MAX16928_12 Datasheet, PDF (14/20 Pages) Maxim Integrated Products – Automotive TFT-LCD Power Supply with Boost Converter and Gate Voltage Regulators

MAX16928
Automotive TFT-LCD Power Supply with Boost
Converter and Gate Voltage Regulators
is the forward-voltage drop of the charge-pump diode,
V_SCHOTTKY is the forward drop of the Schottky diode
of the boost converter, and V_DROPOUT is the dropout
margin for the regulator. Use V_DROPOUT = 0.3V for the
negative voltage regulator and V_DROPOUT = 2V at 20mA
for the positive-gate voltage regulator.
Flying Capacitors
Increasing the flying capacitor (C_X) value lowers the
effective source impedance and increases the output
current capability. Increasing the capacitance indefi-
nitely has a negligible effect on output current capability
because the internal switch resistance and the diode
impedance place a lower limit on the source impedance.
A 0.1FF ceramic capacitor works well in most low-current
applications. The voltage rating of the flying capacitors
for the positive charge pump should exceed V_CP, and
that for the negative charge pump should exceed the
magnitude of V_CN.
Charge-Pump Output Capacitor
Increasing the output capacitance or decreasing the ESR
reduces the output-ripple voltage and the peak-to-peak
transient voltage. With ceramic capacitors, the output-
voltage ripple is dominated by the capacitance value.
Use the following equation to approximate the required
output capacitance for the noninverting charge pump
connected to CP:
C _{OUT_CP}
≥
D ×I_{LOAD_CP}
f_SW × V_{RIPPLE_CP}
where C_{OUT_CP} is the output capacitor of the charge
pump, D is the duty cycle of the boost converter, I_{LOAD_CP}
is the load current of the charge pump, f_SW is the switch-
ing frequency of the boost converter, and V_{RIPPLE_CP} is
the peak-to-peak value of the output ripple.
For the inverting charge pump connected to CN, use the
following equation to approximate the required output
capacitance:
C _{OUT_CN}
≥
(1-D)
f_SW ×
× I_{LOAD_CN}
V_{RIPPLE_CN}
where C_{OUT_CN} is the output capacitor of the charge
pump, D is the duty cycle of the boost converter,
I_{LOAD_CN} is the load current of the charge pump, f_SW
is the switching frequency of the boost converter, and
V_{RIPPLE_CN} is the peak-to-peak value of the output
ripple.
Charge-Pump Rectifier Diodes
Use high-speed silicon switching diodes with a current
rating equal to or greater than two times the average
charge-pump input current. If it helps avoid an extra
stage, some or all of the diodes can be replaced with
Schottky diodes with an equivalent current rating.
Positive-Gate Voltage Regulator
Output Voltage Selection
The output voltage of the positive-gate voltage regula-
tor can be adjusted by using a resistive voltage-divider
formed by R_TOP and R_BOTTOM. Connect R_TOP between
the output and FBGH, and connect R_BOTTOM between
FBGH and GND. Select R_BOTTOM in the 10kI to 50kI
range. Calculate R_TOP with the following equation:
R _TOP
=
R_BOTTOM
× ( ^VGH
V_FBGH
− 1)
where V_GH is the desired output voltage and V_FBGH = 1V
(the regulated feedback voltage for the regulator). Place
both resistors as close as possible to the device.
Avoid excessive power dissipation within the internal
pMOS device of the regulator by paying attention to the
voltage drop across the drain and source. The amount of
power dissipation is given by:
P_GL = (V_CP - V_GH) × I_LOAD(MAX)
where V_CP is the noninverting charge-pump output volt-
age applied to the drain, V_GH is the regulated output
voltage, and I_LOAD(MAX) is the maximum load current.
Stability Requirements
The positive-gate voltage regulator (GH) requires a mini-
mum output capacitance for stability. For an output volt-
age of 5V to (V_CP - 2V) and an output current of 10mA to
15mA, use a minimum capacitance of 0.47FF.
Negative-Gate Voltage Regulator
Output Voltage Selection
The output voltage of the negative-gate voltage regula-
tor can be adjusted by using a resistive voltage-divider
formed by R_TOP and R_BOTTOM. Connect R_TOP between
REF and FBGL and connect R_BOTTOM between FBGL
and the collector of the external npn transistor. Select
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