Datasheet

APPLICATION INFORMATION
Adjusting the Output Voltage of the PTN78000 Wide-Output Adjust Power Modules
General
R =54.9k
SET
W ´
1.25 V
V -V
O min
-R
P
(1)
Input Voltage Considerations
PTN78000W , PTN78000H
SLTS230C – NOVEMBER 2004 – REVISED SEPTEMBER 2008 ...................................................................................................................................... www.ti.com
A resistor must be connected between the V
O
Adjust control (pin 4) and GND (pin 1) to set the output voltage.
The adjustment range is from 2.5 V to 12.6 V for PTN78000W. The adjustment range is from 11.85 V to 22 V for
PTN78000H. 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 several standard voltages, with the actual output
voltage that the value provides. For other output voltages, the value of the required resistor can be calculated
using Equation 1 , and the constants for the applicable product in Table 1 . Alternatilvey, R
SET
can be simply
selected from the range of values given in Table 3 . Figure 27 shows the placement of the required resistor.
Table 1. R
SET
Formula Constants
PRODUCT V
MIN
(V) R
P
(k Ω )
PTN780x0W 2.5 6.49
PTN780x0H 11.824 6.65
The PTN78000 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. (Please refer to the input voltage range
requirements in the electrical characteristics table.)
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 PTN78000x must adhere to
the following requirements.
1. For PTN78000W output voltages lower than 10 V, the minimum input voltage is (V
O
+ 2 V ) or 7 V, whichever
is higher.
2. For PTN78000W output voltages equal to 10 V and higher, the minimum input voltage is (V
O
+ 2.5 V ) .
3. For PTN78000W, the maximum input voltage is (10 x V
O
) or 36 V, whichever is less.
4. For PTN78000H output voltages lower than 19 V, the minimum input voltage is (V
O
+ 3 V ) or 15 V,
whichever is higher.
5. For PTN78000H output voltages equal to 19 V and higher, the minimum input voltage is (V
O
+ 4 V ) .
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