Datasheet

ADP1870/ADP1871 Data Sheet
Rev. B | Page 24 of 44
APPLICATIONS INFORMATION
FEEDBACK RESISTOR DIVIDER
The required resistor divider network can be determined 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 through the
following expression:
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
IN
OUT
SW
L
OUT
IN
V
V
fI
VV
L ×
×∆
−
=
)(
where:
V
IN
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 79).
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%
08730-077
Figure 79. Peak Inductor Current vs. Valley Current Limit for 33%, 40%, and
50% of Inductor Ripple Current
Table 8. Recommended Inductors
L
(µH)
DCR
(mΩ)
I
SAT
(A)
Dimensions
(mm) Manufacturer
Model
Number
0.12 0.33 55 10.2 × 7 Würth Elek. 744303012
0.22 0.33 30 10.2 × 7 Würth Elek. 744303022
0.47 0.67 50 13.2 × 12.8 Würth Elek. 744355147
0.72
1.3
35
10.5 × 10.2
Würth Elek.
744325072
0.9
1.6
28
13 × 12.8
Würth Elek.
744355090
1.2 1.8 25 10.5 × 10.2 Würth Elek. 744325120
1.0 3.3 20 10.5 × 10.2 Würth Elek. 7443552100
1.4 3.2 24 14 × 12.8 Würth Elek. 744318180
2.0 2.6 22 13.2 × 12.8 Würth Elek. 7443551200
0.8 2.5 16.5 12.5 × 12.5 AIC Technology CEP125U-R80
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 possible only during steady state conditions,
not during load transients.)
OUT
RR
VV ×=∆ )01.0(
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
).