Datasheet

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SLUS495H − SEPTEMBER 2001 − REVISED AUGUST 2007
14
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DESIGN PROCEDURE
the shunt resistor
Selection of the shunt resistor is limited by its voltage drop at maximum module output current. This voltage drop
should be much less than the voltage adjustment range of the module:
I
OUT(max)
R
SHUNT
tt D V
ADJ(max)
Other limitations for the sense resistor are the desired minimum power dissipation and available component
ratings.
the CSA gain
The gain of the current sense amplifier is configured by the compensation components between Pin 1, CS−,
and Pin 8, CSO, of the load share device. The voltage at the CSO pin is limited by the saturation voltage of the
internal current sense amplifier and must be at least two volts less than VDD:
V
CSO(max)
t VDD * 2V
The maximum current sense amplifier gain is equal to:
A
CSA
+
V
CSO
ǒ
R
SHUNT
I
OUT(max)
Ǔ
Referring to Figure 5, the gain is equal to R16/R15 and a high-frequency pole, configured with C13, is used for
noise filtering. This impedance is mirrored at the CS+ pin of the differential amplifier as shown.
The current sense amplifier output voltage, V
CSO
, serves as the input to the unity gain LS bus driver. The module
with the highest output voltage forward biases the internal diode at the output of the LS bus driver and determine
the voltage on the load share bus, V
LS
. The other modules act as slaves and represent a load on the I
VDD
of
the module due to the internal 100-kΩ resistor at the LS pin. This increase in supply current for the master
module is equal to N(V
LS
/100 kΩ).
(3)
(4)
(5)