ADP1853 Datasheet, PDF (18/28 Pages) Analog Devices – Synchronous, Step-Down DC-to-DC Controller

ADP1853
VIN PIN FILTER
It is recommended to have a low-pass filter at 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 could be larger than 100 mA when driving
large MOSFETs. A 100 mA 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.
2Ω TO 10Ω
V_IN
1µF
ADP1853
VIN
AGND
Figure 26. 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 by the following equation:
L = ^{VIN − VOUT} × ^VOUT
f _SW × ∆I _L V_IN
where:
L is the inductor value.
f_SW is the switching frequency.
V_OUT is the output voltage.
V_IN is the input voltage.
∆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.
Data Sheet
OUTPUT CAPACITOR SELECTION
For maximum allowed switching ripple at the output, choose an
output capacitor that is larger than
C_OUT
≅
∆I _L
8 f_SW
×
1
∆V_OUT ² − ∆I _L² ×(R_ESR² − (4 f_SW × L_ESL )² )
where:
∆V_OUT is the target maximum output ripple voltage.
∆I_L is the inductor ripple current.
R_ESR is the equivalent series resistance of the output capacitor
(or the parallel combination of ESR of all output capacitors).
L_ESL is the equivalent series inductance of the output capacitor
(or the parallel combination of 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
∆V_OUT ≅ ∆I _L × R_ESR
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 impedance of the
ESR and ESL are small at the switching frequency, for instance,
where the output capacitor is a bank of parallel MLCC capaci-
tors, the capacitive impedance dominates, so the output
capacitance must be larger than
C_OUT
≅
∆I _L
8 ∆V_OUT ×
f_SW
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
C_OUT ≅
∆I _STEP ² L
(V_OUT + ∆V_OVERSHOOT )² − V_OUT ²
where:
∆V_OVERSHOOT is the maximum allowed overshoot.
Select the largest output capacitance given by either of the
previous two equations.
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