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ADM1041
CURRENT SHARE
The current share method is the master–slave type, which
means that the power supply with the highest output current
automatically becomes the master and controls the share bus
signal. All other power supplies become slaves, and the share
bus signal causes them to increase their output voltages slightly
until their output currents are almost equal to that of the master.
This scheme has two major advantages. A failed master power
supply simply allows one of the slaves to become the new
master. A short circuited share signal disables current sharing,
but all power supplies default to their normal voltage setting,
allowing a certain degree of passive sharing. Because this chip
uses a low voltage process, an external bidirectional amplifier is
needed for most existing share bus signal levels. The voltage
between Pins 20 and 23 is always controlled to 2.0 V full scale,
ignoring any offset. By connecting Pins 20 and 23 together, the
chip can produce a 2.0 V share signal directly without any exter-
nal circuits. To improve accuracy, the share signal is referenced
to remote voltage sense negative.
CURRENT SHARE OFFSET
To satisfy some customer specifications, the current share signal
can be offset by a fixed amount by using a trimmable current
generator and a series resistor. The offset is added on top of the
2.0 V full-scale current share output signal. See Figure 14.
I SHARE DRIVE AMPLIFIER
This amplifier is a buffer with enough current source capability
to drive the current share circuits of several slave power supplies.
It has negligible current sink capability. Refer to the Differential
Sense Amplifier section that follows.
DIFFERENTIAL SENSE AMPLIFIER
This amplifier has unity gain and senses the difference between
the share bus voltage and the remote voltage sense negative pin.
When the power supply is the master, it forms a closed loop
with the I SHARE drive amplifier described above, and therefore it
causes the share bus voltage between Pins 20 and 23 to equal the
current share signal at the noninverting input of the I SHARE drive
amplifier. When the power supply is a slave, the output of the
differential sense amplifier exceeds the internal current share
signal, which causes the I SHARE drive amplifier to be driven into
cutoff. Because it is not possible to trim out negative offsets in
the op amps in the current share chain, a 50 mV voltage source
is used to provide a known fixed positive offset. The share bus
I SHARE ERROR AMPLIFIER
This is a low gain transconductance amplifier that measures the
difference between the internal current share voltage and the
signal voltage on the external share bus. If two power supplies
have almost identical current share signals, a 50 mV voltage
source on the inverting input helps arbitrate which power
supply becomes the master and prevents “hunting” between
master and slave roles. The amplifier requires only the output
pin for loop compensation, which typically consists of a series
RC network to common. When the power supply is a slave, the
output of the error amplifier comes out of saturation and begins
to drive a controlled current sink. The control threshold is
nominally 1.0 V. This current flows from a resistor in series with
the trimmed voltage loop signal and thereby attempts to decrease
the voltage signal below the 1.5 V reference for that loop. The
closed voltage loop reacts by increasing the power supply’s
output voltage until current share is achieved. The maximum
current sink is limited so that the power supply voltage can be
increased only a small amount, which is usually limited to be
within the customer’s specified voltage regulation limit. This
small voltage increase also limits the control range of the current
share circuit and is called the capture range. The capture range
may be set via the SMBus to one of four values, from 1% to 4%
nominal. See Figure 14.
I SHARE CLAMP
This clamp keeps the current share-loop compensation capacitor
discharged when the current share is not required to operate.
The clamp is released during power-up when the voltage refer-
ence and therefore the output voltage of the power supply has
risen to either 75% or 88% of its final value. This is configurable
via the SMBus. When the clamp is released, the current share
loop slowly “walks in” the current share and helps to avoid
output voltage spikes during hot swapping. See Figure 14.
Share_OK DETECTOR
Incorrect current sharing is a useful early indicator that there is
some sort of non-catastrophic problem with one of the power
supplies in a parallel system. Two comparators are used to detect
an excessive positive or negative error voltage at the input of the
I SHARE error amplifier, which indicates that the current share
loop has lost control. One of four possible error levels must be
configured via the SMBus. See Figure 14.
offset controlled current source must be trimmed via the
SMBus to take out the resulting overall offset. See Figure 14.
Rev. A | Page 24 of 64
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