MAX16929_12 Datasheet, PDF (21/25 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
where:
f_{POLE_IN}
=
2π
×
C_IN
1
× (R_BE
/R_IN)
C_IN
=
g_m
2πf_T
,
R_IN
=
h_FE
g_m
g_m is the transconductance of the pass transistor, and
f_T is the transition frequency. Both parameters can be
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 component
choices or extra capacitances move one of the other
poles or the zero below 1MHz.
Table 4 is a list of recommended minimum output capaci-
tance for the negative-gate voltage regulator and are
applicable for output currents in the 10mA to 15mA range.
Table 4. 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 buck
converter, boost converter, positive-gate voltage regula-
tor, negative-gate voltage regulator, and the 1.8V/3.3V
regulator controller.
Buck Converter
In the buck converter, conduction and switching losses
in the internal MOSFET are dominant. Estimate these
losses using the following formula:
P_LXB ≈ [(I_OUTB × √D)² × R_{DS_ON(LXB)}] + [0.5 × V_INB ×
I_OUTB × (t_R + t_F) × f_SWB]
where I_OUTB is the output current, D is the duty cycle
of the buck converter, R_{DS_ON(LXB)} is the on-resistance
of the internal high-side FET, V_INB­ is the input voltage,
(t_R + t_F) is the time is takes for the switch current and
voltage to settle to their final values during the rising and
falling transitions, and f_SWB is the switching frequency
of the buck converter. R_{DS_ON(LXB)} is 180mI (typ) and
(t_R + t_F) is 4.4ns + 4.6ns = 9ns at V_INB = 12V.
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