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ADP5076ACBZ-R7 Datasheet(PDF) 19 Page - Analog Devices

Part # ADP5076ACBZ-R7
Description  2 A/1.2 A DC-to-DC Switching Regulator with Independent Positive and Negative Outputs
PDF  23 Pages
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

ADP5076ACBZ-R7 Datasheet(HTML) 19 Page - Analog Devices

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Data Sheet
ADP5076
Rev. A | Page 19 of 23
Boost Regulator
The boost converter produces an undesirable right half plane
zero in the regulation feedback loop. This feedback loop requires
compensating the regulator so that the crossover frequency is
less than the frequency of the right half plane zero. The right
half plane zero frequency is determined by the following
equation:
2
(1
)
()
2
1
LOAD1
Z1
R
DUTY
fRHP
L1
where:
fZ1(RHP) is the right half plane zero frequency.
RLOAD1 is the equivalent resistor load for boost regulator, which
is also equal to the output voltage divided by the load current.
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 boost regulator loop gain is
FB1
IN
VL1
M1
OUT1
COMP1
CS1
OUT1
POS
POS
VV
AG
R
||Z
G
Z
VV

where:
AVL1 is the loop gain.
VFB1 is the feedback regulation voltage.
VPOS is the regulated positive output voltage.
VIN is the input voltage.
GM1 is the error amplifier transconductance gain.
ROUT1 is the output impedance of the error amplifier and is 33 MΩ.
ZCOMP1 is the impedance of the series RC network from the
COMP1 pin to the AGND pin.
GCS1 is the current sense transconductance gain (the inductor
current divided by the voltage at the COMP1 pin), which is
internally set by the ADP5076 and is 12.5 A/V.
ZOUT1 is the impedance of the load in parallel with the output
capacitor.
At crossover frequency (fC1), the ZCOMP1 is dominated by a resistor
(RC1), and the ZOUT1 is dominated by the impedance of an output
capacitor (COUT1). Therefore, when solving for the fC1, the equation
(by definition of the crossover frequency) is simplified to
1
2
1
OUT1
C1
CS1
C1
M1
POS
IN
POS
FB1
VL1
C
f
π
G
R
G
V
V
V
V
A
To solve for RC1, use the following equation:
2
2)
C1
OUT1
POS
C1
FB1
IN
M1
CS1
fC
(V
R
VV
G
G


where GCS1 = 12.5 A/V.
Using typical values for VFB1 and GM1 (see the Specifications
section) results in
2
2094
)
C1
OUT1
POS
C1
IN
fC
(V
R
V

For better accuracy, it is recommended to use the COUT1 value
expected under the dc bias conditions that the COUT1 value
operates under in the calculation for RC1.
After the compensation resistor is known, set the zero formed
by the compensation capacitor and resistor to one-fourth of the
crossover frequency, or
2
C1
C1
C1
C
π fR

where CC1 is the compensation capacitor value.
ERROR
AMPLIFIER
REF1
gM1
FB1
COMP1
RC1
CC1
Figure 44. Compensation Components
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 so that
the crossover frequency is less than the frequency of the right half
plane zero. The right half plane zero frequency is determined by
the following equation:
(1
)
2
2
2
LOAD2
Z2
2
R
DUTY
f (RHP)
π L2 DUTY

where:
fZ2(RHP) is the right half plane zero frequency.
RLOAD2 is the equivalent resistor load for inverting regulator,
which is also equal to the output voltage divided by the load
current.
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
(2 |
|)
FB2
IN
VL2
M2
NEG
IN
NEG
OUT2
COMP2
CS2
OUT2
VV
AG
|V
|
V
V
R||Z
G
Z



where:
AVL2 is the loop gain.
VFB2 is the FB2 reference 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.



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