ADP1879 Analog Devices, ADP1879 Datasheet - Page 25

• Manufacturer Part Number: ADP1879
• Description: Synchronous Buck Controller with Constant On-Time and Valley Current Mode with Power Saving Mode
• Manufacturer: Analog Devices

Data Sheet
Ceramic capacitors are known to have low ESR. However, there
is a trade-off in using the popular X5R capacitor technology
because as much as 80% of its capacitance may be lost due to
derating as the voltage applied across the capacitor is increased
(see Figure 82). Although X7R series capacitors can also be
used, the available selection is limited to 22 μF maximum.
Electrolytic capacitors satisfy the bulk capacitance requirements
for most high current applications. However, because the ESR of
electrolytic capacitors is much higher than that of ceramic capaci-
tors, mount several MLCCs in parallel with the electrolytic
capacitors to reduce the overall series resistance.
COMPENSATION NETWORK
Due to its current-mode architecture, the
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
converter at the unity-gain frequency (f
Examining each variable at high frequency enables the unity-
gain transfer function to be simplified to provide expressions
for the R
Output Filter Impedance (Z
Examining the transfer function of the filter at high frequencies
simplifies to
at the crossover frequency (s = 2πf
series resistance of the output capacitors.
Figure 82. Capacitance vs. DC Voltage Characteristics for Ceramic Capacitors
–100
–10
–20
–30
–40
–50
–60
–70
–80
–90
20
10
0
COMP
0
1 V V
10µF TDK 25V, X7R, 1210 C3225X7R1E106M
22µF MURATA 25V, X7R, 1210 GRM32ER71E226KE15L
47µF MURATA 16V, X5R, 1210 GRM32ER61C476KE15L
⁄
and C
5
COMP
1
1
X5R (16V)
component values.
10
DC VOLTAGE (V
X7R (50V)
FILT
X5R (25V)
15
)
CROSS
SW
DC
). ESR is the equivalent
/10) when H = 1 V/V:
20
)
ADP1878/ADP1879
25
30
Rev. 0 | Page 25 of 40
Error Amplifier Output Impedance (Z
Assuming C
omitted from the output impedance equation of the error
amplifier. The transfer function simplifies to
and
where f
frequency for the ADP1878.
Error Amplifier Gain (G
The error amplifier gain (transconductance) is
Current-Sense Loop Gain (G
The current-sense loop gain is
where:
A
(see the Programming Resistor (RES) Detect Circuit and Valley
Current-Limit Setting sections).
R
Crossover Frequency
The crossover frequency is the frequency at which the overall
loop (system) gain is 0 dB (H = 1 V/V). It is recommended for
current-mode converters, such as the ADP1878, that the user set
the crossover frequency between 1/10
frequency.
The relationship between C
follows:
The zero frequency is set to 1/4
Combining all of the above parameters results in
where ESR is the equivalent series resistance of the output
capacitors.
ON
CS
(V/V) is programmable for 3 V/V, 6 V/V, 12 V/V, and 24 V/V
is the channel impedance of the low-side MOSFET.
G
m
ZERO
= 500 μA/V (μs)
, the zero frequency, is set to be 1/4
C2
2
12
2
12
is significantly smaller than C
1
1
1
1
1
1
⁄
m
)
COMP
1
th
CS
1
and f
)
of the crossover frequency.
ADP1878/ADP1879
th
ZERO
and 1/15
COMP
(zero frequency) is as
COMP
)
th
th
of the switching
of the crossover
, C
C2
can be
1