MAX16929_12 Datasheet, PDF (16/25 Pages) Maxim Integrated Products – Automotive TFT-LCD Power Supply with Boost Converter and Gate Voltage Regulators

MAX16929
Automotive TFT-LCD Power Supply with Boost
Converter and Gate Voltage Regulators
Design Procedure
Buck 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 output
current (LIR) first. For LIR values that are too high, the
RMS currents are high, and therefore I²R losses are high.
Use high-valued inductors to achieve low LIR values.
Typically, inductance is proportional to resistance for a
given package type, which again makes I²R 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 = ^{(VINB-VO) × D} and
LIR ×I_O × f_SWB
D = ^VO
η × V_INB
where V_INB is the input voltage, V_O is the output volt-
age, I_O is the output current, E is the efficiency of the
buck converter, D is the duty cycle, and f_SWB is 2.1MHz
(the switching frequency of the buck converter). The
efficiency of the buck 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.
To ensure the buck converter does not shut down
during load dump input-voltage transients to 42V, an
inductor value larger than calculated above should be
used. Table 2 lists the minimum inductance that should
be used for proper operation during load dump. The
saturation current rating (I_SAT) must be high enough to
ensure that saturation can occur only above the maxi-
mum current-limit value. Find a low-loss inductor having
the lowest possible DC resistance that fits in the allotted
dimensions.
Table 2. Minimum Buck Inductor Value
Required for Normal Operation During
Load Dump
BUCK V_OUTB (V)
3.3
3.3
5
5
BUCK I_OUTB (A)
1.2
2
1.2
2
L_MIN (µH)
3.3
6.8
3.3
4.7
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:
I_INB(RMS) = I_O
D × (1-D + ^LIR2 ) for the input capacitor
12
I_OUTB(RMS)
=
LIR ×I_O
12
for the output capacitor
where D is the duty cycle given above.
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 approxi-
mately given by:
DV_RIPPLE = DV_ESR + DV_CAP
DV_ESR = LIR x I_O x R_ESR
∆V_CAP
=
LIR ×I_O
8 × C × f_SWB
Diode Selection
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
the maximum input voltage in the application. The diode
should have an average forward current rating greater
than:
I_D = I_O × (1-D)
where D is the duty cycle given above. In addition, ensure
that the peak current rating of the diode is greater than:
I_OUTB
×
₁₊
LIR
2


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