ADP3171 データシートの表示（PDF） - Analog Devices
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ADP3171 Datasheet PDF : 13 Pages
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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:
VOUT
= VREF
× 1 +
R4
R3
(9)
COUT Selection
The selection of COUT 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 COUT bulk capacitance value and 75%
from the COUT ESR value. The correct value for COUT can be
determined by:
COUT
=
∆IOUT × tOFF
0.25 × ∆VPP
and:
(10)
ESR = 0.75 × ∆VPP
∆IOUT
(11)
where:
∆IOUT
= VOUT × tOFF
LOUT
and:
(12)
∆VPP = 0.01 ×VOUT
(13)
Solving for this example:
∆VPP = 0.01 × 1.5V = 15 mV
∆IOUT
=
1.5V × 3.5 µs
1.7 µH
=
3
A
ESR = 0.75 × 15 mV = 3.75 mΩ
3A
COUT
=
3 A × 3.5 µs
0.25 × 15 mV
= 2800 µF
Four OSCON 820 µF/4 V capacitors would meet these require-
ments, giving a total capacitance of 3280 µF and an ESR of 3 mΩ.
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 COUT will normally exceed the amount needed to meet the
ripple current requirement.
Feedback Loop Compensation Design
Once the output capacitor COUT and ESR values have been
chosen, the output circuit’s pole (fP) and zero (fZ) frequencies
can be calculated using:
fP =
1
2 π × COUT × (ROUT + ESR)
(14)
fZ
=
1
2 π × COUT × ESR
(15)
where:
ROUT
= VOUT
IOUT
(16)
For this example:
ROUT
= 1.5V
5A
= 0.3 Ω
fP
=
1
= 160 Hz
2 π × 3280 µF × (0.3 Ω + 3 mΩ)
fP
=
1
= 16.2 kHz
2 π × 3280 µF × 3 mΩ
The compensation circuit is simply a capacitor (CC) 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 (fC) should be chosen at one-third the switching frequency,
or 70 kHz. The total gain of the compensation circuit (K) is:
K=
gm
(17)
2 π × ni × C × RS × fC
where error amplifier transconductance (gm) is 2.2 mmho, the
error amplifier gain (ni) is 25, and the sense resistor (RS) is
7.5 mΩ. The value of K is determined using the gain of the
power output circuit at fC. The relationship between gain (GO)
at fC, ESR, and K is:
GO = ESR
K × GO = 1
(18)
K= 1 = 1
GO ESR
As K is now known, the value of CC can be determined by rear-
ranging Equation 17 as follows:
CC =
gm × ESR
2 π × ni × RS × fC
2.2 mmho × 3 mΩ
(19)
CC = 2 π × 25 × 7.5 mΩ × 70 kHz = 80 pF
The closest standard value is 100 pF.
–8–
REV. 0
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