PTB48500A Datasheet, PDF (11/17 Pages) Texas Instruments – DUAL-OUTPUT, 48-V INPUT ISOLATED DC/DC CONVERTER for xDSL

PTB48500A and PTB48502A are Not Recommended for New Designs _PTB48500
PTB48501
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PTB48502
SLTS218C – SEPTEMBER 2003 – REVISED AUGUST 2006
APPLICATION INFORMATION
ADJUSTING THE LOWER OUTPUT
VOLTAGE OF THE PTB4850x
The PTB4850x series of DC/DC converters are
designed to produce two logic-level supply voltages
for use with the AC-7 ADSL chipset. The magnitude
of lowest output voltage (Vo₂) can be adjusted higher
or lower by up to 10% or —20% of the nominal. The
adjustment method uses a single external resistor.¹
The value of the resistor determines the amount of
adjustment, and its placement determines whether
the voltage is increased or decreased. The resistor
values can be calculated using the appropriate
formula (see Equation 1 and Equation 2), or simply
selected from the range of values given in Table 2.
The placement of each resistor is as follows.
Adjust Up: To increase the magnitude of both
output voltages, place a resistor R₁ between Vo₂ Adj
(pin 9) and the Vo₂ (pin 6) voltage rail; see
Figure 17.
PTB4850x
V_O2 ⁶
V_O2Adj ⁹
+V_O
R₁
Adjust
Up
7
COM
Figure 17. Adjust Up
Adjust Down: To decrease the magnitude of both
output voltages, add a resistor (R₂), between Vo₂ Adj
(pin 9) and the COM (pin 7) voltage rail; see
Figure 18.
CALCULATION OF THE ADJUST RESISTOR
The value of the adjust resistor is calculated using
one of the following equations. Use the equation for
R₁ to adjust up, or (R₂) to adjust down.
R1 [Adjust Up] + Rp
ǒ_Va
Va
*
Vo^Ǔ
*
Rs
kW
(1)
ǒ Ǔ R2 [Adjust Down] + Rn
ǒ_Vo
Va
*
Va^Ǔ
*
Rs
kW
(2)
Where:
V_o = Magitude of the original output voltage
V_a = Magnitude of the adjusted voltage
R_p = Adjust-up constant from Table 1
R_n = Adjust-down constant from Table 1
R_s = Internal series resistor from Table 1
Table 1. Adjustment Range and Formula
Parameters
Part No.
V_o(nom)
V_a(min)
V_a(max)
R_p (kΩ)
R_n (kΩ)
R_s (kΩ)
PTB48500(1)A
1.2 V
0.96 V
1.32 V
1.648
4.624
18.2V
PTB48502A
1.2 V
0.84 V
1.32 V
1.196
3.598
13.0
NOTES:
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 in either the R₁ or (R₂) location, as close
to the converter as possible.
2. Never connect capacitors to the Vo₂ Adj pin.
Capacitance added to this pin can affect the
stability of the regulated output.
PTB4850x
V_O2 ⁶
+V_O
V_O2Adj ⁹
7
COM
(R₂)
Adj Down
Figure 18. Adjust Down
Table 2. Adjust Resistor Values
Part No.
% Adjust
–21
–20
–19
–18
–17
–16
–15
–14
V_a (V)
0.848
0.960
0.972
0.984
0.996
1.008
1.020
1.032
PTB4850xA
R₁ / (R₂) ⁽¹⁾
N/A
(0.3) kΩ
(1.5) kΩ
(2.9) kΩ
(4.4) kΩ
(6.1) kΩ
(8.0) kΩ
(10.2) kΩ
PTB48502A
R₁ / (R₂)⁽¹⁾
(0.5) kΩ
(1.4) kΩ
(2.3) kΩ
(3.4) kΩ
(4.6) kΩ
(5.9) kΩ
(7.4) kΩ
(9.1) kΩ
(1) R₁ =Adjust up, (R₂) =Adjust down
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