LTC2920-2 Linear Technology, LTC2920-2 Datasheet - Page 11

• Manufacturer Part Number: LTC2920-2
• Description: Single/Dual Power Supply Margining Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
the I
range, the LTC2920 cannot be used for this application.
Accuracy of Power Supply Voltages when Margining
The accuracy of margined power supply voltages depends
on several factors. Figure 11 shows the magnitude of the
errors discussed in detail below as a function of power
supply margining percentage.
In a typical feedback model (Figure 12), the delta voltage
is a function of the margin current, I
feedback resistor, R
Errors in V
I
a 5% error in V
supply is margined by 2.5%, or 0.0825V to 3.3825V. With
a 5% V
and the actual power supply voltage is 3.3866V. The error
in the expected voltage is then:
Similarly, a 1% inaccuracy in the R
cause only 0.024% error in the expected power supply
margined voltage. In effect, I
R
margining percentage.
The accuracy of the R
margined supply voltage. The first is the error in V
(I
errors described above and is generally quite small (0.024%
MARGIN
SET
MARGIN
V
Error = ⏐1 – (3.3866/3.3825)⏐ • 100 = 0.12%
MARGIN
MACCURACY
resistor or the LTC2920 are attenuated by the voltage
MARGIN
and errors in R
• R
MARGIN
= I
Figure 10. Power Module I
F
). This error is similar in magnitude to the
V
V
MARGIN
error, the actual margin voltage is 0.0866V
IN
IN
specification. If V
+
–
MARGIN
V
REF
are directly proportional to errors in
U
F
F
.
+
–
resistor introduces two errors in the
• R
F
. In this example, a 3.3V power
R
. A 5% error in I
VT
F
U
SENSE
SENSE
TRIM
MARGIN
V
V
O
O
+
+
–
–
T
R
TRIM
does not fall within this
TRIM
W
V
errors caused by the
T
SET
Model
MARGIN
MARGIN
I
2920-1/2 F10
TRIM
resistor would
V
V
PS
O
–
, and the
will cause
U
MARGIN
for this example). The second error is the power supply
initial set point accuracy. In this example the R
has a 1% accuracy error causing a 0.6% initial set point
error in the power supply. Because the margined power
supply voltage is the change in the voltage, V
the power supply initial set point voltage, this error shows
up in the margined power supply voltage. When these two
errors are combined, the error is:
The error caused by a 1% inaccuracy in R
since the dominate error source is the power supply initial
set point voltage.
Errors caused by R
voltage margin errors. Using 0.1% resistors for both R
and R
margin accuracy and power supply initial accuracy.
R
Figure 11. Sources of Power Supply Margined Voltage Errors
SET
Error = ⏐1 – (3.4043/3.3825)⏐ • 100 = 0.65%
= 20k
G
is often the best choice for improving both voltage
Figure 12. Power Supply Voltage Margin Model
LTC2920-1/LTC2920-2
0.7
0.6
0.5
0.4
0.3
0.2
0.1
R
LTC2920
0
S
0
POWER SUPPLY VOLTAGE MARGINING (%)
5% LTC2920
I
I
MARGIN
M
V
I
F
MARGIN
REF
1
and R
R
= 1.2V
G
INACCURACY
1% FEEDBACK
RESISTOR INACCURACY
= 944k
= ± 50μA
2
G
+
–
can be a major contributor to
3
–
+
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I
4
FB
R
1% R
RESISTOR
INACCURACY
F
= 1.27mA
= 1.65k
SET
5
2920-1/2 F11
G
will be similar
MARGIN
6
V
PSOUT
F
resistor
11
2920-1/2 F12
, from
292012fa
= 3.3V
F