Rainbow Electronics MAX1748 - Manual (Page 11)

MAX1748
Triple-Output TFT LCD DC-DC Converter
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converter reaches regulation, the negative charge
pump turns on. When the negative output voltage
reaches approximately 88% of its nominal value (V
FBN
< 110mV), the positive charge pump starts up. 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 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 125
Ω on-resis-
tance. If a fault is detected, the internal open-drain
MOSFET appears as a high impedance. Connect a
100k
Ω pull-up 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
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
Typical Operating Circuit). Select R2 in the 10k
Ω to
20k Ω 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 may 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:
The inductance value is then given by:
Considering the typical application circuit, the maxi-
mum DC load current (I
MAIN(MAX) ) is 200mA with a 10V
output. A 6.8µH inductance value is then chosen,
based on the above equations and using 85% efficien-
cy and a 1MHz operating frequency. Smaller induc-
tance values typically offer a smaller physical size for a
given series resistance and current rating, allowing the
smallest overall circuit dimensions. However, due to
higher peak inductor currents, the output voltage ripple
(I
PEAK
✕ output filter capacitor ESR) will be higher.
Use inductors with a ferrite core or equivalent; powder
iron cores are not recommended for use with the
MAX1748’s high switching frequencies. The inductor’s
maximum current rating should exceed I
PEAK . Under
fault conditions, inductor current may reach up to 2.0A.
L
V Efficiency (V V )
V LIR I f
IN(MIN)
2
MAIN IN(MIN)
(MAIN)
2
MAIN(MAX) OSC
=
Ч Ч −
Ч Ч Ч
I
I V
Efficiency V
1 (LIR/2)
PEAK
MAIN(MAX) MAIN
IN(MIN)
=
Ч
Ч
Ч +
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