Datasheet

ADP1882/ADP1883
Rev. 0 | Page 24 of 40
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 0.8 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 using the
following expression:
R
T
= R
B
×
V8.0
V)8.0( −
OUT
V
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 ≈×=Δ
where K
I
is typically 0.33.
The equation for the inductor value is given by
IN
SW
L
VfI
L ×
×Δ
=
6 8 10 12 14 16 18 20 22 24 26 28 30
VALLEY CURRENT LIMIT (A)
OUTOUT
IN
VVV − )(
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
PEAK INDUCTOR CURRENT (A)
I = 50%
I = 40%
I = 33%
08901-078
Figure 79. 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
Number
0.12 0.33 55 10.2 × 7 Wurth Electronics 744303012
0.22 0.33 30 10.2 × 7 Wurth Electronics 744303022
0.47 0.8 50 14.2 × 12.8 Wurth Electronics 744355147
0.72 1.65 35 10.5 × 10.2 Wurth Electronics 744325072
0.9 1.6 28 13 × 12.8 Wurth Electronics 744355090
1.2 1.8 25 10.5 × 10.2 Wurth Electronics 744325120
1.0 3.3 20 10.5 × 10.2 Wurth Electronics 7443552100
1.4 3.2 24 14 × 12.8 Wurth Electronics 744318180
2.0 2.6 22 13.2 × 12.8 Wurth Electronics 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 deter-
mined using the following equation. Note that an accuracy of
1.0% is possible only during steady state conditions, not during
load transients.
V
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:
[]
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×Δ−Δ××
×Δ=
)(8
1
ESRIVF
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
×Δ−Δ×
Δ
×=
where 
V
DROOP
is the amount that V
OUT
is allowed to deviate for
a given positive load current step (I
LOAD
).