Datasheet

Data Sheet ADP1872/ADP1873
Rev. B | Page 23 of 40
APPLICATIONS INFORMATION
FEEDBACK RESISTOR DIVIDER
The required resistor divider network can be determine for a
given V
OUT
value because the internal band gap reference (V
REF
)
is fixed at 0.6 V. Selecting values for R
T
and R
B
determines the
minimum output load current of the converter. Therefore, for a
given value of R
B
, the R
T
value can be determined by
V6.0
V)6.0(
−
×=
OUT
B
T
V
RR
INDUCTOR SELECTION
The inductor value is inversely proportional to the inductor
ripple current. The peak-to-peak ripple current is given by
3
LOAD
LOAD
IL
I
IKI ≈×=∆
where K
I
is typically 0.33.
The equation for the inductor value is given by
VIN
V
fI
VVIN
L
OUT
SW
L
OUT
×
×∆
−
=
)(
where:
VIN is the high voltage input.
V
OUT
is the desired output voltage.
f
SW
is the controller switching frequency (300 kHz, 600 kHz,
and 1.0 MHz).
When selecting the inductor, choose an inductor saturation rating
that is above the peak current level and then calculate the
inductor current ripple (see the Valley Current-Limit Setting
section and Figure 78).
52
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
6 8 10 12 14 16 18 20 22 24 26 28 30
PEAK INDUCTOR CURRENT (A)
VALLEY CURRENT LIMIT (A)
ΔI = 50%
ΔI = 40%
ΔI = 33%
08297-077
Figure 78. Peak Current vs. Valley Current Threshold for
33%, 40%, and 50% of Inductor Ripple Current
Table 7. Recommended Inductors
L
(µH)
DCR
(mΩ)
I
SAT
(A)
Dimensions
(mm)
Manufacturer Model No.
0.12 0.33 55 10.2 × 7 Würth Elektronic 744303012
0.22 0.33 30 10.2 × 7 Würth Elektronic 744303022
0.47 0.8 50 14.2 × 12.8 Würth Elektronic 744355147
0.72 1.65 35 10.5 × 10.2 Würth Elektronic 744325072
0.9 1.6 28 13 × 12.8 Würth Elektronic 744355090
1.2 1.8 25 10.5 × 10.2 Würth Elektronic 744325120
1.0 3.3 20 10.5 × 10.2 Würth Elektronic 7443552100
1.4 3.2 24 14 × 12.8 Würth Elektronic 744318180
2.0 2.6 22 13.2 × 12.8 Würth Elektronic 7443551200
0.8 27.5 Sumida CEP125U-0R8
OUTPUT RIPPLE VOLTAGE (ΔV
RR
)
The output ripple voltage is the ac component of the dc output
voltage during steady state. For a ripple error of 1.0%, the output
capacitor value needed to achieve this tolerance can be determined
using the following equation. (Note that an accuracy of 1.0% is
only possible during steady state conditions, not during load
transients.)
ΔV
RR
= (0.01) × V
OUT
OUTPUT CAPACITOR SELECTION
The primary objective of the output capacitor is to facilitate
the reduction of the output voltage ripple; however, the output
capacitor also assists in the output voltage recovery during load
transient events. For a given load current step, the output voltage
ripple generated during this step event is inversely proportional
to the value chosen for the output capacitor. The speed at which
the output voltage settles during this recovery period depends
on where the crossover frequency (loop bandwidth) is set. This
crossover frequency is determined by the output capacitor, the
equivalent series resistance (ESR) of the capacitor, and the
compensation network.
To calculate the small signal voltage ripple (output ripple
voltage) at the steady state operating point, use the following
equation:
[ ]








×∆−∆××
×∆=
)(8
1
ESRIV
f
IC
LRIPPLE
SW
L
OUT
where ESR is the equivalent series resistance of the output
capacitors.
To calculate the output load step, use the following equation:
))((
2
ESRIVf
I
C
LOADDROOPSW
LOAD
OUT
×∆−∆×
∆
×=
where ΔV
DROOP
is the amount that V
OUT
is allowed to deviate for
a given positive load current step (ΔI
LOAD
).