Datasheet

ADP1850 Data Sheet
Rev. A | Page 18 of 32
INPUT CAPACITOR SELECTION
The input current to a buck converter is a pulse waveform. It is
zero when the high-side switch is off and approximately equal
to the load current when it is on. The input capacitor carries the
input ripple current, allowing the input power source to supply
only the direct current. The input capacitor needs sufficient
ripple current rating to handle the input ripple, as well as an
ESR that is low enough to mitigate input voltage ripple. For the
usual current ranges for these converters, it is good practice to
use two parallel capacitors placed close to the drains of the
high-side switch MOSFETs (one bulk capacitor of sufficiently
high current rating and a 10 F ceramic decoupling capacitor,
typically).
Select an input bulk capacitor based on its ripple current rating.
First, determine the duty cycle of the output.
IN
OUT
V
V
D =
The input capacitor RMS ripple current is given by
)1( DDII
ORMS
−=
where:
I
O
is the output current.
D is the duty cycle
The minimum input capacitance required for a particular load is
SWESRO
PP
O
MININ
fDRIV
DDI
C
)(
)1(
,
×−
−×
=
where:
V
PP
is the desired input ripple voltage.
R
ESR
is the equivalent series resistance of the capacitor.
If an MLCC capacitor is used, the ESR is near 0, then the
equation is simplified to
SW
PP
O
MININ
fV
DD
IC
×
−
×=
)1(
,
The capacitance of MLCC is voltage dependent. The actual
capacitance of the selected capacitor must be derated according to
the manufacturer’s specification. In addition, add more bulk
capacitance, such as by using electrolytic or polymer capacitors,
as necessary for large step load transients. Make sure the
current ripple rating of the bulk capacitor exceeds the
maximum input current ripple of a particular design.
INPUT FILTER
Normally a 0.1 µF or greater value bypass capacitor from the
input pin (VIN) to AGND is sufficient for filtering out any
unwanted switching noise. However, depending on the PCB
layout, some switching noise can enter the ADP1850 internal
circuitry; therefore, it is recommended to have a low pass filter
at the VIN pin. Connecting a resistor, between 2 Ω and 5 Ω, in
series with VIN and a 1 µF ceramic capacitor between VIN and
AGND creates a low pass filter that effectively filters out any
unwanted glitches caused by the switching regulator. Keep in
mind that the input current could be larger than 100 mA when
driving large MOSFETs. A 100 mA across a 5 Ω resistor creates
a 0.5 V drop, which is the same voltage drop in VCCO. In this
case, a lower resistor value is desirable.
ADP1850
VIN
V
IN
AGND
2Ω TO 5Ω
1µF
09440-033
Figure 32. Input Filter Configuration
BOOST CAPACITOR SELECTION
To lower system component count and cost, the ADP1850 has
an integrated rectifier (equivalent to the boost diode) between
VCCO and BSTx. Choose a boost ceramic capacitor with a
value between 0.1 µF and 0.22 µF; this capacitor provides the
current for the high-side driver during switching.
INDUCTOR SELECTION
The output LC filter smoothes the switched voltage at SWx. For
most applications, choose an inductor value such that the
inductor ripple current is between 20% and 40% of the
maximum dc output load current. Generally, a larger inductor
current ripple generates more power loss in the inductor and
larger voltage ripples at the output. Check the inductor data
sheet to make sure that the saturation current of the inductor is
well above the peak inductor current of a particular design.
Choose the inductor value by the following equation:
IN
OUT
L
SW
OUT
IN
V
V
If
VV
L ×
Δ×
−
=
where:
L is the inductor value.
f
SW
is the switching frequency.
V
OUT
is the output voltage.
V
IN
is the input voltage.
ΔI
L
is the peak-to-peak inductor ripple current.