Datasheet

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APPLICATION INFORMATION
Adjusting the Output Voltage of the PTN78020A Wide-Output Adjust Power Modules
General
R = 54.9 k
SET
W ´
1.25 V
| |V - 3 V
O
- 5.62 kW
Input Voltage Considerations
GND GND
PTN78020A
6
1, 72
Adj
GND
V
O
V
I
V
I
V
O
4
+
C2C1
R
SET
0.05 W
1%
C3
+
PTN78020A
SLTS244A – APRIL 2005 – REVISED MAY 2005
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.
Table 1 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 Table 2 . Figure 17 shows the placement of the
required resistor.
The PTN78020A 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.
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's input and output voltage magnitudes.
For satisfactory performance, the maximum operating input voltage range must be equal to (32 - |V
O
|) volts.
As an example, Table 1 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
R
SET
V
O
Operating
(Required) (Standard Value) (Actual) V
I
Range
–15 V 100 Ω –14.997 V 9 V to 17 V
–12 V 2 k Ω –12.006 V 9 V to 20 V
–5 V 28.7 k Ω –5.000 V 9 V to 27 V
–3.3 V 221 k Ω –3.303 V 9 V to 28.7 V
(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.
(2) Never connect capacitors from V
O
Adjust to either GND or V
out
. Any capacitance added to the V
O
Adjust pin affects
the stability of the regulator.
Figure 17. V
O
Adjust Resistor Placement
8