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ADP5070 Fiches technique(PDF) 14 Page - Analog Devices

No de pièce ADP5070
Description  1.2 A, DC-to-DC Inverting Regulator
PDF  17 Pages
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Fabricant  AD [Analog Devices]
Site Internet  http://www.analog.com
Logo AD - Analog Devices

ADP5070 Fiches technique(HTML) 14 Page - Analog Devices

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ADP5073
Data Sheet
Rev. A | Page 14 of 17
Loop Compensation
The ADP5073 uses external components to compensate the
regulator loop, allowing the optimization of the loop dynamics
for a given application. It is recommended to use the ADIsimPower
tool to calculate compensation components.
The inverting converter, produces a right half plane zero in the
regulation feedback loop. This feedback loop requires compensat-
ing the regulator such that the crossover frequency occurs well
below the frequency of the right half plane zero. The right half
plane zero frequency is determined by the following equation:
Duty
L1
π
Duty)
(
R
(RHP)
f
2
LOAD
Z
×
×
−
=
2
1
where:
fZ (RHP) is the right half plane zero frequency.
RLOAD is the equivalent load resistance or the output voltage
divided by the load current.
+
+
+
=
DIODE
OUT
IN
DIODE
OUT
V
|
|V
V
V
|
|V
Duty
where VDIODE is the forward voltage drop of the Schottky diode
(D1).
To stabilize the regulator, ensure that the regulator crossover
frequency is less than or equal to one-tenth of the right half
plane zero frequency.
The regulator loop gain is
OUT
CS
COMP
OUT
M
OUT
IN
IN
OUT
FB
VL
Z
G
Z
||
R
G
V
V
V
|
|V
V
A
×
×
×
×
×
+
×
=
|)
|
2
(
where:
AVL is the regulator loop gain.
VFB is the feedback regulation voltage.
VOUT is the regulated negative output voltage.
VIN is the input voltage.
GM is the error amplifier transconductance gain.
ROUT is the output impedance of the error amplifier and is 33 MΩ.
ZCOMP is the impedance of the series RC network from COMP
to GND.
GCS is the current sense transconductance gain (the inductor
current divided by the voltage at COMP), which is internally
set by the ADP5073 and is 6.25 A/V.
ZOUT is the impedance of the load in parallel with the output
capacitor.
To determine the crossover frequency, it is important to note
that, at that frequency, the compensation impedance (ZCOMP) is
dominated by a resistor, RC, and the output impedance (ZOUT) is
dominated by the impedance of the output capacitor (COUT).
Therefore, when solving for the crossover frequency, the equation
(by definition of the crossover frequency) is simplified to
1
2
1
|)
|
2
(
=
×
×
×
×
×
×
×
+
×
=
OUT
C
CS
C
M
OUT
IN
IN
OUT
FB
VL
C
f
π
G
R
G
V
V
V
|
|V
V
A
where fC is the crossover frequency.
To solve for RC, use the following equation:
CS
M
IN
FB
OUT
IN
OUT
OUT
C
C
G
G
V
V
V
(V
|
|V
C
f
π
R
×
×
×
×
+
×
×
×
×
=
|)
|
2
(
2
where GCS = 6.25 A/V.
Using typical values for VFB and GM results in
IN
OUT
IN
OUT
OUT
C
C
V
V
V
V
C
f
R
|)
|
2
(
(
|
|
4188
×
+
×
×
×
×
=
For better accuracy, it is recommended to use the value of
output capacitance (COUT) that takes into account the capacitance
reduction from dc bias in the calculation for RC.
After the compensation resistor is known, set the zero formed
by CC and RC to one-fourth of the crossover frequency, or
C
C
C
R
f
π
C
×
×
=
2
where CC is the compensation capacitor.
ERROR
AMPLIFIER
REF
gM
FB
COMP
RC
CB
CC
Figure 24. Compensation Components
The optional capacitor, CB, is chosen to cancel the zero
introduced by the ESR of the output capacitor. For low ESR
capacitors such as ceramic chip capacitors, CB can be omitted
from the design.
Solve for CB as follows:
C
OUT
B
R
C
ESR
C
×
=
For optimal transient performance, RC and CC may need to be
adjusted by observing the load transient response of the ADP5073.
For most applications, RC is within the range of 1 kΩ to 200 kΩ,
and CC is within the range of 1 nF to 68 nF.



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