UG-653
EVAL-ADP2311 User Guide
Measuring Output Voltage Ripple
To observe the output voltage ripple, place the oscilloscope probe
across the output capacitor with the probe ground lead connected
to the negative (−) capacitor terminal and the probe tip placed at
the positive (+) capacitor terminal. Set the oscilloscope to ac,
10 mV/division, 2 µs/division time base, and 20 MHz bandwidth.
A standard oscilloscope probe has a long wire ground clip. For
high frequency measurements, this ground clip picks up high
frequency noise and injects it into the measured output ripple.
Figure 2 shows an easy way to measure the output ripple properly.
It requires removing the oscilloscope probe sheath and wrapping
an unshielded wire around the oscilloscope probe. By keeping
the ground length of the oscilloscope probe as short as possible,
the true ripple can be measured.
Figure 2. Measuring Output Voltage Ripple
MODIFYING THE EVALUATION BOARD
To modify the
ADP2311
evaluation board configuration, unsolder
and/or replace/remove the appropriate passive components or
jumpers on the board.
Changing the Output Voltages
The output voltage setpoints of the
ADP2311
can be changed by
replacing the R1, R2, R5, and R8 resistors with the resistor values
shown in Table 1.
Table 1. Resistive Divider for Various Output Voltages
V
OUT
(V)
R1/R5, ±1% (kΩ)
R2/R8, ±1% (kΩ)
1.0
10
15
1.2
10
10
1.5
15
10
1.8
20
10
2.5
47.5
15
3.3
10
2.21
5.0
22
3
To limit the output voltage accuracy degradation due to the FBx
bias current (0.1 µA maximum) to less than 0.5% (maximum),
ensure that the bottom divider string resistors, R2 and R8, is less
than 30 kΩ.
The top resistors, R1 and R5, value is calculated using the following
equations:
For Channel 1,
−
V
0.6
V
0.6
×
=
OUT1
V
R2
R1
For Channel 2,
−
V
0.6
V
0.6
×
=
OUT2
V
R8
R5
When the output voltage of Channel 1 is changed, the values of
the inductor (L1) and the output capacitors (C4 and C5) must
be recalculated and changed to ensure stable operation (see the
ADP2311
data sheet for details on external component selection).
Likewise, if the output voltage of Channel 2 is changed, the values
of the inductor (L2) and the output capacitors (C8 and C9)
must be recalculated and changed.
Changing the VIN Threshold of Power Fail (PFI)
The evaluation board sets the V
PFI_RISING
at 8.99 V and the
V
PFI_FALLING
at 8.61 V. It can be changed with different R4 and
R7 value.
Using the following equation to calculate the R4 and R7:
7
7
4
6
.
0
_
R
R
R
V
V
RISING
PFI
+
×
=
7
7
4
575
.
0
_
R
R
R
V
V
FALLING
PFI
+
×
=
Changing the VIN Threshold of Voltage Monitoring (VM2)
The evaluation board sets the V
VM2_RISING
at 5.41 V and the
V
VM2_FALLNG
at 5 V. It can be changed with different R3 and R6 value.
Using the following equation to calculate R3 and R6:
6
6
3
65
.
0
_
2
R
R
R
V
V
RISING
VM
+
×
=
6
6
3
6
.
0
_
2
R
R
R
V
V
FALLING
VM
+
×
=
1
1995-
002
Rev. 0 | Page 4 of 8
