Datasheet

ADP1882/ADP1883
Rev. 0 | Page 25 of 40
–90
–100
0 5 10 15 20 25 30
DC VOLTAGE (V
DC
)
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 may 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
CAPACITANCE CHARGE (%)
X7R (50V)
X5R (25V)
X5R (16V)
10µF TDK 25V, X7R, 1210 C3225X7R1E106M
22µF MURATA 25V, X7R, 1210 GRM32ER71E226KE15L
47µF MURATA 16V, X5R, 1210 GRM32ER61C476KE15L
08901-079
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 its current-mode architecture, the ADP1882/ADP1883
require Type II compensation. To determine the component values
needed for compensation (resistance and capacitance values),
it is necessary to examine the overall loop gain (H) of the con-
verter at the unity gain frequency (f
SW
/10) when H = 1 V/V,
as follows:
H = 1 V/V = G
M
× A
CS
×
REF
OUT
V
V
× Z
COMP
× Z
FILT
Examining each variable at high frequency enables the unity-
gain transfer function to be simplified to provide expressions
for the R
COMP
and C
COMP
component values.
Output Filter Impedance (Z
FILT
)
Examining the transfer function of the filter at high frequencies
simplifies to
Z
FILT
=
OUT
sC
1
at the crossover frequency (s = 2πf
CROSS
).
Error Amplifier Output Impedance (Z
COMP
)
Assuming C
C2
is significantly smaller than C
COMP
, C
C2
can be
omitted from the output impedance equation of the error
amplifier. The transfer function simplifies to
CROSS
ZERO
CROSSCOMP
COMP
f
ffR
Z
)( +
=
and
SWCROSS
ff ×=
12
1
where f
ZERO
, the zero frequency, is set to be 1/4 of the crossover
frequency for the ADP1882.
Error Amplifier Gain (G
M
)
The error amplifier gain (transconductance) is
G
M
= 500 µA/V
Current-Sense Loop Gain (G
CS
)
The current-sense loop gain is
ONCS
CS
RA
G
×
=
1
(A/V)
where:
A
CS
(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 Val le y
Current-Limit Setting sections).
R
ON
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 ADP1882, it is recommended that the
user set the crossover frequency between 1/10 and 1/15 of the
switching frequency.
SWCROSS
ff
12
1
=
The relationship between C
COMP
and f
ZERO
(zero frequency) is as
follows:
COMPCOMP
ZERO
CR
f
××π
=
2
1
)
The zero frequency is set to 1/4 of the crossover frequency.
Combining all of the above parameters results in
REF
OUT
CS
M
OUT
CROSS
ZERO
CROSS
CROSS
COMP
V
V
AG
Cf
ff
f
R
×
π
×
+
=
2
ZERO
COMP
COMP
fR
C
××π×
=
2
1