UC1907
UC2907
UC3907
PIN/BLOCK DESCRIPTIONS (cont.)
external resistor tolerances. The Adjust Amplifier has a
built-in 50mV offset on its inverting input which will force
the unit acting as the master to have a low output result-
ing in no change to the reference. While this 50mV offset
represents an error in current sharing, the gain of the
current amplifier reduces it to only 2.5mV across the cur-
rent sense resistor. It should also be noted that when the
module is acting independently with no connection to the
Share Bus node, or when the Share Bus node is shorted
to ground, its reference voltage will be unchanged. Since
only the circuit acting as a master will have a low output
from the Adjust Amplifier, this signal is used to activate a
flag output to identify the master should some corrective
action be needed.
BUFFER AMPLIFIER (Pins 1, 15) - This amplifier is a
uni-directional buffer which drives the CURRENT SHARE
BUS - the line which will interconnect all power modules
paralleled for current sharing. Since the Buffer Amplifier
will only source current, it insures that the module with
the highest output current will be the master and drive
the bus with a low-impedance drive capability. All other
Buffer Amplifiers will be inactive with each exhibiting a
10kohm load impedance to ground. The Share Bus ter-
minal is protected against both shorts to ground and ac-
cidental voltages in excess of 50 Volts.
ADJUST AMPLIFIER (Pins 13, 14, 15) - This amplifier
adjusts the individual module’s reference voltage to
maintain equal current sharing. It is a transconductance
type in order that its bandwidth may be limited, and noise
kept out of the reference adjust circuitry, with a simple
capacitor to ground. The function of this amplifier is to
compare its own module output current to the Share Bus
signal - which represents the highest output current - and
force an adjust command which is capable of increasing
the reference voltage as seen by the voltage amplifier by
as much as 100 millivolts. This number stems from the
17.5:1 internal resistor ratio between the Adjust Ampli-
fier’s clamped output and the reference, and represents a
5% total range of adjustment - a value which should be
adequate to compensate for unit-to-unit reference and
STATUS INDICATE (Pin 16) - This pin is an open collec-
tor output intended to indicate the unit which is acting as
the master. It achieves this by sensing when the adjust
amp is in its low state and pulling the status indicate pin
low.
ADDITIONAL INFORMATION
Please refer to additional application information.
[1] Application Note U-129, UC3907 Load Share IC Simplifies
Parallel Power Supply Design by Mark Jordan.
[2] Application Note U-163, UC3902 Load Share Controller and
its Performance in Distributed Power Systems by Laszlo
Balogh.
UDG-94103
Figure 1. Load system diagram.
5
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