Datasheet

MAX17113
Low-Cost, Multiple-Output
Power Supply for LCD TVs
______________________________________________________________________________________ 27
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 MAX17113’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 2A Schottky
diode complements the internal MOSFET well.
Output-Voltage Selection
The output voltage of the step-up regulator can be
adjusted by connecting a resistive voltage-divider from
the output (V
AVDD
) to AGND with the center tap con-
nected to FB1 (see Figure 1). Select R4 in the 10kΩ to
50kΩ range. Calculate R3 with the following equation:
where V
FB1
, the step-up regulator’s feedback set point,
is 1.25V. Place R4 and R3 close to the IC.
Loop Compensation
Choose R
COMP
(R5 in Figure 1) to set the high-frequen-
cy integrator gain for fast transient response. Choose
C
COMP
(C17 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 number
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, V
SWO
is the supply voltage of the positive
charge-pump regulators, V
D
is the forward voltage drop
of the charge-pump diode, and V
DROPOUT
is the dropout
margin for the regulator. Use V
DROPOUT
= 300mV.
The number of negative charge-pump stages is given by:
where n
NEG
is the number of negative charge-pump
stages and V
GOFF
is the output of the negative charge-
pump regulator.
The above equations are derived based on the
assumption that the first stage of the positive charge
pump is connected to V
AVDD
and the first stage of the
negative charge pump is connected to ground.
Sometimes fractional stages are more desirable for bet-
ter efficiency. This can be done by connecting the first
stage to V
OUT
or another available supply. If the first
charge-pump stage is powered from V
OUT
, then the
above equations become:
Flying Capacitors
Increasing the capacitance of the flying capacitors
(connected to DRVN and DRVP) value lowers the effec-
tive source impedance and increases the output-current
capability. Increasing the capacitance indefinitely has a
negligible effect on output-current capability because
the internal switch resistance and the diode impedance
place a lower limit on the source impedance. A 0.1μF
ceramic capacitor works well in most low-current appli-
cations. The flying capacitor’s voltage rating must
exceed the following:
where n is the stage number in which the flying capaci-
tor appears.
Charge-Pump Output Capacitor
Increasing the output capacitance or decreasing the
ESR reduces the output ripple voltage and the peak-to-
peak transient voltage. With ceramic capacitors, the
output-voltage ripple is dominated by the capacitance
value. Use the following equation to approximate the
required capacitor value:
C
I
fV
OUT CP
LOAD CP
OSC RIPPLE CP
_
_
_
≥
2
VnV
CX SWO
>×
n
VV V
VV
NEG
GOFF DROPOUT OUT
VIN D
=
++
×
-
-2
n
VV V
VV
POS
GON DROPOUT OUT
SWO D
=
+
×
-
-2
n
VV
VV
NEG
GOFF DROPOUT
VIN D
=
+
×
-
-2
n
VV V
VV
POS
GON DROPOUT AVDD
SWO D
=
+
×
-
-2
C
VC
IR
COMP
AVDD AVDD
AVDD MAX COMP
≈
×
××1250
()
R
VV C
LI
COMP
VIN AVDD AVDD
AVDD AVDD MAX
≈
×× ×
×
125
()
RR
V
V
AVDD
FB
34 1
1
=×
⎛
⎝
⎜
⎞
⎠
⎟
-