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參數(shù)資料
| 型號： | ADP3171JR |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Charger front end protection IC with 30V max Vin and 4.5V LDO output 8-WSON 0 to 125 |
| 中文描述： | SWITCHING CONTROLLER, PDSO14 |
| 封裝： | SOIC-14 |
| 文件頁數(shù)： | 8/12頁 |
| 文件大小： | 310K |
| 代理商： | ADP3171JR |
REV. 0
–8–
ADP3171
Setting the Switcher Output Voltage
For this example, the resistor divider R3 and R4 set the output
voltage at 1.5 V by comparing the divided-down output to the
internal 1.2 V reference using:
V
V
R
R
OUT
REF
=
×
+
1
4
3
(9)
C
OUT
Selection
The selection of C
OUT
is driven by the required effective series
resistance (ESR) and the desired output ripple. A good rule of
thumb is to limit the ripple voltage to 1% of the nominal output
voltage. It is assumed that the total ripple has two main con-
tributors: 25% from the C
OUT
bulk capacitance value and 75%
from the C
OUT
ESR value. The correct value for C
OUT
can be
determined by:
C
I
.
t
V
OUT
OUT
OFF
PP
=
×
×
0 25
(10)
and:
ESR
V
I
PP
OUT
=
×
0 75
.
(11)
where:
I
V
t
L
OUT
OUT
OFF
OUT
=
×
(12)
and:
V
V
PP
OUT
=
×
0 01
.
(13)
Solving for this example:
V
V
.
mV
I
V
1 7
.
s
H
mV
A
ESR
A
3 5
.
m
C
A
s
mV
F
PP
OUT
OUT
=
=
μ
=
×
=
=
3
=
=
×
=
0 01 1 5
1 5
.
15
3 5
μ
3
0 75 15
.
3 75
.
3
.
0 25 15
2800
.
.
μ
μ
Four OSCON 820
μ
F/4 V capacitors would meet these require-
ments, giving a total capacitance of 3280
μ
F and an ESR of 3 m
.
Manufacturers such as Vishay, AVX, Elna, WIMA, and Sanyo
provide good high-performance capacitors. Sanyo’s OSCON
capacitors have lower ESR for a given size at a somewhat higher
price. Choosing sufficient capacitors to meet the ESR requirement
for C
OUT
will normally exceed the amount needed to meet the
ripple current requirement.
Feedback Loop Compensation Design
Once the output capacitor C
OUT
and ESR values have been
chosen, the output circuit’s pole (f
P
) and zero (f
Z
) frequencies
can be calculated using:
f
C
R
ESR
P
OUT
OUT
=
×
×
+
1
(
2
π
)
(14)
f
C
ESR
Z
OUT
=
×
×
1
2
π
(15)
where:
R
V
I
OUT
OUT
OUT
=
(16)
For this example:
R
V
A
f
F
m
Hz
f
F
m
kHz
OUT
P
P
=
=
=
×
×
+
=
=
×
×
=
1 5
5
0 3
.
1
2
3280
(0 3
3
160
1
2
3280
3
16 2
.
)
.
π
μ
π
μ
The compensation circuit is simply a capacitor (C
C
) connected
to the COMP pin. This makes the converter have a fast dynamic
response to load changes.
The switching frequency of the converter is 200 kHz. The crossover
frequency (f
C
) should be chosen at one-third the switching frequency,
or 70 kHz. The total gain of the compensation circuit (K) is:
K
g
C
n
R
f
m
i
S
C
=
×
×
×
×
2
π
(17)
where error amplifier transconductance (g
m
) is 2.2 mmho, the
error amplifier gain (
n
i
) is 25, and the sense resistor (
R
S
) is
7.5 m
. The value of
K
is determined using the gain of the
power output circuit at
f
C
. The relationship between gain (G
O
)
at f
C
, ESR, and K is:
G
K
ESR
G
O
1
K
G
ESR
O
O
=
×
=
=
=
1
1
(18)
As K is now known, the value of C
C
can be determined by rear-
ranging Equation 17 as follows:
C
g
×
ESR
R
mmho
×
7 5
n
.
f
C
m
×
m
kHz
pF
C
m
i
S
C
C
=
×
×
×
=
×
×
=
2
2 2
25
3
2
70
80
π
π
.
(19)
The closest standard value is 100 pF.
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