Rainbow Electronics MAX8758 - Manual (Page 17)

Step-Up Regulator Input Capacitor Selection
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 Applications Circuit.
Step-Up Regulator Rectifier Diode
The MAX8758’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.
Step-Up Regulator Output Voltage Selection
The output voltage of the step-up regulator can be
adjusted by connecting a resistive voltage-divider from
the output (V
OUT
) to GND with the center tap connect-
ed to FB (see Figure 1). Select R2 in the 10kΩ to 50kΩ
range. Calculate R1 with the following equation:
where V FB , the step-up regulator’s feedback set point,
is 1.25V. Place R1 and R2 close to the IC.
Step-Up Regulator Loop Compensation
Choose R COMP (R3 in Figure 1) to set the high-frequen-
cy integrator gain for fast transient response. Choose
C
COMP (C7 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.
Place C COMP2 (C8 in Figure 1) from COMP to GND to
add an additional high-frequency pole. UseC COMP2
between 10pF and 47pF.
Step-Up Regulator Soft-Start Capacitor
The soft-start capacitor should be large enough that it
does not reach final value before the output has
reached regulation. Calculate the soft-start capacitor
(C
SS ) value using:
where C MAIN is the total output capacitance, V MAIN is
the maximum output voltage, and I INRUSH is the peak
inrush current allowed, I MAIN is the maximum output
current, and V
IN
is the minimum input voltage.
The load must wait for the soft-start cycle to finish
before drawing a significant amount of load current.
The duration after which the load can begin to draw
maximum load current is:
t MAX = 6.77 x 10 5 x C SS
Charge Pumps
Selecting the Number of Charge-Pump Stages
For highest efficiency, always choose the lowest num-
ber of charge-pump stages that meet the output volt-
age 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 positive-charge-pump output,
V
MAIN
is the main step-up regulator output, and V
D
is
the forward voltage drop of the charge-pump diode.
The number of negative charge-pump stages is given by:
where n NEG is the number of negative-charge-pump
stages, V GOFF is the negative charge-pump output,
V
MAIN
is the main step-up regulator output, and V
D
is
the forward voltage drop of the charge-pump diode.
n
V
V V
NEG
GOFF
MAIN D
=
×
−
− 2
n
V V
V V
POS
GON MAIN
MAIN D
=
×
−
− 2
C C
V V V
V I I V
SS MAIN
MAIN IN MAIN
IN INRUSH MAIN MAIN
= Ч Ч
Ч
Ч
Ч Ч






−
−
−
21 10
6
2
C
V C
I R
COMP
MAIN MAIN
MAIN MAX COMP ( )
≈
Ч
Ч Ч 10
R
V V C
L I
COMP
IN MAIN MAIN
MAIN MAX ( )
≈
Ч Ч Ч
Ч
315
R R
V
V
MAIN
FB
1 2 1 = ×





 −
MAX8758
Step-Up Regulator with Switch Control and
Operational Amplifier for TFT LCD
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