MAX8795A datasheet(21/31 Pages) MAXIM

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TFT-LCD DC-DC Converter with

Operational Amplifiers

______________________________________________________________________________________

a typical input voltage of 5V. Choosing an LIR of 0.5 and

estimating efficiency of 85% at this operating point:

Using the circuit’s minimum input voltage (4.5V) and

estimating efficiency of 80% at that operating point:

The ripple current and the peak current are:

Output-Capacitor Selection

The total output voltage ripple has two components: the

capacitive ripple caused by the charging and discharging

of the output capacitance, and the ohmic ripple due to the

capacitor’s equivalent series resistance (ESR):

where IRIPPLE is the RIPPLE inductor current (see the

Inductor Selection section). For ceramic capacitors, the

output voltage ripple is typically dominated by

VRIPPLE(C). The voltage rating and temperature charac-

teristics of the output capacitor must also be considered.

Input-Capacitor Selection

The input capacitor (CIN) reduces the current peaks

drawn from the input supply and reduces noise injection

into the IC. A 22µF ceramic capacitor is used in the typi-

cal applications circuit (Figure 1) because of the high

source impedance seen in typical lab setups. Actual

applications usually have much lower source impedance

since the step-up regulator often runs directly from the

output of another regulated supply. Typically, CIN can

be reduced below the values used in the typical applica-

tions circuit. Ensure a low-noise supply at IN by using

adequate CIN. Alternately, greater voltage variation can

be tolerated on CIN if IN is decoupled from CIN using an

RC lowpass filter (see R10 and C13 in Figure 1).

Rectifier Diode

The MAX8795A’s high switching frequency demands a

high-speed rectifier. Schottky diodes are recommended

for most applications because of their fast recovery time

and low forward voltage. In general, a 2A Schottky

diode complements the internal MOSFET well.

Output-Voltage Selection

The output voltage of the main step-up regulator can be

adjusted by connecting a resistive voltage-divider from the

output (VMAIN) to AGND with the center tap connected to

FB (see Figure 1). Select R2 in the 10k

Ω to 50kΩ range.

Calculate R1 with the following equation:

where VFB, the step-up regulator’s feedback set point,

is 1.233V. Place R1 and R2 close to the IC.

Loop Compensation

Choose RCOMP to set the high-frequency integrator

gain for fast transient response. Choose CCOMP to set

the integrator zero to maintain loop stability.

For low-ESR output capacitors, use the following equa-

tions to obtain stable performance and good transient

response:

To further optimize transient response, vary RCOMP in

20% steps and CCOMP in 50% steps while observing

transient-response waveforms.

Charge Pumps

Selecting the Number of Charge-Pump Stages

For highest efficiency, always choose the lowest num-

ber of charge-pump stages that meet the output

requirement. Figures 8 and 9 show the positive and

negative charge-pump output voltages for a given

VMAIN for one-, two-, and three-stage charge pumps.

COMP

OUT

MAIN MAX

COMP

OUT

MAIN MAX

COMP

≈

××

≈

××

253

()

MAIN

=×

−

⎛

⎝⎜

⎞

⎠⎟

()

(

)

()

(

)

and

RIPPLE

RIPPLE C

RIPPLE ESR

RIPPLE C

MAIN

OUT

MAIN

MAIN OSC

RIPPLE ESR

PEAK ESR COUT

≈

−

⎛

⎝⎜

⎞

⎠⎟

≈

MHz

RIPPLE

PEAK

×−

µ×

≈

077

194

077

233

)

IN DCMAX

)

≈

194

MHz

= ⎛

⎝⎜

⎞

⎠⎟

−

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

≈µ

085

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