF 25 V
330 mF
+
PWM1
SRE1
FF1
CS1
PWM2
SRE2
FF2
CS2
Temp
EAp1
EAn1
10 W
800 nH
Vout2
GND
+
EAp2
EAn2
800 nH
0.22 mF
22 mF 25 V
0.22 mF
4.7 mF
1 mF
10kW
26
25
18
20
1
2
9
7
19
5
8
29
28
27
14
24
23
15
16
17
30
31
PWM-A
SRE-A
FLT-A
IMON-A
PWM-B
SRE-B
FLT-B
IMON-B
TMON
V
GG
DIS
Test
Vin
NC
SW-A
BST-A
BSW-A
PGND
NC
PGND
Vin
Vin
NC
Vin
32
13
3
4
10
11
12
6
SW-B
BST-B
BSW-B
PGND
NC
PGND
V
GG
22 BP3
21 AGND
UCD7242
Vin
+
330 Fm
10 W
10 W
10 W
47 mF
330 mF
10kW
R
BIAS
R
BIAS
UCD7242
SLUS962B –JANUARY 2010–REVISED AUGUST 2012
www.ti.com
APPLICATION INFORMATION
A partial schematic of a power supply application using the UCD7242 power stage is provided below. Although
not shown the IC controlling the output is from the UCD92XX family of digital controllers.
PRE-BIAS OPERATION
The UCD7242 has no problem starting up into pre-biased output voltages. However, when one channel is held in
tri-state and the second channel is actively switching, the tri-stated channel may generate a DC voltage through
weak capacitive coupling between SW-A and SW-B. This coupling comes principally from the close proximity of
the switch nodes on the silicon and the PWB layout.
There are several options to address this concern.
1. The device(s) that the UCD7242 is powering on a 3-stated channel has a known current draw at sub-
regulation voltage levels. This current draw may be sufficient to hold the voltage down.
2. Instead of holding the off channel in a 3-state condition, drive PWM actively low. This forces the synchronous
rectifier to turn on and prevent the pre-bias voltage from rising. If this option is elected, it is important to verify
that there are no other sources of leakage in the system.
3. Add a small load resistor, R
BIAS
. In most cases a value of 1kΩ should keep the output voltage below 200mV.
Some experimentation may be needed to determine the appropriate value. In many cases, the feedback
divider may provide a sufficient load.
It is important that V
BIAS
be less than or equal to the steady state output voltage during regulation. If this
condition is not enforced the controller in charge of regulating this rail will be unable to start up. If start up is
forced, damage may result.
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