Rainbow Electronics MAX8728 User Manual
Page 25
MAX8728
Low-Cost, Multiple-Output
Power Supply for LCD Monitors/TVs
______________________________________________________________________________________
25
output voltage ripple is typically dominated by
V
AVDD
_
RIPPLE(C)
. The voltage rating and temperature
characteristics of the output capacitor must also be
considered.
Input-Capacitor Selection
The input capacitor reduces the current peaks drawn
from the input supply and reduces noise injection into
the IC. Two 10µF ceramic capacitors are used in the
Typical Applications Circuit (Figure 1) because of the
high-source impedance seen in typical lab setups.
Actual applications usually have much lower source
impedance since the step-up regulator often runs
directly from the output of another regulated supply.
Typically, the input capacitance can be reduced below
the values used in the Typical Operating Circuit.
Rectifier Diode
The MAX8728’s high-switching frequency demands a
high-speed rectifier. Schottky diodes are recommend-
ed for most applications because of their fast recovery
time and low forward voltage. In general, a 1A to 2A
Schottky diode complements the internal MOSFET well.
Output-Voltage Selection
The output voltage of the step-up regulator is adjusted
by connecting a resistive voltage-divider from the out-
put (V
AVDD
) to GND with the center tap connected to
FB2 (see Figure 1). Select R2 in the 10k
Ω to 50kΩ
range. Calculate R1 with the following equation:
where V
FB2
, the step-up regulator’s feedback set point,
is 2.0V. Place R1 and R2 close to the IC.
Loop Compensation
Choose R
COMP
(R3 in Figure 1) to set the high-frequen-
cy integrator gain for fast-transient response. Choose
C
COMP
(C13 in Figure 1) to set the integrator zero to
maintain loop stability.
For low-ESR output capacitors, use the following equa-
tions to obtain stable performance and good transient
response:
To further optimize transient response, vary R
COMP
in
20% steps and C
COMP
in 50% steps while observing
transient response waveforms.
Charge-Pump Regulators
Selecting the Number of Charge-Pump Stages
For highest efficiency, always choose the lowest num-
ber of charge-pump stages that meet the output
requirement.
The number of positive charge-pump stages is given by:
where n
POS
is the number of positive charge-pump
stages, V
GON
is the output of the positive charge-pump
regulator, I
GON
is the positive charge-pump output cur-
rent, V
SUPP
is the supply voltage of the charge-pump
regulators, V
D
is the forward voltage drop of the
charge-pump diode, and R
EFF
is the effective output
resistance of the charge-pump switches (10
Ω typ.)
The number of negative charge-pump stages is given by:
where n
NEG
is the number of negative charge-pump
stages, V
GOFF
is the output of the negative charge-
pump regulator, and I
GOFF
is the negative charge-
pump output current.
The above equations assume that the flying capacitors
are large enough to not further limit the output current.
Flying Capacitors
Increasing the flying capacitor (C
X
) value lowers the
effective source impedance and increases the output
current capability. Increasing the capacitance indefi-
nitely has a negligible effect on output current capabili-
ty because the internal switch resistance and the diode
impedance place a lower limit on the source imped-
ance. A 0.1µF ceramic capacitor works well, except in
cases of low frequency, low headroom, and high cur-
rent. The flying capacitor’s voltage rating must exceed
the following:
V
CX
> n x V
SUPP
where n is the stage number in which the flying capaci-
tor appears.
n
V
V
x V
I
x R
NEG
GOFF
SUPP
D
GOFF
EFF
(
)
(
)
=
−
−
−
2
n
V
V
V
V
I
x R
POS
GON
SUPP
SUPP
D
GON
EFF
(
)
(
)
=
−
−
×
−
2
R
V
V
C
L
I
C
V
C
I
R
COMP
IN
AVDD
AVDD
AVDD
AVDD MAX
COMP
AVDD
AVDD
AVDD MAX
COMP
(
)
(
)
≈
Ч
Ч
Ч
Ч
≈
Ч
Ч
Ч
250
20
R
R
V
V
AVDD
FB
1
2
1
2
=
Ч
−
⎛
⎝⎜
⎞
⎠⎟