MAX8728ETJ+T Maxim Integrated Products, MAX8728ETJ+T Datasheet - Page 25

• Manufacturer Part Number: MAX8728ETJ+T
• Description: Display Drivers Low-Cost Multiple-Ou tput Power Supply fo
• Manufacturer: Maxim Integrated Products

Specifications of MAX8728ETJ+T

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

output voltage ripple is typically dominated by
V
characteristics of the output capacitor must also be
considered.
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.
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.
The output voltage of the step-up regulator is adjusted
by connecting a resistive voltage-divider from the out-
put (V
FB2 (see Figure 1). Select R2 in the 10kΩ to 50kΩ
range. Calculate R1 with the following equation:
where V
is 2.0V. Place R1 and R2 close to the IC.
Choose R
cy integrator gain for fast-transient response. Choose
C
maintain loop stability.
For low-ESR output capacitors, use the following equa-
tions to obtain stable performance and good transient
response:
AVDD
COMP
R
C
AVDD
COMP
_
COMP
RIPPLE(C)
FB2
(C13 in Figure 1) to set the integrator zero to
COMP
) to GND with the center tap connected to
, the step-up regulator’s feedback set point,
≈
≈
R
(R3 in Figure 1) to set the high-frequen-
1
250
. The voltage rating and temperature
20
______________________________________________________________________________________
=
×
L
×
R
AVDD
V
2
V
IN
I
AVDD MAX
AVDD
×
×



Input-Capacitor Selection
Output-Voltage Selection
×
V
(
V
Power Supply for LCD Monitors/TVs
V
AVDD
AVDD
×
FB
I
AVDD MAX
2
Loop Compensation
C
)
AVDD
×
−
(
×
1
R
Rectifier Diode



C
COMP
AVDD
)
Low-Cost, Multiple-Output
To further optimize transient response, vary R
20% steps and C
transient response waveforms.
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
stages, V
regulator, I
rent, V
regulators, V
charge-pump diode, and R
resistance of the charge-pump switches (10Ω typ.)
The number of negative charge-pump stages is given by:
where n
stages, V
pump regulator, and I
pump output current.
The above equations assume that the flying capacitors
are large enough to not further limit the output current.
Increasing the flying capacitor (C
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:
where n is the stage number in which the flying capaci-
tor appears.
n
n
POS
NEG
Selecting the Number of Charge-Pump Stages
SUPP
NEG
POS
GON
GOFF
GON
=
=
is the supply voltage of the charge-pump
is the number of negative charge-pump
is the number of positive charge-pump
V
is the output of the positive charge-pump
D
V
SUPP
SUPP
is the positive charge-pump output cur-
is the output of the negative charge-
is the forward voltage drop of the
COMP
V
CX
−
−
Charge-Pump Regulators
V
(
GOFF
> n x V
2
(
GON
in 50% steps while observing
2
×
x V
−
EFF
V
V
GOFF
D
is the negative charge-
−
D
SUPP
)
)
is the effective output
−
V
−
SUPP
(
I
(
X
I
GOFF
GON
) value lowers the
Flying Capacitors
x R
x R
EFF
EFF
COMP
)
)
25
in