Datasheet
APPLICATION INFORMATION
ADJUSTING THE OUTPUT VOLTAGE
SET
O
0.7
R 8.25 (k )
V 0.7
0.7
30.1 10
æ ö
ç ÷
ç ÷
= - W
ç ÷
-
æ ö
-
ç ÷
ç ÷
è ø
è ø
(1)
+Sense
V
O
-Sense
GND
V
O
Adj
PTH08T255W
10
17
11
16
13 4 181298
V
O
GND GND GND AGND
R
SET
1%
0.05 W
C
O
+
+Sense
V
O
-Sense
GND
UDG-08145
PTH08T255W
SLTS290 – NOVEMBER 2008 ...........................................................................................................................................................................................
www.ti.com
The V
o
Adjust control sets the output voltage of the PTH08T255W. The adjustment range of the PTH08T255W is
3.0 V to 5.25 V. The adjustment method requires the addition of a single external resistor, R
SET
, that must be
connected directly between pins V
o
Adjust (pin 18) and AGND (pin 4). Figure 6 shows the placement of the
required resistor.
To set the output voltage, the value of the required resistor can either be calculated using the following formula,
or simply selected from the range of standard resistor values given in Table 2 .
Table 2. Output Voltage Set-Point Resistor Values
V
O
Required (V) R
SET
(k Ω ) V
O
Required (V) R
SET
(k Ω )
3.0 100 4.2 6.81
3.1 63.4 4.3 5.90
3.2 45.3 4.4 4.99
3.3 34.0 4.5 4.22
3.4 27.4 4.6 3.48
3.5 22.1 4.7 2.87
3.6 18.2 4.8
(3)
2.32
3.7 15.4 4.9
(3)
1.82
3.8 13.0 5.0
(3)
1.37
3.9 11.0 5.1
(3)
0.931
4.0 9.31 5.2
(3)
0.549
4.1 8.06 5.25
(3)
0.374
(1) R
SET
: Use a 0.05-W resistor with a tolerance of 1% and temperature stability of 100 ppm/ ° C (or better). Connect the
resistor directly between pins 18 and 4, as close to the regulator as possible, using dedicated PCB traces.
(2) Any capacitance added to the V
O
Adjust pin affects the stability of the regulator.
(3) When setting the output voltage to 4.8V or greater, the Auto-Track pin must be directly connected to V
I
if the feature
is not used. If the Auto-Track feature is utilized, a 100k Ω resistor must connect the Auto-Track pin to V
I
.
Figure 6. V
O
Adjust Resistor Placement
8 Copyright © 2008, Texas Instruments Incorporated
Product Folder Link(s): PTH08T255W