UC1902
UC2902
UC3902
PRELIMINARY
Load Share Controller
FEATURES
DESCRIPTION
• 2.7V to 20V Operation
The UC3902 load share controller is an 8-pin device that balances the cur-
rent drawn from independent, paralleled power supplies. Load sharing is
accomplished by adjusting each supply’s output current to a level propor-
tional to the voltage on a share bus.
• 8-Pin Package
• Requires Minimum Number of
External Components
The master power supply, which is automatically designated as the supply
that regulates to the highest voltage, drives the share bus with a voltage
• Compatible with Existing Power
Supply Designs Incorporating Remote proportional to its output current. The UC3902 trims the output voltage of
Output Voltage Sensing
the other paralleled supplies so that they each support their share of the
load current. Typically, each supply is designed for the same current level
although that is not necessary for use with the UC3902. By appropriately
scaling the current sense resistor, supplies with different output current ca-
pability can be paralleled with each supply providing the same percentage
of their output current capability for a particular load.
• Differential Share Bus
• Precision Current Sense Amplifier with
Gain of 40
• UVLO (Undervoltage Lockout)
Circuitry
A differential line is used for the share bus to maximize noise immunity and
accommodate different voltage drops in each power converter’s ground re-
turn line. Trimming of each converter’s output voltage is accomplished by
injecting a small current into the output voltage sense line, which requires a
small resistance (typically 20Ω – 100Ω) to be inserted.
• User Programmable Share Loop
Compensation
BLOCK DIAGRAM
LOAD SHARE CONTROLLER
GND
1
8
VCC
BIAS
UVLO
–
SHARE DRIVE AMPLIFIER
+
40R
R
SENSE
2
–
+
7
6
SHARE+
SHARE–
+
CURRENT SENSE
AMPLIFIER
SHARE SENSE AMPLIFIER
–
ERROR AMPLIFIER
+
ADJ
3
4
+
35mV
–
0.6V
ADJ AMPLIFIER
+
+
5
COMP
–
2.3V
ADJR
UDG-99042
SLUS232 - FEBRUARY 1999