MAX1748/MAX8726
Triple-Output TFT-LCD
DC-DC Converters
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In the MAX1748, the reference powers up first, then the
main DC-DC step-up converter powers up with soft-
start enabled. Once the main step-up converter reach-
es regulation, the negative charge pump turns on.
When the negative output voltage reaches approxi-
mately 88% of its nominal value (V
FBN
< 110mV), the
positive charge pump starts up. Finally, when the posi-
tive output voltage reaches 90% of its nominal value
(V
FBP
> 1.125V), the active-low ready signal (RDY)
goes low (see the Power Ready section).
In the MAX8726, the reference powers up first. After the
reference is in regulation, the main DC-DC step-up con-
verter powers up with soft-start enabled. The negative
charge pump is enabled when the main step-up con-
verter reaches regulation, and at least 16ms (typ) after
the main step-up converter has been enabled. The posi-
tive charge pump is enabled when the negative output
voltage reaches approximately 88% of its nominal value
(V
FBN
< 110mV), and at least 4ms (typ) after the nega-
tive charge pump has been enabled. Finally, when the
positive output voltage reaches 90% of its nominal value
(V
FBP
> 1.125V), the active-low ready signal (RDY) goes
low (see the Power Ready section).
Power Ready
Power ready is an open-drain output. When the power-
up sequence is properly completed, the MOSFET turns
on and pulls RDY low with a typical 125Ω on-resis-
tance. If a fault is detected, the internal open-drain
MOSFET appears as a high impedance. Connect a
100kΩ pullup resistor between RDY and IN for a logic-
level output.
Fault Detection
Once RDY is low and if any output falls below its fault-
detection threshold, RDY goes high impedance.
For the reference, the fault threshold is 1.05V. For the
main boost converter, the fault threshold is 88% of its
nominal value (V
FB
< 1.1V). For the negative charge
pump, the fault threshold is approximately 90% of its
nominal value (V
FBN
< 130mV). For the positive charge
pump, the fault threshold is 88% of its nominal value
(V
FBP
< 1.11V).
Once an output faults, all outputs later in the power
sequence shut down until the faulted output rises
above its power-up threshold. For example, if the nega-
tive charge-pump output voltage falls below the fault-
detection threshold, the main boost converter remains
active while the positive charge pump stops switching
and its output voltage decays, depending on output
capacitance and load. The positive charge-pump out-
put will not power up until the negative charge-pump
output voltage rises above its power-up threshold (see
the Power-Up Sequencing section).
Voltage Reference
The voltage at REF is nominally 1.25V. The reference
can source up to 50µA with good load regulation (see
the Typical Operating Characteristics). Connect a
0.22µF bypass capacitor between REF and GND.
Design Procedure
Main Boost Converter
Output Voltage Selection
Adjust the output voltage by connecting a voltage-
divider from the output (V
MAIN
) to FB to GND (see the
Typical Operating Circuit). Select R2 in the 10kΩ to
20kΩ range. Higher resistor values improve efficiency
at low output current but increase output voltage error
due to the feedback input bias current. Calculate R1
with the following equations:
R1 = R2 [(V
MAIN
/ V
REF
) - 1]
where V
REF
= 1.25V. V
MAIN
can range from V
IN
to 13V.
Feedback Compensation
For stability, add a pole-zero pair from FB to GND in the
form of a series resistor (R
COMP
) and capacitor
(C
COMP
). The resistor should be half the value of the
R2 feedback resistor.
Inductor Selection
Inductor selection depends on input voltage, output
voltage, maximum current, switching frequency, size,
and availability of inductor values. Other factors can
include efficiency and ripple voltage. Inductors are
specified by their inductance (L), peak current (I
PEAK
),
and resistance (R
L
). The following boost-circuit equa-
tions are useful in choosing inductor values based on
the application. They allow the trading of peak current
and inductor value while allowing for consideration of
component availability and cost.
The following equation includes a constant LIR, which
is the ratio of the inductor peak-to-peak AC current to
maximum average DC inductor current. A good com-
promise between the size of the inductor, loss, and out-
put ripple is to choose an LIR of 0.3 to 0.5. The peak
inductor current is then given by:
