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ADP1871 Datasheet(PDF) 25 Page - Analog Devices

Part # ADP1871
Description  Synchronous Buck Controller with Constant On-Time and Valley Current Mode
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

ADP1871 Datasheet(HTML) 25 Page - Analog Devices

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ADP1870/ADP1871
Rev. 0 | Page 25 of 44
Ceramic capacitors are known to have low ESR. However, the
trade-off of using X5R technology is that up to 80% of its capaci-
tance might be lost due to derating as the voltage applied across
the capacitor is increased (see Figure 80). Although X7R series
capacitors can also be used, the available selection is limited to
only up to 22 μF.
20
10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
0
5
10
15
20
25
30
DC VOLTAGE (VDC)
X7R (50V)
X5R (25V)
X5R (16V)
10µF TDK 25V, X7R, 1210 C3225X7R1E106M
22µF MURATA 25V, X7R, 1210 GRM32ER71E226KE15L
47µF MURATA 16V, X5R, 1210 GRM32ER61C476KE15L
Figure 80. Capacitance vs. DC Voltage Characteristics for Ceramic Capacitors
Electrolytic capacitors satisfy the bulk capacitance requirements
for most high current applications. Because the ESR of electrolytic
capacitors is much higher than that of ceramic capacitors, when
using electrolytic capacitors, several MLCCs should be mounted
in parallel to reduce the overall series resistance.
COMPENSATION NETWORK
Due to their current-mode architecture, the ADP1870/ADP1871
require Type II compensation. To determine the component
values needed for compensation (resistance and capacitance
values), it is necessary to examine the converter’s overall loop
gain (H) at the unity gain frequency (fSW/10) when H = 1 V/V:
FILT
COMP
REF
OUT
CS
M
Z
Z
V
V
G
G
H
×
×
×
×
=
= V/V
1
Examining each variable at high frequency enables the unity-
gain transfer function to be simplified to provide expressions
for the RCOMP and CCOMP component values.
Output Filter Impedance (ZFILT)
Examining the filter’s transfer function at high frequencies
simplifies to
OUT
FILTER
sC
Z
1
=
at the crossover frequency (s = 2πfCROSS).
Error Amplifier Output Impedance (ZCOMP)
Assuming that CC2 is significantly smaller than CCOMP, CC2 can
be omitted from the output impedance equation of the error
amplifier. The transfer function simplifies to
CROSS
ZERO
CROSS
COMP
COMP
f
f
f
R
Z
)
(
+
=
and
SW
CROSS
f
f
×
=
12
1
where fZERO, the zero frequency, is set to be 1/4th of the crossover
frequency for the ADP1870.
Error Amplifier Gain (GM)
The error amplifier gain (transconductance) is
GM
= 500 μA/V
Current-Sense Loop Gain (GCS)
The current-sense loop gain is
ON
CS
CS
R
A
G
×
=
1
(A/V)
where:
ACS
(V/V) is programmable for 3 V/V, 6 V/V, 12 V/V, and 24 V/V
(see the Programming Resistor (RES) Detect Circuit and Valley
Current-Limit Setting sections).
RON
is the channel impedance of the lower-side MOSFET.
Crossover Frequency
The crossover frequency is the frequency at which the overall
loop (system) gain is 0 dB (H = 1 V/V). For current-mode
converters, such as the ADP1870, it is recommended that the
user set the crossover frequency between 1/10th and 1/15th of the
switching frequency.
SW
CROSS
f
f
12
1
=
The relationship between CCOMP and fZERO (zero frequency) is as
follows:
COMP
COMP
ZERO
C
R
f
×
×
π
=
2
1
)
The zero frequency is set to 1/4th of the crossover frequency.
Combining all of the above parameters results in
REF
OUT
CS
M
OUT
CROSS
ZERO
CROSS
CROSS
COMP
V
V
G
G
C
f
f
f
f
R
×
π
×
+
=
2
ZERO
COMP
COMP
f
R
C
×
×
π
×
=
2
1


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