Rainbow Electronics MAX17075 - 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
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