Rainbow Electronics MAX17075 User Manual
Page 20
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 10k
Ω 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 35k
Ω 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 35k
Ω
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 10k
Ω 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.
R
R
V
V
INXAO
RSTIN
11
12
1
=
×
−
⎛
⎝⎜
⎞
⎠⎟
R
R
V
V
V
V
FBN
GOFF
REF
FBN
7
6
=
×
−
−
R
R
V
V
GON
FBP
15
16
1
=
×
−
⎛
⎝⎜
⎞
⎠⎟
C
I
f
V
OUT CP
LOAD CP
OSC RIPPLE CP
_
_
_
≥
2
V
n V
CX
SUP
> ×
η
NEG
GOFF
DROPOUT
SUP
D
V
V
V
V
=
−
+
− ×
2
η
POS
GON
DROPOUT
AVDD
SUP
D
V
V
V
V
V
=
+
−
− ×
2
Boost Regulator with Integrated Charge Pumps,
Switch Control, and High-Current Op Amp
20
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