APPLICATION INFORMATION
Adjusting the Output Voltage of the PTN78020x Series of Wide-Output Adjust Power Modules
General
R
=54.9k
SET
W
1.25 V
V
-V
O
min
-R
P
(1)
Input Voltage Considerations
SLTS228B – DECEMBER 2004 – REVISED APRIL 2008 .................................................................................................................................................. www.ti.com
A resistor must be connected between the VO Adjust control (pin 4) and GND (pin 7) to set the output voltage.
The adjustment range is from 2.5 V to 12.6 V for PTN78020W. The adjustment range is from 11.85 V to 22 V for
PTN78020H. If pin 4 is left open, the output voltage defaults to the lowest value.
Table 2 gives the preferred value of the external resistor for a number of standard voltages, with the actual
output voltage that the value provides. For other output voltages, the value of the required resistor can either be
placement of the required resistor.
Table 1. RSET Formula Constants
PRODUCT
VMIN
RP
PTN780x0W
2.5 V
6.49 k
PTN780x0H
11.824 V
6.65 k
The PTN78020 is a step-down switching regulator. In order that the output remains in regulation, the input
voltage must exceed the output by a minimum differential voltage.
Another consideration is the pulse width modulation (PWM) range of the regulator's internal control circuit. For
stable operation, its operating duty cycle should not be lower than some minimum percentage. This defines the
maximum advisable ratio between the regulator input and output voltage magnitudes.
As an example, for satisfactory performance, the operating input voltage range of the PTN78020x must adhere to
the following requirements.
1. For PTN78020W output voltages lower than 10 V, the minimum input voltage is (VO + 2 V ) or 7 V, whichever
is higher.
2. For PTN78020W output voltages equal to 10 V and higher, the minimum input voltage is (VO + 2.5 V ) .
3. For PTN78020W, the maximum input voltage is (10
× V
O ) or 36 V, whichever is less.
4. For PTN78020H output voltages lower than 19 V, the minimum input voltage is (VO + 3 V ) or 15 V,
whichever is higher.
5. For PTN78020H output voltages equal to 19 V and higher, the minimum input voltage is (VO + 4 V ) .
Table 2 gives the operating input voltage range for the common output bus voltages. In addition, the Electrical
Characteristics table defines the available output voltage adjust range for various input voltages.
Table 2. Standard Values of Rset for Common Output Voltages
VO
RSET
VO
Operating
PRODUCT
(Required)
(Standard Value)
(Actual)
VI Range
2.5 V
Open
2.5 V
7 V to 25 V
3.3 V
78.7 k
3.306 V
7 V to 33 V
PTN780x0W
5 V
21 k
4.996 V
7 V to 36 V
12 V
732
12.002 V
14.5 V to 36 V
12 V
383 k
12.000 V
15 V to 36 V
15 V
15 k
14.994 V
18 V to 36 V
PTN780x0H
18 V
4.42 k
18.023 V
21 V to 36 V
22 V
95.3
21.998 V
26 V to 36 V
10
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