SLTS092B Datasheet, PDF (5/9 Pages) Texas Instruments – 20-W 48-V Input Isolated DC/DC Converter

Application Notes
PT4120/4140 Series
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 regulator¹ 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, R₂ between pin 6 (V_o adjust), and pin
4 (-V_out).
Adjust Down: Add a resistor (R₁), 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, (R₁) or R₂.
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:-
I_o(max) = ²⁰ A, or 5A, whichever is less.
V_a
Where V_a is the adjusted ouput voltage.
3. Use only a single 1% resistor in either the (R₁) or R₂
location. Place the resistor as close to the ISR as possible.
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 (R₁) [adjust down], and R₂ [adjust up], can also
be calculated using the following formulas.
(R₁) =
K_o (V_a – V_r)
V_r (V_o – V_a)
– R_s
kΩ
R₂
=
K_o
(V_a – V_o)
– R_s
kΩ
Where V_o = Original output voltage
V_a = Adjusted output voltage
V_r = Reference voltage (Table 1)
K_o = Multiplier constant (Table 1)
R_s = Internal series resistance (Table 1)
Figure 1
+ V_in
– V _in
3
+V in
5
+V out
2
–Vin
PT4120/40
Remote
On/Off
1
4
–V out
V_o(adj)
6
(R1)
Adj Down
R2
Adj
Up
+V_o
–V_o
Table 1
DC/DC CONVERTER ADJUSTMENT RANGE AND FORMULA PARAMETERS
Series Pt #
48V Bus
PT4126
PT4129
PT4127
PT4128
24V Bus
PT4146
PT4147
PT4148
Max Current 2
5A
5A
5A
5A
Vo(nom)
1.5
1.65
1.8
2.5
Va(min)
1.35
1.49
1.62
2.25
Va(max)
1.65
1.81
1.98
2.75
Vr
Ko (V·kΩ)
1.225
67.07
1.225
63.9
1.225
69.7
1.225
64.2
Rs (kΩ)
43.2
66.5
110.0
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
3.8A
5.2V
4.75
5.75
2.5
134.7
243.0
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
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