PTN78000A
www.ti.com
SLTS246A – APRIL 2005 – REVISED MAY 2005
APPLICATION INFORMATION
Adjusting the Output Voltage of the PTN78000A Wide-Output Adjust Power Modules
General
A resistor must be connected directly between the
V
O
Adjust
control (pin 4) and the output
voltage
(pin 7) to set
the output voltage lower than –3 V. The adjustment range is from –15 V to –3 V. If pin 4 is left open, the output
voltage defaults to the highest value, –3 V.
gives the standard resistor value for a number of common voltages, and with the actual output voltage
that the value produces. For other output voltages, the resistor value can either be calculated using the following
formula, or simply selected from the range of values given in
shows the placement of the
required resistor.
R
SET
= 54.9 k
W
´
1.25 V
|
V
O
|
- 3 V
- 5.62 kW
Input Voltage Considerations
The PTN78000A is a buck-boost switching regulator. In order that the output remains in regulation, the input
voltage must not exceed the output by a maximum differential voltage.
For satisfactory performance, the maximum operating input voltage is (32 - |V
O
|) volts.
As an example,
gives the operating input voltage range for the common output bus voltages. In addition,
the Electrical Characteristics define the available output voltage adjust range for various input voltages.
Table 1. Standard Values of R
set
for Common Output
Voltages
V
O
(Required)
–15 V
–12 V
–5 V
–3.3 V
R
SET
(Standard Value)
100
Ω
2 kΩ
28.7 kΩ
221 kΩ
V
O
(Actual)
–14.997 V
–12.006 V
–5.000 V
–3.303 V
Operating
V
I
Range
9 V to 17 V
9 V to 20 V
9 V to 27 V
9 V to 28.7 V
V
I
2
PTN78000A
V
I
Adj
GND
V
O
1
5
GND
+
+
C1
C2
R
SET
0.05 W
1%
4
C3
V
O
GND
(1)
A 0.05-W rated resistor may be used. The tolerance should be 1%, with a temperature stability of 100 ppm/°C (or
better). Place the resistor as close to the regulator as possible. Connect the resistor directly between pins 4 and 1
using dedicated PCB traces.
Never connect capacitors from
V
O
Adjust
to either
GND
or
V
O
. Any capacitance added to the
V
O
Adjust
pin affects the
stability of the regulator.
(2)
Figure 17. V
O
Adjust Resistor Placement
8
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