Datasheet

UC2902
UC3902
SLUS232E − DECEMBER 19, 2002 − REVISED JULY 2011
6
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APPLICATION INFORMATION
The values of five passive components must be determined to configure the UC3902 load share controller. The
output and return lines of each converter are connected together at the load, with current sense resistor R
SENSE
inserted in each negative return line. Another resistor, R
ADJ
, is also inserted in each positive remote sense line.
The differential share bus terminals (SHARE+ and SHARE−) of each UC3902 are connected together
respectively, and the SHARE− node is also connected to the system ground. A typical application is illustrated
in Figure 1.
The load share controller design can be executed by following the next few steps:
Step 1.
R
SENSE
+
V
SHARE(max)
A
CSA
I
O(max)
D where A
CSA
is 40, the gain of the current sense amplifier
At full load, the voltage drop across the R
SENSE
resistor is I
O(max)
× R
SENSE
. Taking into account the gain of the
current sense amplifier, the voltage at full load on the current share bus,
V
SHARE(max)
+
A
CSA
I
O(max)
R
SENSE
This voltage must stay 1.5-V below V
CC
or below 10 V whichever is smaller. V
SHARE
represents an upper limit
but the designer should select the full scale share bus voltage keeping in mind that every volt on the load share
bus increases the master controller’s supply current by approximately 100 μA times the number of slave units
connected parallel.
Step 2.
R
G
+
V
ADJ(max)
I
ADJ(max)
Care must be taken to ensure that I
ADJ(max)
is low enough so that both the drive current and power dissipation
are within the device’s capability. For most applications, an I
ADJ(max)
current between 5 mA and 10 mA is
acceptable. In a typical application, a 360-Ω R
G
resistor from the ADJR pin to ground sets I
ADJ(max)
to
approximately 5 mA.
Step 3.
R
ADJ
+
DV
O(max)
*
ǒ
I
O(max)
R
SENSE
Ǔ
I
ADJ(max)
R
ADJ
must be low enough to not affect the normal operation of the converter’s voltage feedback loop. Typical
R
ADJ
values are between 20 Ω to100 Ω depending on V
O
, ΔV
O(max)
and the selected I
ADJ(max)
value.
Step 4.
C
C
+
g
M
2p f
C
R
ADJ
R
G
R
SENSE
R
LOAD
A
CSA
A
PWR
ǒ
f
C
Ǔ
(1)
(2)
(3)
(4)
(5)