Datasheet

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SLUS495H − SEPTEMBER 2001 − REVISED AUGUST 2007
15
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DESIGN PROCEDURE
determining R
ADJUST
The Sense+ terminal of the module is connected to the ADJ pin of the load-share controller. By placing a resistor
between this ADJ pin and the load, an artificial Sense+ voltage is created from the voltage drop across R
ADJUST
due to the current sunk by the internal NPN transistor. The voltage at the ADJ pin must be maintained at
approximately 1 V above the voltage at the EAO pin. This is necessary in order to keep the transistor at the
output of the internal adjust amplifier from saturating. To fulfill this requirement, R
ADJUST
is first calculated using
the following equation:
R
ADJUST
w
ǒ
DV
ADJ(max)
* I
OUT(max)
R
SHUNT
Ǔ
500 W
ƪ
V
OUT
* DV
ADJ(max)
* 1V*
ǒ
DV
ADJ(max)
R
SENSE
500 W
Ǔ
ƫ
Where R
SHUNT
is the current sense resistor, and R
SENSE
is the internal resistance between V
OUT+
and SENSE+
within the module.
Also needed for consideration is the actual adjust pin current. The maximum sink current for the ADJ pin,
I
ADJmax
, is 6 mA as determined by the internal 500-Ω emitter resistor and 3-V clamp. The value of adjust resistor,
R
ADJUST
, is based upon the maximum adjustment range of the module, ∆V
ADJmax
. This adjust resistor is
determined using the following formula:
R
ADJUST
w
ƪ
DV
ADJ(max)
* I
OUT(max)
R
SHUNT
ƫ
I
ADJ(max)
*
DV
ADJ(max)
R
SENSE
By selecting a resistor that meets both of these minimum requirements, the ADJ pin will be at least 1 V greater
than the EAO voltage and the adjust pin sink current will not exceed its 6 mA maximum.
(6)
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