MAX16929DGUI datasheet(19/25 Pages) MAXIM

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�� Maxim Integrated Products 19

MAX16929

Automotive TFT-LCD Power Supply with Boost

Converter and Gate Voltage Regulators

For highest efficiency, choose the lowest number of

charge-pump stages that meets the output requirement.

The number of positive charge-pump stages needed is

given by:

DROPOUT

SCHOTTKY

2 V

−

− ×

and the number of negative charge-pump stages is

given by:

DROPOUT

SCHOTTKY

|+V

2 V

− ×

where nCP is the number of positive charge-pump stag-

es, nCN is the number of negative charge-pump stages,

VGH is the positive-gate voltage regulator output volt-

age, VGL is the negative-gate voltage regulator output

voltage, VSH is the boost converter’s output voltage, VD

is the forward-voltage drop of the charge-pump diode,

VSCHOTTKY is the forward drop of the Schottky diode

of the boost converter, and VDROPOUT is the dropout

margin for the regulator. Use VDROPOUT = 0.3V for the

negative voltage regulator and VDROPOUT = 2V at 20mA

for the positive-gate voltage regulator.

Flying Capacitors

Increasing the flying capacitor (CX) 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 VCP, and

that for the negative charge pump should exceed the

magnitude of VCN.

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:

LOAD_CP

OUT_CP

RIPPLE_CP

D I

≥

where COUT_CP is the output capacitor of the charge

pump, D is the duty cycle of the boost converter, ILOAD_CP

is the load current of the charge pump, fSW is the switch-

ing frequency of the boost converter, and VRIPPLE_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:

LOAD_CN

OUT_CN

RIPPLE_CN

(1-D) I

≥

where COUT_CN is the output capacitor of the charge

pump, D is the duty cycle of the boost converter,

ILOAD_CN is the load current of the charge pump, fSW

is the switching frequency of the boost converter, and

VRIPPLE_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 RTOP and RBOTTOM. Connect RTOP between

the output and FBGH, and connect RBOTTOM between

FBGH and GND. Select RBOTTOM in the 10kI to 50kI

range. Calculate RTOP with the following equation:

TOP

BOTTOM

FBGH

(

−

where VGH is the desired output voltage and VFBGH = 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:

PGL = (VCP - VGH) × ILOAD(MAX)

where VCP is the noninverting charge-pump output volt-

age applied to the drain, VGH is the regulated output

voltage, and ILOAD(MAX) is the maximum load current.

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