ADP1875 Analog Devices, ADP1875 Datasheet - Page 27

• Manufacturer Part Number: ADP1875
• Description: Synchronous Buck Controller with Constant On-Time, Valley Current Mode, and Power Saving Mode
• Manufacturer: Analog Devices

APPLICATIONS INFORMATION
FEEDBACK RESISTOR DIVIDER
The required resistor divider network can be determined for a
given V
is fixed at 0.6 V. Selecting values for R
minimum output load current of the converter. Therefore, for
a given value of R
following expression:
INDUCTOR SELECTION
The inductor value is inversely proportional to the inductor
ripple current. The peak-to-peak ripple current is given by
where K
The equation for the inductor value is given by
where:
V
V
f
or 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 85).
SW
Figure 85. Peak Inductor Current vs. Valley Current Limit for 33%, 40%, and
IN
OUT
is the controller switching frequency (300 kHz, 600 kHz,
is the high voltage input.
R
Δ
L
is the desired output voltage.
T
I
52
50
48
46
44
42
40
38
36
34
32
30
28
26
24
22
20
18
16
14
12
10
=
OUT
8
L
I
=
6
is typically 0.33.
(
=
V
R
Δ
value because the internal band gap reference (V
K
IN
B
I
8
L
×
I
−
×
×
(
V
V
10
I
f
B
OUT
OUT
LOAD
SW
, the R
50% of Inductor Ripple Current
0
12
6 .
)
−
VALLEY CURRENT LIMIT (A)
×
≈
0
V
14
6 .
V
T
I
V
value can be determined through the
OUT
LOAD
V)
IN
3
16
18
20
T
and R
∆I = 40%
22
∆I = 50%
24
B
determines the
∆I = 33%
26
28
30
REF
Rev. 0 | Page 27 of 44
)
Table 8. Recommended Inductors
L
(μH)
0.12
0.22
0.47
0.72
0.9
1.2
1.0
1.4
2.0
0.8
OUTPUT RIPPLE VOLTAGE (ΔV
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.)
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:
where ESR is the equivalent series resistance of the output
capacitors.
To calculate the output load step, use the following equation:
where ΔV
a given positive load current step (ΔI
ΔV
C
C
OUT
OUT
DCR
(mΩ)
0.33
0.33
0.8
1.65
1.6
1.8
3.8
3.2
2.0
RR
DROOP
= (0.01) × V
=
=
Δ
2
×
I
(A)
55
30
50
35
32
25
16
24
23
27.5
I
is the amount that V
SAT
L
f
×
SW
⎛
⎜
⎜
⎝
8
×
Dimensions
(mm)
10.2 × 7
10.2 × 7
14.2 × 12.8
10.5 × 10.2
14 × 12.8
10.5 × 10.2
10.2 × 10.2
14 × 12.8
10.2 × 10.2
×
OUT
(
Δ
f
V
SW
DROOP
×
[
Δ
Δ
I
V
LOAD
−
RIPPLE
ADP1874/ADP1875
(
Δ
Manufacturer
Würth Elek.
Würth Elek.
Würth Elek.
Würth Elek.
Würth Elek.
Würth Elek.
Würth Elek.
Würth Elek.
Würth Elek.
Sumida
OUT
1
LOAD
I
LOAD
RR
is allowed to deviate for
−
)
).
(
Δ
×
I
L
ESR
×
ESR
))
Model
Number
744303012
744303022
744355147
744325072
744318120
744325120
7443552100
744318180
7443551200
CEP125U-0R8
)
]
⎞
⎟
⎟
⎠