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Application Notes
For technical support and more information, see inside back cover or visit www.ti.com
Adjusting the Output Voltage of the PT4660 and
PT4680 Dual Output Voltage DC/DC Converters
The output voltages Vo
1
and Vo
2
from the PT4680
(24V Bus) and PT4660 (48V Bus) series of DC/DC
converters can be independantly adjusted higher or
lower than the factory trimmed pre-set voltage by up to
±10%. The adjustment requires the addition of a single
external resistor
1
. Table 1 gives the adjustment range of
Vo
1
and Vo
2
for each model in the series as V
a
(min) and
V
a
(max).
Vo
1
Adjust Down:
Add a resistor
(R
1
)
, between pin 13
(V
1
Adj) and pin 12 (Vo
1
)
2
.
Vo
1
Adjust Up:
To increase the output, add a resistor R
2
between pin 13 (V
1
Adj) and pin 14 (COM)
2, 4
.
Vo
2
Adjust Down:
Add a resistor
(R
3
)
between pin 20
(V
2
Adj) and pin 21 (Vo
2
)
2
.
Vo
2
Adjust Up:
Add a resistor R
4
between pin 20
(V
2
Adj) and pins 19 (COM)
2, 4
.
Refer to Figure 1 and Table 2 for both the placement and value
of the required resistor.
Notes:
1. Adjust resistors are not required if Vo
1
and Vo
2
are to
remain at their respective nominal set-point voltage.
In this case, V
1
Adj (pin 13) and V
2
Adj (pin 20) are
left open-circuit
2. Use only a single 1% resistor in either the
(R
1
)
or R
2
location to adjust Vo
1
, and in the
(R
3
)
or R
4
location
to adjust Vo
2
. Place the resistor as close to the DC/
DC/DC converter as possible.
Figure 1
3. Vo
2
must always be at least 0.3V lower than Vo
1
.
4.
The over-voltage protection
threshold is fixed, and is set
nominally 25% above the set-point output voltage.
Adjusting Vo
1
or Vo
2
higher will reduce the voltage
margin between the respective steady-state output
voltage and its over-voltage (OV) protection threshold.
This could make the module sensitive to OV fault
detection, as a result of random noise and load
transients.
Note: An OV fault is a latched condition that shuts down
both outputs of the converter. The fault can only be cleared
by cycling one of the Enable control pins (EN
1
* / EN
2
), or
by momentarily removing the input power to the module.
5. Never connect capacitors to either the Vo
1
Adjust or
Vo
2
Adjust pins. Any capacitance added to these
control pins will affect the stability of the respective
regulated output.
The adjust up and adjust down resistor values can also be
calculated using the following formulas. Be sure to select
the correct formula parameter from Table 1 for the out-
put and model being adjusted.
(R
1
)
or
(R
3
)
=
K
o
(V
a
– V
r
)
V
r
(V
o
– V
a
)
– R
s
k
R
2
or R
4
=
K
o
V
a
– V
o
– R
s
k
Where: V
o
= Original output voltage, (Vo
1
or Vo
2
)
= Adjusted output voltage
= The reference voltage from Table 1
K
o
= The multiplier constant in Table 1
R
s
= The series resistance from Table 1
V
a
V
r
PT4660 & PT4680 Series
PT4660/80
+Vin
EN 1*
EN 2
–Vin
Vo
1
adj
Vo
2
adj
Vo
1
Vo
2
COM
9–12
21–24
14–19
L
O
A
D
L
O
A
D
Vo
1
Vo
2
+V
IN
–V
IN
20
13
1
4
3
2
COM
* Inverted logic
R2
(R1)
(R3)
R4