add8754dacpz-reel7 Analog Devices, Inc., add8754dacpz-reel7 Datasheet - Page 15

• Manufacturer Part Number: add8754dacpz-reel7
• Description: Lcd Panel Power, Vcom, And Gate Modulation
• Manufacturer: Analog Devices, Inc.

To stabilize the regulator, make sure that the regulator crossover
frequency is less than or equal to one-fifth of the right-half
plane zero and less than or equal to one-fifteenth of the
switching frequency.
The regulator loop gain is
where:
A
V
V
V
G
Z
GND.
G
current divided by the voltage at COMP), which is internally set
by the ADD8754.
Z
To determine the crossover frequency, it is important to note
that at that frequency the compensation impedance (Z
dominated by the resistor and the output impedance (Z
dominated by the impedance of the output capacitor. Therefore,
when solving for the crossover frequency, (by definition of the
crossover frequency) the equation is simplified to
where:
f
R
Solving for R
Table 12. Recommended External Components for Various Input/Output Voltage Conditions
V
5
5
5
5
3.3
3.3
3.3
3.3
C
A
COMP
OUT
C
IN
VL
FB
OUT
IN
MEA
CS
is the crossover frequency.
VL
is the compensation resistor.
(V)
is the feedback regulation voltage, 1.210 V.
is the input voltage.
is the loop gain.
is the current sense transconductance gain (the inductor
is the impedance of the load and output capacitor.
A
is the regulated output voltage.
is the error amplifier transconductance gain.
R
=
is the impedance of the series RC network from COMP to
VL
C
V
V
OUT
=
FB
=
2
V
9
9
12
12
9
9
12
12
V
π
V
×
OUT
C
OUT
×
V
,
FB
V
V
FB
f
(V)
OUT
IN
C
×
×
×
V
V
×
C
V
IN
OUT
G
OUT
IN
f
650 kHz
1.2 MHz
650 kHz
1.2 MHz
650 kHz
1.2 MHz
650 kHz
1.2 MHz
MEA
×
SW
G
×
×
×
V
MEA
G
R
OUT
MEA
C
×
×
G
×
G
×
V
CS
CS
L (μH)
10
4.7
10
4.7
10
4.7
10
4.7
Z
OUT
×
COMP
2
π
×
×
f
C
G
1
×
CS
C
10
10
10
10
10
10
10
10
C
OUT
OUT
×
Z
(μF)
=
OUT
COMP
OUT
1
) is
) is
(16)
(14)
(15)
C
10
10
10
10
10
10
10
10
Rev. 0 | Page 15 of 28
IN
(μF)
R
63.4
63.4
88.7
88.7
63.4
63.4
88.7
88.7
1
For V
Once the compensation resistor is known, set the zero formed
by the compensation capacitor and resistor to one-fourth of the
crossover frequency, or
where C
The capacitor C2 is chosen to cancel the zero introduced by
output capacitance ESR.
Solving for C2,
For low ESR output capacitance, such as with a ceramic capaci-
tor, C2 is optional. For optimal transient performance, the R
and C
transient response of the ADD8754. For most applications, the
compensation resistor should be in the range of 30 kΩ to
400 kΩ, and the compensation capacitor should be in the range
of 100 pF to 1.2 nF. Table 12 shows external component values
for several applications.
(kΩ)
C
C2
R
FB
C
C
C
might need to be adjusted by observing the load
= 1.21 V, G
C
=
=
=
is the compensation capacitor.
R
10
10
10
10
10
10
10
10
ESR
. 2
π
2
×
55
(kΩ)
f
R
×
×
C
FB
2
Figure 19. Compensation Components
REF
C
10
C
×
OUT
R
MEA
4
C
×
= 100 μs, and G = 2 sec,
ERROR AMP
f
R
84.5
178
140
300
71.5
150
130
280
C
C
×
(kΩ)
G
V
MEA
C
IN
OUT
×
V
OUT
C
390
100
220
100
820
180
420
100
R
CS
c
C
C
×
(pF)
C
V
OUT
C2
ADD8754
I
450
450
350
350
350
350
250
250
OUT_MAX
(19)
(mA)
(17)
(18)
C