Datasheet

ADP1864-EVALZ
Rev. A | Page 10 of 12
MODIFYING THE EVALUATION BOARD
The ADP1864 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 listed in this section.
CHANGING THE MOSFET
The ADP1864 evaluation board is supplied with a Siliconix
Si5435 power MOSFET. The layout can accommodate
MOSFETs placed in parallel to accommodate higher current
levels. Additionally, SO-8, thermally-enhanced SO-8, TSOP-6,
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 m 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 figure in the
ADP1864 data sheet called “Slope Factor (SF) vs. Duty Cycle”
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 evaluation board is supplied with a Diodes, Inc.
PDS1040L Schottky diode. The board can accommodate 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 suggested 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 evaluation board is populated with a Coilcraft
DO3316P-332ML 3.3 µH inductor with a saturation current of
6.4 A. In order to operate at currents higher than this, the
inductor would have to be modified to accommodate at least
the higher current plus half the inductor ripple current. If the
current demand is to be less, it could be advantageous to go
with a physically smaller and lower saturation current inductor
for cost considerations. Changing the inductance value can
affect the stress on the transistor and diode, the output voltage
ripple, and load transient response. The
ADP1864 Buck Design
Software
accounts for all these changes when the new induc-
tance value is selected from the L1 pull-down menu. It will then
provide new suggestions for component values. The Applications
Information section of the 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 evaluation board is supplied with a 100 F
ceramic capacitor on the output. If the capacitance is insufficient
to meet load transient requirements, a D-case tantalum capacitor
footprint and 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. After supplying the
ADP1864
Buck Design Software
tool with the new output capacitance
values and the effective series resistance of the capacitors, the
tool will provide recommended values for compensation
components (R
C
, C
C0
, and C
C1
). It is recommended to always
refer to the manufacturer’s data for capacitance derating over
applied voltage and temperature.
CHANGING THE OUTPUT VOLTAGE
The ADP1864 evaluation board output regulation voltage can
be changed by altering the voltage divider consisting of R
F2
and
R
F1
. The output regulation voltage is determined by the
equation
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
×=
2F
1F2F
OUT
R
RR
V 8.0
Modifying the output voltage changes the inductor ripple
current and subsequently the output voltage ripple, transient
response, and stress on the PFET. The design tool will consider
this and if necessary, provide new component values.