Xilinx Virtex-4 ML455, User Manual

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22 www.xilinx.com Virtex-4 ML455 PCI/PCI-X Board
UG084 (v1.0) May 17, 2005
Chapter 3: Hardware Description
R
V
OUT
= 1.21V (1 + (R2/R1)) + (I
ADJ
)(R2)
Where I
ADJ
= 3
µ
A at T
J
= 25 °C where T
J
= junction temperature, R1 < 4.17 K
Ω
, and the
output range is from 1.21V to 20V.
Choosing a 169
Ω
, standard 1% resistor for R1, the formula can be re-arranged to:
R2 = V
OUT
– 1.21/.007162
Using this formula with a V
OUT
of 2.50V gives an R2 value of 180
Ω
. The closest standard
1% resistor is 182
Ω
, which, when plugged back into the original formula, gives
V
OUT
= 2.513613V (0.5% difference).
Table 3-10 summarizes the adjustment resistor values for the LT1764A regulators U2, U3,
and U4.
Figure 3-5: LT1764A Voltage Regulator Circuit
Table 3-10: LT1764 Resistor Calculations
V
OUT
R1 R2 V
CALC
Calc Diff % Diff
U2 2.50V
–5% 2.375 169 162 2.370367657 –0.051852937 –5.2%
Nom. 2.500 169 182 2.513622923 0.005449169 +0.5%
+5% 2.625 169 200 2.642552663 0.057021065 +5.7%
U3 3.0V
–5% 2.850 169 226 2.828784509 –0.05707183 –5.7%
Nom. 3.000 169 255 3.036504645 0.012168215 +1.2%
+5% 3.150 169 274 3.172597148 0.057532383 +5.7%
U4 1.2V
–5% 1.140 N/A N/A
Nom. 1.200 none none 1.21 0.008333333 +0.8%
+5% 1.260 N/A N/A
Notes:
1. The LT1764A minimum voltage out is 1.21V, achieved with no adjust resistor network. Adj pin 5 is
wired to V
OUT
pin 4. Because the minimum V
OUT
= 1.21V, this regulator cannot be adjusted
±
5%.
LT1764AEQ
U2
3.3V
2.5V
2
1
4
5
3
GND
SHDN#
IN
OUT
ADJ
+
C20
3.3
µ
F
+
C19
10
µ
F
UG084_c3_03_032005
R1
169
R2
182
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