UG-863 ADP2389-EVALZ/ADP2390-EVALZ User Guide
Rev. 0 | Page 4 of 8
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 BOARD
To modify the ADP2389/ADP2390 evaluation board configura-
tion, unsolder and/or replace or remove the appropriate passive
components or jumpers on the board.
Changing the Output Voltages
The output voltage setpoints of the ADP2389/ADP2390 can be
changed by replacing the R9 and R5 resistors with the resistor
values shown in Table 1.
Table 1. Resistive Divider for Various Output Voltages
OUT
2.5 47.5 15
To limit output voltage accuracy degradation due to the FB pin
bias current (0.1 µA maximum) to less than 0.5% (maximum),
ensure that the bottom divider string resistor, R5, is less than
30 kΩ.
The top resistor, R9, value is calculated using the following
equation:
−
×=
V6.0
V6.0
OUT
V
R5R9
When the output voltage is changed, the values of the inductor
(L1), the output capacitors (C13, C14, C15, C16, and C17), and
the compensation components (R10, C5, and C4) must be
recalculated and changed to ensure stable operation (see the
ADP2389/ADP2390 data sheet for details on external component
selection).
Changing the Switching Frequency
The switching frequency (f
SW
) setpoint can be changed by
replacing the R6 resistor with a different value, as shown in
the following equation:
f
SW
(kHz) = 67,000/(R6 (kΩ) +12)
A 210 kΩ resistor sets the frequency to 300 kHz, and a 100 kΩ
resistor sets the frequency to 600 kHz.
When the switching frequency is changed, the values of the
inductor (L1), the output capacitors (C13, C14, C15, C16,
and C17), and the compensation networks (R10, C5, and C4)
must be recalculated and changed for stable operation (see
the ADP2389/ADP2390 data sheet for details on external
component selection).
Changing the Soft Start Time
The soft start time of the ADP2389/ADP2390 on the evaluation
board is programmed to 4 ms.
To change the soft start time, t
SS
, replace the C3 capacitor value
using the following equation:
C3 (nF) = 5.67 × t
SS
(ms)
Changing the Peak Current-Limit Threshold
The peak current-limit threshold of ADP2389/ADP2390 can be
changed by replacing the R7 resistor with a different value, as
shown in the following equation:
I
OCP
(A) = 1000/(R7 (kΩ) + 0.5)
A 54.9 kΩ resistor sets the current-limit threshold to 18 A, an
82.5 kΩ resistor sets the current-limit threshold to 12 A, and a
110 kΩ resistor sets the current-limit threshold to 9 A.
By programming the peak current-limit threshold at different
levels, the value and size of the inductor (L1) can be optimized
based on actual applications.
