PTB48510 Datasheet, PDF (9/13 Pages) Texas Instruments – Dual Complementary-Output DC/DC Converter for DSL

Application Notes
PTB48510 & PTB48511
Adjusting the Output Voltages of the
PTB4851x Series of DC/DC Converters
The PTB48510 & PTB48511 DC/DC converters produce
a balanced pair of complimentary output voltages. They
are identified +V_OUT and -V_OUT, respectively. The mag-
nitude of both output voltages can be adjusted together
as a pair, higher or lower, by up to ±10% of their nominal.
The adjustment method uses a single external resistor. ¹
The value of the resistor determines the adjustment
magnitude, and its placement determines whether the
magnitude is increased or decreased. The resistor values
can be calculated using the appropriate formula (see
below). The formula constants are given in Table 1-1.
The placement of each resistor is as follows.
Adjust Up: ³ To increase the magnitude of both output
voltages, place a resistor R₁ between ±Vo₁ Adj (pin 7) and
the -V_OUT (pin 8) voltage rail; see Figure 1-1(a).
Figure 1-1a
Calculation of Resistor Adjust Values
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]
=
V_r R_o
2 (V_a – V_o )
– R_s kΩ
(R₂) [Adjust Down]
=
R_o (2 V_a – V_r )
2 (V_o – V_a )
– R_s
kΩ
Where: V_o
V_a
V_r
R_o
R_s
= Magitude of the original ±V_OUT
= Magnitude of the adjusted voltage
= The reference voltage from Table 1-1
= The resistance value in Table 1-1
= The series resistance from Table 1-1
PTB48510
+V_OUT ⁵
+V_OUT
±V_o Adj ⁷
6
COM
8
–V_OUT
R₁
Adjust Up
–V_OUT
Table 1-1
ADJUSTMENT RANGE AND FORMULA PARAMETERS
Series Pt. No.
PTB4851xA
PTB4851xB
Vo(nom)
Va(min)
Va(max)
Vr
Ro (kΩ)
Rs (kΩ)
5V
3.5 V
5.5 V
2.495 V
7.5
9.09
12 V
6.5 V
13.4 V
2.495 V
18.2
16.9
PTB48510C
15 V
7.2 V
16.7 V
2.495 V
22.1
16.9
Adjust Down: To decrease the magnitude of both output
voltages, add a resistor (R₂), between Vo Adj (pin 7) and
the +V_OUT (pin 5) voltage rail; see Figure 1-1(b).
Figure 1-1b
PTB48510
+V_OUT ⁵
±V_o Adj ⁷
6
COM
(R₂)
Adj Down
+V_OUT
8
–V_OUT
–V_OUT
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 ±Vo Adj pin. Capacitance
added to this pin can affect the stability of the regulated
output.
3. The over-voltage protection (PTB48511x) is nominally
set to 25% above the original output voltage set-point.
Increasing the magnitude of the output voltages reduces
the margin between the output voltage and the over-
voltage (OV) protection threshold. This could make the
module more sensitive to OV faults, as a result of
random noise and load transients.
Note: An OV fault is a latched condition that shuts down the
converter’s outputs. The fault can only be cleared by cycling the
Enable pin, or by momentarily removing input power to the
module.
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