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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 PT4120/
PT4140 Series of Isolated DC/DC Converters
The factory pre-set output voltage of Power Trends’
PT4120 and PT4140 series of isolated DC/DC converters
may be adjusted within ±10% of nominal. Adjustment is
made from the secondary side of the regulator
1
with a
single external resistor. For the input voltage range
specified in the data sheet Table 1 gives the allowable
adjustment range for each model, as V
o
(min) and V
o
(max).
Adjust Up:
An increase in the output voltage is obtained
by adding a resistor, R
2
between pin 6 (V
o
adjust), and pin
4 (-V
out
).
Adjust Down:
Add a resistor
(R
1
)
, between pin 6 (V
o
ad-
just) and pin 5 (+V
out
).
Refer to Figure 1 and Table 2 for both the placement
and value of the required resistor,
(R
1
)
or R
2
.
Notes:
1. The PT4120 and PT4140 series of DC/DC converters
incorporate isolation between the V
in
and V
o
terminals.
Adjustment of the output voltage is made to the
regulation circuit on the secondary or output side of
the converter.
2. The maximum rated output power for this series is
20W. An increase in the output voltage may therefore
require a corresponding reduction in the maximum
output current (
see Table 1
). The revised maximum
output current must be determined as follows:-
=20 A,
or 5A, whichever is less.
V
a
I
o
(max)
Where V
a
is the adjusted ouput voltage.
3. Use only a single 1% resistor in either the
(R
1
)
or R
2
location. Place the resistor as close to the ISR as possible.
Table 1
DC/DC CONVERTER ADJUSTMENT RANGE AND FORMULA PARAMETERS
Series Pt #
48V Bus
PT4126
PT4129
24V Bus
PT4146
Max Current
2
5A
5A
V
o
(nom)
1.5
1.65
Va(min)
1.35
1.49
Va(max)
1.65
1.81
V
r
1.225
1.225
K
o
(V·k
)
67.07
63.9
R
s
(k
)
43.2
66.5
PT4127
PT4147
5A
1.8
1.62
1.98
1.225
69.7
110.0
PT4128
PT4148
5A
2.5
2.25
2.75
1.225
64.2
187.0
PT4121
PT4141
5A
3.3
2.95
3.65
1.225
69.3
187.0
PT4122
PT4142
4A
5.0
4.5
5.5
2.5
125.2
187.0
PT4125
PT4123
PT4143
1.6A
12.0
10.8
13.2
2.5
139.8
110.0
PT4124
PT4144
1.3A
15.0
13.5
16.5
2.5
137.6
90.9
3.8A
5.2V
4.75
5.75
2.5
134.7
243.0
Figure 1
(R1)
Adj Down
R2
Adj
Up
+V
in
–
V
in
+V
o
–
V
o
PT4120/40
+Vout
–
Vout
+Vin
–
Vin
Remote
On/Off
V
o
(adj)
1
2
3
5
4
6
4. Never connect capacitors to V
o
adjust. Any capacitance
added to the V
o
adjust control pin will affect the stability of
the ISR.
The values of
(R
1
)
[adjust down], and R
2
[adjust up], can also
be calculated using the following formulas.
(R
1
)
=
K
o
(V
a
– V
r
)
V
r
(V
o
– V
a
)
– R
s
k
R
2
=
K
o
(V
a
– V
o
)
– R
s
k
Where V
o
= Original output voltage
= Adjusted output voltage
= Reference voltage (Table 1)
K
o
= Multiplier constant (Table 1)
R
s
= Internal series resistance (Table 1)
V
a
V
r
PT4120/4140 Series