MAX8727
Soft-Start
The MAX8727 can be programmed for soft-start upon
power-up with an external capacitor. When the shutdown
pin is taken high, the soft-start capacitor (C
SS
) is immedi-
ately charged to 0.4V. Then the capacitor is charged at a
constant current of 4.5µA (typ). During this time, the SS
voltage directly controls the peak inductor current, allow-
ing 0A at V
SS
= 0.4V to the full current limit at V
SS
= 1.5V.
The maximum load current is available after the soft-start
is completed. When the SHDN pin is taken low, the soft-
start capacitor is discharged to ground.
Frequency Selection
The MAX8727’s frequency can be user selected to
operate at either 640kHz or 1.2MHz. Connect FREQ to
GND for 640kHz operation. For a 1.2MHz switching fre-
quency, connect FREQ to IN. This allows the use of
small, minimum-height external components while
maintaining low output noise. FREQ has an internal
pulldown, allowing the user the option of leaving FREQ
unconnected for 640kHz operation.
Shutdown
The MAX8727 shuts down to reduce the supply current
to 0.1µA when SHDN is low. In this mode, the internal
reference, error amplifier, comparators, and biasing cir-
cuitry turn off, and the n-channel MOSFET is turned off.
The step-up regulator’s output is connected to IN by
the external inductor and rectifier diode.
Applications Information
Step-up regulators using the MAX8727 can be
designed by performing simple calculations for a first
iteration. All designs should be prototyped and tested
prior to production. Table 1 provides a list of power
components for the typical applications circuit. Table 2
lists component suppliers.
External-component-value choice is primarily dictated
by the output voltage and the maximum load current,
as well as maximum and minimum input voltages.
Begin by selecting an inductor value. Once L is known,
choose the diode and capacitors.
Inductor Selection
The minimum inductance value, peak current rating, and
series resistance are factors to consider when selecting
the inductor. These factors influence the converter’s effi-
ciency, maximum output load capability, transient-
response time, and output voltage ripple. Physical size
and cost are also important factors to be considered.
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very high inductance values minimize the cur-
rent ripple and therefore reduce the peak current,
which decreases core losses in the inductor and I
2
R
losses in the entire power path. However, large induc-
tor values also require more energy storage and more
turns of wire, which increase physical size and can
increase I
2
R losses in the inductor. Low inductance val-
ues decrease the physical size but increase the current
ripple and peak current. Finding the best inductor
involves choosing the best compromise between circuit
efficiency, inductor size and cost.
TFT-LCD Step-Up DC-DC Converter
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