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MAX16929
Automotive TFT-LCD Power Supply with Boost
Converter and Gate Voltage Regulators
? V ESR INP × (1+
) × R ESR
=I
Boost Converter
Inductor Selection
Three key inductor parameters must be specified for
operation with the device: inductance value (L), induc-
tor saturation current (I SAT ), and DC resistance (R DC ).
To determine the inductance value, select the ratio of
inductor peak-to-peak ripple current to average input
current (LIR) first. For LIR values that are too high, the
The output voltage contains a ripple component whose
peak-to-peak value depends on the value of the ESR and
capacitance of the output capacitor and is approximately
given by:
D V RIPPLE = D V ESR + D V CAP
LIR
2
RMS currents are high, and therefore I 2 R losses are high.
Use high-valued inductors to achieve low LIR values.
Typically, inductance is proportional to resistance for a
? V CAP =
I O × D
C OUT × f SW
and I INP = O O
V INA × D V × I
η V INA
I INP × ? 1+
LIR × I INP
I RMS = for the input capacitor
R TOP = R BOTTOM × (
V O
given package type, which again makes I 2 R losses high
for very low LIR values. A good compromise between
size and loss is to select a 30%-to-60% peak-to-peak
ripple current to average-current ratio. If extremely thin
high-resistance inductors are used, as is common for
LCD-panel applications, the best LIR can increase
between 0.5 and 1.0. The size of the inductor is deter-
mined as follows:
L=
LIR × I INP × f SW η V INA
D =1 ?
V O
where V INA is the input voltage, V O is the output voltage,
I O is the output current, I INP is the average boost input
current, E is the efficiency of the boost converter, D is the
duty cycle, and f SW is 2.2MHz (the switching frequency
of the boost converter). The efficiency of the boost
converter can be estimated from the Typical Operating
Characteristics and accounts for losses in the internal
switch, catch diode, inductor R DC , and capacitor ESR.
Capacitor Selection
The input and output filter capacitors should be of a low-
ESR type (tantalum, ceramic, or low-ESR electrolytic) and
should have I RMS ratings greater than:
12
where I INP and D are the input current and duty cycle
given above.
Rectifier Diode
The catch diode should be a Schottky type to minimize
its voltage drop and maximize efficiency. The diode must
be capable of withstanding a reverse voltage of at least
V SH . The diode should have an average forward current
rating greater than:
I D = I INP × (1-D)
where I INP and D are the input current and duty cycle
given above. In addition ensure that the peak current rat-
ing of the diode is greater than:
? LIR ?
?
? 2 ?
Output-Voltage Selection
The output voltage of the boost converter can be adjust-
ed by using a resistive voltage-divider formed by R TOP
and R BOTTOM . Connect R TOP between the output and
FBP and connect R BOTTOM between FBP and GND.
Select R BOTTOM in the 10k I to 50k I range. Calculate
R TOP with the following equation:
? 1)
V FBP
where V FBP , the boost converter’s feedback set point, is
1V. Place both resistors as close as possible to the device
and connect R BOTTOM to the analog ground plane.
LIR 2
I RMS =I O
D+
12
1 ? D
for the output capacitor
Loop Compensation
Choose R COMPV to set the high-frequency integrator
gain for fast transient response. Choose C COMPV to set
the integrator zero to maintain loop stability. For low-ESR
where I INP and D are the input current and duty cycle
given above.
output capacitors, use Table 3 to select the initial values
for R COMPV and C COMPV . Use a 22pF capacitor in paral-
lel with R COMPV + C COMPV .
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