EVAL-ADP1864_15 Datasheet, PDF (10/16 Pages) Analog Devices – Evaluation Board for ADP1864 with Web Design Tool

EVAL-ADP1864
MODIFYING THE EVALUATION BOARD
The ADP1864-EVALZ evaluation board is complete and tested
for operation. Due to the versatility of the ADP1864 step-down
dc-to-dc controller, the ADP1864 evaluation board can be
modified for a variety of external components. Some of the
most common modifications are described in this section.
CHANGING THE MOSFET
The ADP1864-EVALZ evaluation board is supplied with a
Siliconix Si5435 power MOSFET. The layout can accommodate
MOSFETs placed in parallel to accommodate higher current
levels. Additionally, 8-lead SOIC, thermally enhanced 8-lead
SOIC, 6-lead TSOP, 1206-8, and SOT-23 packages all have
footprints available on the ADP1864 evaluation board. On
resistances, gate charges and capacitances, gate-to-source
thresholds, and maximum drain to source voltage ratings
should all be considered before changing the MOSFET.
CHANGING THE SENSE RESISTOR
If an increase in current capability is desired, it may be necessary
to change the current limit via the sense resistor. As supplied,
two 33 mΩ resistors in parallel are used to sense current. For
duty cycles <40%, the current limit voltage is 125 mV typically.
For duty cycles >40%, use the slope factor (SF) vs. duty cycle
plot in the ADP1864 data sheet to determine the actual current
sense limit.
Note that across the full temperature range and input voltage range
of the ADP1864, the current limit voltage can be as low as 80 mV.
CHANGING THE DIODE
The ADP1864-EVALZ evaluation board is supplied with a
Diodes, Inc. PDS1040L Schottky diode. The board can accom-
modate SMC, DPAK (TO-252), and other popular packages.
The two primary factors to consider when changing the diode
are the current handling capabilities as well as the maximum
reverse dc blocking voltage. Because of switch node voltage
excursions, it is recommended to select a diode with at least three
times the reverse dc blocking voltage as the maximum input
voltage for the application when no snubber circuit is used.
CHANGING THE OUTPUT INDUCTOR
The ADP1864-EVALZ evaluation board is populated with a
Coilcraft DO3316P-332ML 3.3 μH inductor with a saturation
current of 6.4 A. To operate at currents higher than this, the
inductor needs to be modified to accommodate at least the higher
current plus half the inductor ripple current. If the current
demand is less, it could be advantageous to select a physically
smaller and lower saturation current inductor for cost consider-
ations. Changing the inductance value can affect the stress on
the transistor and diode, the output voltage ripple, and the load
transient response. The ADP1864 Buck Design Software accounts
for all these changes when the new inductance value is selected
from the L1 pull-down menu. It then provides new suggestions
for component values. The Applications Information section of
ADP1864 data sheet also provides information concerning the
implications of changing the output inductor. If the duty cycle is
to exceed 40%, keep the ripple current to 30% to 50% of the
output current so that the slope compensation remains
effective. See the ADP1864 data sheet for more details.
CHANGING THE OUTPUT CAPACITORS
The ADP1864-EVALZ evaluation board is supplied with a 100 μF
ceramic capacitor on the output. If the capacitance is insufficient
to meet load transient requirements, a D-case tantalum capacitor
footprint and an 8 mm electrolytic capacitor footprint are
available to provide the capability to greatly increase the output
capacitance. Any change in output capacitance also requires a
change in compensation component values. To compensate for
a change in output capacitor, use the following equations:
R_C
=
2π ×
f _UN
× C _OUT × (ESR + R _LOAD )² × V_OUT
G _M × R _LOAD ² × V _REF
× R_CL
×G_C
C _C1
=
2π
3
× R_C
×
f _UN
C _C0
=
2π ×
f _UN
G _M × R _LOAD × ESR × V _REF
× (ESR + R _LOAD ) × R_CL × G _C × V_OUT
or
C _C0
=
3
2π × R_C × f _SW
, whichever is larger.
where:
fUN is the desired unity gain frequency in hertz (usually 40 kHz).
COUT is output capacitance in farads.
ESR is the effective series resistance of COUT in ohms.
RLOAD is VOUT (volts)/maximum load current (amps).
RCL is the parallel resistance of the current sense resistors in ohms.
GC is the nominal gain of the current sense amplifier (12).
GM is the nominal gain of the error amplifier (2.4 × 10^−4 mhos).
VREF is the nominal error amplifier reference voltage (0.8 V).
fSW is the nominal switching frequency of the ADP1864
(580 kHz).
If multiple output capacitors are used and of the same type and
value, ESR is defined as the parallel effective series resistance of
all the capacitors. If multiple output capacitors are used that are
not of the same type and value, the analysis to calculate optimal
compensation components is considerably more complicated
and beyond the scope of this document.
It is recommended to always refer to the manufacturer’s data for
capacitance derating over applied voltage and temperature.
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