Datasheet

ADP1851 Data Sheet
Rev. 0 | Page 16 of 24
VIN PIN FILTER
It is recommended that a low-pass filter be connected to the
VIN pin. Connecting a resistor, between 2 Ω and 10 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 may be larger than 100 mA when
driving large MOSFETs. A 100 mA current across a 10 Ω
resistor creates a 1 V drop, which is the same voltage drop in
VCCO. In this case, a lower resistor value is desirable.
Figure 25. Input Filter Configuration
BOOST CAPACITOR SELECTION
Connect a boost capacitor between the SW and BST pins to
provide the current for the high-side driver during switching.
Choose a ceramic capacitor with a value between 0.1 µF and
0.22 µF.
INDUCTOR SELECTION
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.
Choose the inductor value using the following equation:
IN
OUT
L
SW
OUT
IN
V
V
If
VV
L ×
∆×
−
=
where:
L is the inductor value.
V
IN
is the input voltage.
V
OUT
is the output voltage.
f
SW
is the switching frequency.
∆I
L
is the peak-to-peak inductor ripple current.
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.
OUTPUT CAPACITOR SELECTION
For maximum allowed switching ripple at the output, choose an
output capacitor that is larger than
))4((
1
8
2
222
ESLSWESR
L
OUT
SW
L
OUT
LfRIV
f
I
C
×−×∆−∆
×
∆
≅
where:
∆I
L
is the inductor ripple current.
∆V
OUT
is the target maximum output ripple voltage.
R
ESR
is the equivalent series resistance of the output capacitor
(or the parallel combined ESR of all output capacitors).
L
ESL
is the equivalent series inductance of the output capacitor
(or the parallel combined ESL of all capacitors).
The impedance of the output capacitor at the switching
frequency multiplied by the ripple current gives the output
voltage ripple. The impedance is made up of the capacitive
impedance plus the nonideal parasitic characteristics, the
equivalent series resistance (ESR), and the equivalent series
inductance (ESL).
Usually the capacitor impedance is dominated by ESR. The
maximum ESR rating of the capacitor, such as in electrolytic
or polymer capacitors, is provided in the manufacturer’s data
sheet; therefore, the output ripple reduces to
ESR
L
OUT
RIV ×∆≅∆
Electrolytic capacitors also have significant ESL, on the order
of 5 nH to 20 nH, depending on type, size, and geometry. PCB
traces contribute some ESR and ESL as well. However, using the
maximum ESR rating from the capacitor data sheet usually
provides some margin such that measuring the ESL may not be
required.
In the case of output capacitors where the impedances of the
ESR and ESL are small at the switching frequency, for example,
where the output capacitor is a bank of parallel MLCC capaci-
tors, the capacitive impedance dominates; therefore, the output
capacitance must be larger than
SW
OUT
L
OUT
fV
I
C
×∆
∆
≅
8
(1)
Make sure that the ripple current rating of the output capacitors
is greater than the maximum inductor ripple current.
To meet the requirement of the output voltage overshoot during
load release, the output capacitance should be larger than
2
2
2
)(
OUTOVERSHOOTOUT
STEP
OUT
VVV
LI
C
−∆+
∆
≅
(2)
where:
∆V
OVERSHOOT
is the maximum allowed overshoot.
Select the largest output capacitance given by either Equation 1
or Equation 2.
VIN
V
IN
AGND
2Ω TO 10Ω
1µF
ADP1851
10595-025