User Manual

MAX17075
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, V
SUP
is the supply voltage of the 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
= 600mV.
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.
Flying Capacitors
Increasing the flying capacitor C
X
(connected to DRVN
and DRVP) value lowers the effective 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 applications. 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:
where C
OUT
_
CP
is the output capacitor of the charge
pump, I
LOAD
_
CP
is the load current of the charge
pump, and V
RIPPLE_CP
is the peak-to-peak value of the
output ripple, and f
OSC
is the switching frequency.
Output Voltage Selection
Adjust the positive charge-pump regulator’s output volt-
age by connecting a resistive voltage-divider from the
REG P output to GND with the center tap connected to
FBP (Figure 1). Select the lower resistor of divider R16
in the 10kΩ to 30kΩ range. Calculate the upper resistor
R15 with the following equation:
where V
FBP
= 1.25V (typical).
Adjust the negative charge-pump regulator’s output
voltage by connecting a resistive voltage-divider from
V
GOFF
to REF with the center tap connected to FBN
(Figure 1). Select R6 in the 35kΩ to 68kΩ range.
Calculate R7 with the following equation:
where V
FBN
= 250mV, V
REF
= 1.25V. Note that REF can
only source up to 50µA, using a resistor less than 35kΩ
for R6 results in higher bias current than REF can supply.
Set the XAO Threshold Voltage
XAO threshold voltage can be adjusted by connecting
a resistive voltage-divider from input V
IN
to GND with
the center tap connected to RSTIN (see Figure 1).
Select R12 in the 10kΩ to 50kΩ range. Calculate R11
with the following equation:
where V
RSTIN
, the RSTIN threshold set point, is 1.25V.
V
INXAO
is the desired XAO threshold voltage. Place
R11 and R12 close to the IC.
RR
V
V
INXAO
RSTIN
11 12 1=× −
⎛
⎝
⎜
⎞
⎠
⎟
RR
VV
VV
FBN GOFF
REF FBN
76=×
−
−
RR
V
V
GON
FBP
15 16 1=× −
⎛
⎝
⎜
⎞
⎠
⎟
C
I
fV
OUT CP
LOAD CP
OSC RIPPLE CP
_
_
_
≥
2
VnV
CX SUP
>×
η
NEG
GOFF DROPOUT
SUP D
VV
VV
=
−+
−×2
η
POS
GON DROPOUT AVDD
SUP D
VV V
VV
=
+−
−×2
Boost Regulator with Integrated Charge Pumps,
Switch Control, and High-Current Op Amp
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