MAX16928_12 Datasheet, PDF (16/20 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
found in the transistor’s data sheet. Because R_BE is
much greater than R_IN, the above equation can be
simplified:
f_{POLE_IN}
=
2π
×
1
C_IN
×
R_IN
Substituting for C_IN and R_IN yields:
f_POLE
=
f_T
h_FE
4) Next, calculate the pole set by the regulator’s feed-
back resistance and the capacitance between FBGL
and GND (including stray capacitance):
f_{POLE_FBGL}
=
2π
× C_FBGL
1
× (R_TOP /R_BOTTOM)
where C_FBGL is the capacitance between FBGL and
GND and is equal to 30pF, R_TOP is the upper resistor
of the regulator’s feedback divider, and R_BOTTOM is
the lower resistor of the divider.
5) Next, calculate the zero caused by the output capaci-
tor’s ESR:
f_{ZERO_ESR}
=
2π
×
1
C _{OUT_LR}
×
R_ESR
where R_ESR is the equivalent series resistance of
C_{OUT_LR}. To ensure stability, make C_{OUT_LR} large
enough so the crossover occurs well before the poles
and zero calculated in steps 2 to 5. The poles in steps
3 and 4 generally occur at several MHz and using
ceramic capacitors ensures the ESR zero also occurs
at several MHz. Placing the crossover frequency
below 500kHz is sufficient to avoid the amplifier delay
pole and generally works well, unless unusual compo-
nent choices or extra capacitances move one of the
other poles or the zero below 1MHz.
Table 3 is a list of recommended minimum output capaci-
tance for the negative-gate voltage regulator and is appli-
cable for output currents in the 10mA to 15mA range.
Table 3. Minimum Output Capacitance vs.
Output Voltage Range for Negative-Gate
Voltage Regulator (I_OUT = 10mA to 15mA)
OUTPUT VOLTAGE
RANGE
-2V R V_GL R -4V
-5V R V_GL R -7V
-8V R V_GL R -13V
MINIMUM OUTPUT
CAPACITANCE (µF)
2.2
1.5
1
Applications Information
Power Dissipation
An IC’s maximum power dissipation depends on the ther-
mal resistance from the die to the ambient environment
and the ambient temperature. The thermal resistance
depends on the IC package, PCB copper area, other
thermal mass, and airflow. More PCB copper, cooler
ambient air, and more airflow increase the possible dis-
sipation, while less copper or warmer air decreases the
IC’s dissipation capability. The major components of
power dissipation are the power dissipated in the boost
converter, positive-gate voltage regulator, negative-gate
voltage regulator, and the 1.8V/3.3V regulator controller.
Boost Converter
Power dissipation in the boost converter is primarily due
to conduction and switching losses in the low-side FET.
Conduction loss is produced by the inductor current
flowing through the on-resistance of the FET during the
on-time. Switching loss occurs during switching transi-
tions and is a result of the finite time needed to fully turn
on and off the FET. Power dissipation in the boost con-
verter can be estimated with the following formula:
P_LXP ≈ [(I_IN(DC,MAX) × √D)² × R_{DS_ON(LXP)}] + V_SH ×
I_IN(DC,MAX) × f_SW × [(t_R-V + t_F-I) + (t_R-I + t_F-V)]
where I_IN(DC,MAX) is the maximum expected average
input (i.e., inductor) current, D is the duty cycle of the
boost converter, R_{DS_ON(LXP)} is the on-resistance of
the internal low-side FET, V_SH­ is the output voltage, and
f_SW is the switching frequency of the boost converter.
R_{DS_ON(LXP)} is 110mI (typ) and f_SW is 2.2MHz.
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