UG-466 Datasheet, PDF (4/12 Pages) Analog Devices – Evaluation Board for the ADP2384/ADP2386, 20 V, 4 A/6 A Synchronous Step-Down DC-to-DC Regulators

UG-466
MeasuringOutput 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 ADP2384/ADP2386 evaluation board configura-
tion, unsolder and/or replace/remove the appropriate passive
components or jumpers on the board.
Changingthe Output Voltages
The output voltage setpoints of the ADP2384/ADP2386 can be
changed by replacing the R7 and R5 resistors with the resistor
values shown in Table 1.
Table 1. Resistive Divider for Various Output Voltages
V_OUT (V)
1.0
R7, ±1% (kΩ)
10
R5, ±1% (kΩ)
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
Evaluation Board User Guide
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, R7, value is calculated using the following
equations:
R7 = R5 ×
 ^{VOUT − 0.6 V} 
 _{0.6 V} 
When the output voltage is changed, the values of the inductor
(L1), the output capacitors (C9, C10, and C11), and the com-
pensation components (R6, C2, and C3) must be recalculated
and changed to ensure stable operation (see the ADP2384 and
ADP2386 data sheets for details on external component selection).
Changingthe SwitchingFrequency
The switching frequency (f_SW) set point can be changed by
replacing the R4 resistor with a different value, as shown in
the following equation:
f_SW [kHz] = 69, 120 / (R4 [kΩ] + 15)
A 215 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 (C9, C10, C11), the
compensation networks (R6, C2, C3) must be recalculated and
changed for stable operation (see the ADP2384 and ADP2386
data sheets for details on external component selection).
Changingthe Soft Start Time
The soft start time of the ADP2384/ADP2386 on the evaluation
board is programmed to 4 ms.
To change the soft start time, t_SS, replace the C1 capacitor value
using the following equation:
C1 [nF] = 5.33 × t_SS [ms]
External Synchronization
To synchronize the regulator to an external clock signal, apply
an external clock signal to J1 (SYNC) of the evaluation board.
The clock signal must have a logic high level from 1.3 V to 5 V
and a logic low level below 0.4 V. Set the external clock pulse
width to more than 100 ns and the frequency range between
200 kHz and 1.4 MHz.
During synchronization, the regulator operates in CCM mode
and the switching waveform’s rising edge runs 180° out of phase
to the external clock rising edge.
For reliable synchronization, connect a resistor from the RT pin
to GND (R4) to program the internal oscillator to run at 90% to
110% of the external synchronization clock signal.
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