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ADP5071ACPZ-R7 数据表(PDF) 22 Page - Analog Devices

部件名 ADP5071ACPZ-R7
功能描述  Independent Positive and Negative Outputs
PDF  28 Pages
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制造商  AD [Analog Devices]
网页  http://www.analog.com
标志 AD - Analog Devices

ADP5071ACPZ-R7 数据表(HTML) 22 Page - Analog Devices

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Data Sheet
ADP5071
Rev. A | Page 21 of 27
For low ESR output capacitance such as with a ceramic capacitor,
CB1 is optional. For optimal transient performance, RC1 and CC1
may need to be adjusted by observing the load transient response
of the ADP5071. For most applications, RC1 must be within the
range of 1 kΩ to 200 kΩ, and CC1 must be within the range of
1 nF to 68 nF.
Inverting Regulator
The inverting converter, like the boost converter, produces an
undesirable right half plane zero in the regulation feedback loop.
This feedback loop requires compensating 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:
2
2
2
LOAD2
Z2
DUTY
L2
π
)
DUTY
(
R
(RHP)
f
×
×
−
=
2
1
where:
fZ2(RHP) is the right half plane zero frequency.
RLOAD2 is the equivalent load resistance or the output voltage
divided by the load current.


+
+
+
=
DIODE2
NEG
IN
DIODE2
NEG
2
V
|
|V
V
V
|
|V
DUTY
where VDIODE2 is the forward voltage drop of the Schottky diode
(D2).
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 inverting regulator loop gain is
OUT2
CS2
COMP2
OUT2
M2
NEG
IN
IN
NEG
FB2
VL2
Z
G
Z
||
R
G
V
V
V
|
|V
V
A
×
×
×
×
×
+
×
=
|)
|
2
(
where:
AVL2 is the loop gain.
VFB2 is the feedback regulation voltage.
VNEG is the regulated negative output voltage.
VIN is the input voltage.
GM2 is the error amplifier transconductance gain.
ROUT2 is the output impedance of the error amplifier and is 33 MΩ.
ZCOMP2 is the impedance of the series RC network from COMP2
to AGND.
GCS2 is the current sense transconductance gain (the inductor
current divided by the voltage at COMP2), which is internally
set by the ADP5071 and is 12.5 A/V.
ZOUT2 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 (ZCOMP2) is
dominated by a resistor, RC2, and the output impedance (ZOUT2)
is dominated by the impedance of the output capacitor, COUT2.
Therefore, when solving for the crossover frequency, the equation
(by definition of the crossover frequency) is simplified to
1
2
1
2
|)
|
2
(
=
×
×
×
×
×
×
×
+
×
=
OUT2
C2
CS
C2
M2
NEG
IN
IN
NEG
FB2
VL2
C
f
π
G
R
G
V
V
V
|
|V
V
A
where fC2 is the crossover frequency.
To solve for RC2, use the following equation:
CS2
M2
IN
FB2
NEG
IN
NEG
OUT2
C2
C2
G
G
V
V
V
(V
|
|V
C
f
π
R
×
×
×
×
+
×
×
×
×
=
|)
|
2
(
2
where GCS2 = 12.5 A/V.
Using typical values for VFB2 and GM2 results in
IN
NEG
IN
NEG
OUT2
C2
C2
V
V
V
V
C
f
R
|)
|
2
(
(
|
|
2094
×
+
×
×
×
×
=
For better accuracy, it is recommended to use the value of output
capacitance, COUT2, expected under the dc bias conditions that it
operates under in the calculation for RC2.
After the compensation resistor is known, set the zero formed
by the CC2 and RC2 to one-fourth of the crossover frequency, or
C2
C2
C2
R
f
π
C
×
×
=
2
where CC2 is the compensation capacitor.
ERROR
AMPLIFIER
REF2
gM2
FB2
COMP2
RC2
CB2
CC2
Figure 46. Compensation Component
The capacitor, CB2, is chosen to cancel the zero introduced by
output capacitance, ESR.
Solve for CB2 as follows:
C2
OUT2
B2
R
C
ESR
C
×
=
For low ESR output capacitance, such as with a ceramic capacitor,
CB2 is optional. For optimal transient performance, RC2 and CC2
may need to be adjusted by observing the load transient response
of the ADP5071. For most applications, RC2 must be within the
range of 1 kΩ to 200 kΩ, and CC2 must be within the range of
1 nF to 68 nF.



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