MAX16929HGUI datasheet(21/25 Pages) MAXIM

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MAX16929

Automotive TFT-LCD Power Supply with Boost

Converter and Gate Voltage Regulators

3) Next, calculate the pole set by the transistor’s input

capacitance, the transistor’s input resistance, and the

base-to-emitter pullup resistor:

POLE_IN

/R )

π ×

where:

, R

2 f

gm is the transconductance of the pass transistor, and

fT is the transition frequency. Both parameters can be

found in the transistor’s data sheet. Because RBE is

much greater than RIN, the above equation can be

simplified:

POLE_IN

π ×

Substituting for CIN and RIN yields:

POLE

4) Next, calculate the pole set by the regulator’s feed-

back resistance and the capacitance between FBGL

and GND (including stray capacitance):

POLE_FBGL

FBGL

TOP

BOTTOM

)

π ×

where CFBGL is the capacitance between FBGL and

GND and is equal to 30pF, RTOP is the upper resistor

of the regulator’s feedback divider, and RBOTTOM is

the lower resistor of the divider.

5) Next, calculate the zero caused by the output capaci-

tor’s ESR:

ZERO_ESR

OUT_LR

ESR

π ×

where RESR is the equivalent series resistance of

COUT_LR. To ensure stability, make COUT_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.

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:

PLXB ≈ [(IOUTB × √D)2 × RDS_ON(LXB)] + [0.5 × VINB ×

IOUTB × (tR + tF) × fSWB]

where IOUTB is the output current, D is the duty cycle

of the buck converter, RDS_ON(LXB) is the on-resistance

of the internal high-side FET, VINB is the input voltage,

(tR + tF) is the time is takes for the switch current and

voltage to settle to their final values during the rising and

falling transitions, and fSWB is the switching frequency

of the buck converter. RDS_ON(LXB) is 180mI (typ) and

(tR + tF) is 4.4ns + 4.6ns = 9ns at VINB = 12V.

Table 4. Minimum Output Capacitance vs.

Output Voltage Range for Negative-Gate

Voltage Regulator (IOUT = 10mA to 15mA)

OUTPUT VOLTAGE

RANGE

MINIMUM OUTPUT

CAPACITANCE (µF)

-2V R VGL R -4V

2.2

-5V R VGL R -7V

1.5

-8V R VGL R -13V

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