PTB48500 Datasheet, PDF (9/14 Pages) Texas Instruments – Dual-Output, 48-V Input Isolated DC/DC Converter for xDSL

Application Notes
PTB48500, PTB48501, & PTB48502
Adjusting the Lower Output Voltage of the
PTB4850x Series of DC/DC Converters
The PTB4850x series of DC/DC converters are de-
signed 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 adjust-
ment 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 calcu-
lated using the appropriate formula (see below), or simply
selected from the range of values given in Table 1-2. The
placement of each resistor is as follows.
Adjust Up: To increase the magnitude of both output volt-
ages, place a resistor R₁ between Vo₂ Adj (pin 9) and the
Vo₂ (pin 6) 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]
=
R_p
·
V_a
(V_a – V_o
)
– R_s
kΩ
(R₂) [Adj Down] =
R_n ·
V_a
(V_o – V_a )
– R_s
kΩ
Where: V_o
V_a
R_p
R_n
R_s
= Magitude of the original output voltage
= Magnitude of the adjusted voltage
= Adjust-up constant from Table 1-1
= Adjust-down constant from Table 1-1
= Internal series resistor from Table 1-1
PTB4850x
+VO
Vo2 ⁶
Vo2 Adj ⁹
R1
Adjust Up
7
COM
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 1-1(b).
Figure 1-1b
Table 1-1
ADJUSTMENT RANGE AND FORMULA PARAMETERS
Part No.
PTB48500(1)A PTB48502A
V_o(nom)
V_a(min)
V_a(max)
R_p (kΩ)
R_n (kΩ)
R_s (kΩ)
1.2 V
0.96 V
1.32 V
1.648
4.624
18.2
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
+VO
Vo2 ⁶
Vo2 Adj ⁹
7
COM
(R2)
Adj Down
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