
Application Notes
For technical support and more information, see inside back cover or visit www.ti.com
Adjusting the Output Voltage of the PT4840
Dual Output Voltage DC/DC Converters
The Vo2 and Vo3 output voltages from the PT4840 se-
ries of DC/DC converters can be independantly adjusted
by up to ±10% from their factory pre-set voltage. The
method of adjustment uses a single external resistor. 1
The value of the resistor determines the magnitude of
adustment, and the placement of the resistor determines
the direction of adjustment (up or down). The resistor
value can be calculated directly from a simple formula.
The constants are given in Table 1. Alternatively, Table 2
provides the resistor values for a select number of output
voltages. The placement of each resistor is as follows.
Vo2 Adjust Up:
To increase the output, add a resistor R1
between pin 17 (Vo2 Adjust) and pin 18 (COM).
Vo2 Adjust Down: Add a resistor (R2), between pin 17
(Vo2 Adjust) and pin 16 (+Vo1).
Vo3 Adjust Up:
Add a resistor R3 between pin 21
(Vo3 Adjust) and pins 20 (COM).
Vo3 Adjust Down: Add a resistor (R4) between pin 21
(Vo3 Adjust) and pin 22 (+Vo3).
Refer to Figure 1 for further information on resistor
placement.
Figure 1
Notes:
1. Use only a single 1% resistor in either the R1 or (R2)
location to adjust Vo2, and in the R3 or (R4) location to
adjust Vo3. Place the resistor as close to the ISR as
possible.
2. Never connect capacitors to either the Vo2 Adjust or
Vo3 Adjust pins. Any capacitance added to these control
pins will affect the stability of the respective regulated
output.
The adjust resistor values may be calculated. Use the
applicable formula and select the appropriate constants
from Table 1 for the output and model being adjusted.
R1 or R3
=
Ro Vr
– Rs
k
Va – Vo
(R2) or (R4)
=
Ro (Va – Vr )
– Rs
k
Vo – Va
Where: Vo = Original output voltage, (Vo2 or Vo3)
Va = Adjusted output voltage
Vr = The reference voltage from Table 1
Ro = The resistance value in Table 1
Rs = The series resistance from Table 1
PT4840 Series
+V
IN
– V
IN
+Vo
1
+Vo
2
+Vo
3
PT4840
1
2
11
15, 16
22, 23
3
Vo
1
COM
Vo
2
Vo
3
+V
IN
-V
IN
EN 1
Vo
2 ad j
Vo
3 ad j
17
21
C
OUT 1
C
OUT 3
C
OUT 2
C
IN
4
EN 2
( R
4 )
Ad j Down
R
3
Ad j ust Up
( R
2 )
Ad j Down
R
1
Ad j ust Up
12, 13
18 – 20
COM