MAX16929 Datasheet, PDF (17/25 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
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
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 = ^{VINA × D}
LIR ×I_INP × f_SW
and
I_INP
=
V_O ×I_O
ηV_INA
D =1− ^ηVINA
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:
I_RMS
=
LIR
× I_INP
12
for the input capacitor
D + ^LIR2
I_RMS = I_O
12 _{for the output capacitor}
1− D
where I_INP and D are the input current and 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 approximately
given by:
DV_RIPPLE = DV_ESR + DV_CAP
∆V_ESR
=
I_INP
×
(1+
LIR)
2
×
R_ESR
∆V_CAP
=
C
I_O ×D
OUT ^×fSW
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:
I_INP
×
₁₊
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 10kI to 50kI range. Calculate
R_TOP with the following equation:
R _TOP
= R_BOTTOM
× ( ^VO
V_FBP
− 1)
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.
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
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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