PT6644 Datasheet, PDF (3/6 Pages) Texas Instruments – 24W 12V Input Positive to Negative Voltage Converter

Application Notes
PT6640 Series
Adjusting the Output Voltage of the PT6640 24W
Positive to Negative ISR Series
The negative output voltage of the Power Trends
PT6640 series ISRs may be adjusted higher or lower than
the factory trimmed pre-set voltage with the addition of a
single external resistor. Table 1 gives the allowable ad-
justment range for each model in the series as V_a (min)
and V_a (max).
Adjust Up:
An increase in the negative output
voltage is obtained by adding a resistor R2, between pin
14 (V_o adjust) and pins 7-10 (-V_out).
Adjust Down:
Adding a resistor (R1), between pin 14
(V_o adjust) and pins 11-13 (GND), decreases the output
voltage magnitude.
Refer to Figure 1 and Table 2 for both the placement and
value of the required resistor, either (R1) or R2 as appro-
priate.
Figure 1
+V _in
4,5,6
+V in
PT6640
7,8,9,10
V out
C _in
+
5 6 0µF
GND
SNS(+)
11,12,13 1
V _o(adj)
14
R2
|-V_o | Up
(R1)
|-V_o | Down
COM
V _out
L
C _out
O
+ ^{3 3 0µF}
A
D
COM
The values of (R1) [adjust down], and R2 [adjust up], can
also be calculated using the following formulas.
Notes:
1. Use only a single 1% resistor in either the (R1) or R2
location. Place the resistor as close to the ISR as possible.
2. Never connect capacitors from V_o adjust to either GND,
V_out, or the Remote Sense pin. Any capacitance added to
the V_o adjust pin will affect the stability of the ISR.
3. If the Remote Sense feature is being used, connecting the
resistor (R1) between pin 14 (V_o adjust) and pin 1 (Remote
Sense) can benefit load regulation.
4. The maximum allowed input voltage (V_in) will change as
V_out is adjusted. The difference between the input voltage
(V_in) and the output voltage (V_out) must not exceed 30V or
10 × V_out, whichever is less. Use one of the following
formulas to determine the maximum allowed input voltage
for the PT6640.
V_out greater than 2.73V,
V_in(max) = 30 – V_out
Vdc
For example, if V_out = -12V,
V_in(max) = 30 – -12 = 18Vdc
V_out less than 2.73V,
V_in(max) = 10 × V_out
Vdc
(R1)
=
R_o (V_o – 1.25)(V_a – 1.25)
1.25 (V_o – V_a)
– R_s
kΩ
R2
=
R_o (V_o – 1.25)
V_a - V_o
– R_s
kΩ
Where: V_o
V_a
R_o
R_s
= Original V_out (magnitude)
= Adjusted V_out (magnitude)
= The resistance value in Table 1
= The series resistance from Table 1
Table 1
PT6640 ADJUSTMENT AND FORMULA PARAMETERS
Series Pt #
PT6646
PT6641
Vo (nom)
Va (min)
Va (max)
Ro (kΩ)
Rs (kΩ)
–2.5V
–1.8V
–4.3V
4.99
2.49
–3.3V
–2.2V
–4.7V
4.22
4.99
PT6642
–5.0V
–3.0V
–6.5V
2.49
4.99
PT6644
–9.0V
–6.0V
–10.2V
2.0
12.7
PT6643
–12.0V
–9.0V
–13.6V
2.0
12.7
PT6645
–15.0V
–10.0V
–17.0V
2.0
12.7
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