Datasheet

SLUU166A - December 2003 − Revised February 2004
22
UCC39002 Advanced Load-Share Controller User’s Guide, HPA027A
Determining R
ADJ
I
ADJ(max)
is equal to the maximum current that the internal adjust amplifier can sink:
I
ADJ(max)
+
3V
500 W
I
ADJ(max)
+ 6mA
A resistor, R
ADJ
, is placed between the power module’s Sense+ terminal and the load. The load share
controller’s adjust amplifier will sink current through this resistor proportional to the error amplifier output voltage,
setting up an artificial sense voltage at the module and resulting in the module adjusting its output voltage, and
current, accordingly until the error amplifier output turns off the adjust amplifier. The value of R
ADJ
can be
calculated first by assuming the maximum voltage drop across it can be no greater than the maximum output
voltage adjustment range of the module by the Sense pins, ∆V
OUTADJ(max)
, minus the voltage drop across the
sense resistor:
R
ADJ
+
DV
OUTADJ(max)
* I
OUT(max)
R
SENSE
I
ADJ(max)
R
ADJ
+ 13.3 W
This sets up a voltage at the ADJ pin, V
ADJ
, equal to:
V
ADJ
+ V
OUT
* R
ADJ
I
ADJ
I
ADJ
+ 6mA
V
ADJ
+ 4.92 V
V
ADJ
is not only equal to the voltage at the ADJ pin but it must also be greater than or equal to the error amplifier
output voltage, V
EAO
, by at least 1 V in order to keep the internal transistor from saturating:
V
ADJ
w V
EAO
) 1V
V
EAO
is clamped to a maximum of 3 V and will be equal to the voltage drop across the internal 500-Ω resistor
due to the current sunk by the adjust amplifier at ADJ:
V
EAO
+ I
ADJ
500 W
With V
EAO
clamped to a maximum of 3 V, and V
ADJ
approximately equal to the output voltage (minus a small
voltage drop across the sense resistor), designs with output voltages of 4 V or greater, such as this one, will
easily meet the 1 V greater than V
EAO
requirement for V
ADJ
. However, paralleling modules whose output voltage
is greater than 1 V but less than 4 V may risk saturating the internal adjust amplifier transistor if the R
ADJ
does
not also meet the following requirement:
R
ADJ
w
ǒ
DV
OUTADJ(max)
* I
OUT(max)
R
SENSE
Ǔ
500 W
ƪ
V
OUT
*
ǒ
DV
OUTADJ(max)
* I
OUT(max)
R
SENSE
Ǔ
* 1V
ƫ
R
ADJ
w 10.2 W
As pointed out above, this design has an output voltage of 5 V so ADJ pin is approximately equal to 4.92 V which
naturally exceeds the maximum voltage on the EAO pin, equal to 3 V, by greater than 1 V by using an adjust
resistor that is greater than 10.2 Ω. Utilizing the full range of available adjust amplifier sink current of 6 mA,
without exceeding it, requires using an adjust resistor equal to 13.3 Ω, as calculated earlier.