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MAX16929

Automotive TFT-LCD Power Supply with Boost

Converter and Gate Voltage Regulators

Design Procedure

Buck Converter

Inductor Selection

Three key inductor parameters must be specified for

operation with the device: inductance value (L), induc-

To determine the inductance value, select the ratio of

inductor peak-to-peak ripple current to average output

current (LIR) first. For LIR values that are too high, the

Use high-valued inductors to achieve low LIR values.

Typically, inductance is proportional to resistance for a

for very low LIR values. A good compromise between

size and loss is to select a 30%-to-60% peak-to-peak

ripple current to average-current ratio. If extremely thin

high-resistance inductors are used, as is common for

LCD-panel applications, the best LIR can increase

between 0.5 and 1.0. The size of the inductor is deter-

mined as follows:

INB O

O

SWB

(V

-V ) D

L

and

LIR I

f

×

=

×

×

O

INB

V

D

V

=

η×

(the switching frequency of the buck converter). The

efficiency of the buck converter can be estimated from

the Typical Operating Characteristics and accounts for

and capacitor ESR.

To ensure the buck converter does not shut down

during load dump input-voltage transients to 42V, an

inductor value larger than calculated above should be

used. Table 2 lists the minimum inductance that should

be used for proper operation during load dump. The

ensure that saturation can occur only above the maxi-

mum current-limit value. Find a low-loss inductor having

the lowest possible DC resistance that fits in the allotted

dimensions.

Capacitor Selection

The input and output filter capacitors should be of a low-

ESR type (tantalum, ceramic, or low-ESR electrolytic) and

2

INB(RMS)

O

LIR

I

I

D (1-D

)

12

=

×

+

for the input capacitor

O

OUTB(RMS)

LIR I

I

12

×

=

for the output capacitor

where D is the duty cycle given above.

The output voltage contains a ripple component whose

peak-to-peak value depends on the value of the ESR

and capacitance of the output capacitor, and is approxi-

mately given by:

D

D

O

CAP

SWB

LIR I

V

8 C f

×

∆

=

× ×

Diode Selection

The catch diode should be a Schottky type to minimize

its voltage drop and maximize efficiency. The diode must

be capable of withstanding a reverse voltage of at least

the maximum input voltage in the application. The diode

should have an average forward current rating greater

than:

where D is the duty cycle given above. In addition, ensure

that the peak current rating of the diode is greater than:

OUTB

LIR

I

1

2





× +









Table 2. Minimum Buck Inductor Value

Required for Normal Operation During

Load Dump

3.3

1.2

3.3

3.3

2

6.8

5

1.2

3.3

5

2

4.7