SLUS495H − SEPTEMBER 2001 − REVISED AUGUST 2007
UCC29002
UCC29002/1
UCC39002
FUNCTIONAL DESCRIPTION
differential current sense amplifier (CS+, CS−, CSO)
The UCC39002 features a high-gain and high-precision amplifier to measure the voltage across a low-value
current sense resistor. Since the amplifier is fully uncommitted, the current sense gain is user programmable.
The extremely low input offset voltage of the UCC39002 current sense amplifier makes it suitable to measure
current information across a low value sense resistor. Furthermore, the input common mode range includes
ground and the positive supply rail of the UCC39002 (V
DD
). Accordingly, the current sense resistor can be
placed in the ground return path or in the positive output rail of the power supply V
O
as long as V
O
≤
V
DD
. The
current sense amplifier is not unity gain stable and must have a minimum gain of three.
load share bus driver amplifier (CSO)
This is a unity-gain buffer amplifier to provide separation between the load share bus voltage and the output
of the current sense amplifier. The circuit implements an ideal diode with virtually 0 V forward voltage drop by
placing the diode inside the feedback loop of the amplifier. The diode function is used to automatically establish
the role of the master module in the system. The UCC39002 which is assigned to be the master uses the load
share bus driver amplifier to copy its output current information on to the load share bus.
All slave units, with lower output current levels by definition, have this “ideal diode” reversed biased
(V
CSO
< V
LS
). Consequently, the V
CSO
and V
LS
signals will be separated. That allows the error amplifier of the
UCC39002 to compare its respective module’s output current to the master module’s output current and make
the necessary corrections to achieve a balanced current distribution.
Since the bus is always driven by a single load share bus driver amplifier, the number of modules (n) are limited
by the output current capability of the amplifier according to:
n
+
100 kW
I
OUT,MIN
(1)
V
LS,FULL_SCALE
where 100 kΩ is the input impedance of the LS pin as shown in the block diagram, I
OUT,MIN
is given in the data
sheet and V
LS,FULL_SCALE
is the maximum voltage on the load share bus at full load.
Note that the number of parallel units can be increased by reducing the full scale bus voltage, i.e. by reducing
the current sense gain.
load share bus receiver amplifier (LS)
The load share bus receiver amplifier is a unity gain buffer monitoring the load share bus voltage. Its primary
purpose is to ensure that the load share bus is not loaded by the internal impedances of the UCC39002. The
LS pin is already internally compensated and has an internal 15-kHz filter. Adding external capacitance,
including stray capacitance, should be avoided to maintain stability.
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