MAX1997 datasheet(23/31 Pages) MAXIM

23 / 31 page

Quintuple/Triple-Output TFT LCD Power Supplies

with Fault Protection and VCOM Buffer

______________________________________________________________________________________

The frequencies of the zero and pole for the lead com-

pensation are:

At high frequencies, R9 is effectively in parallel with R7,

determining the amount of added high-frequency gain. If

R9 is very large, there is no added gain and as R9

approaches zero, the added gain approaches the inverse

of the feedback divider’s attenuation. A typical value for

R9 is greater than half of R7. The value of C9 determines

the frequency placement of the zero and pole. A typical

value of C9 is between 100pF and 10nF. When adding

lead compensation, always check the loop stability by

monitoring the transient response to a pulsed output load.

Adding lag compensation (an RC network from FB to

ground) decreases the loop bandwidth and improves

FB noise immunity. Lag compensation slows the tran-

sient response but can increase stability margin, which

can be needed for particular component choices, a

poor layout, or high values of FB divider resistors (R8

greater than 1.5k

Ω).

Lag compensation adds a pole-zero pair, attenuating

gain at higher frequencies and lowering loop band-

width. The frequencies of the pole and zero for lag com-

pensation depend on the feedback divider resistors and

the RC network between FB and GND (Figure 9).

The frequencies of the pole and zero for the lag com-

pensation are:

At high frequencies, R10 is effectively in parallel with

R8, increasing the divider attenuation ratio. If R10 is

very large, the attenuation ratio remains unchanged

and as R10 approaches zero, the attenuation ratio

approaches infinity. A typical value for R10 is greater

than 0.1 times R8. If high-value divider resistors are

used, choose R10 < 1.5k

Ω for FB noise immunity. The

value of C10 determines the frequency placement of

the pole and zero. A typical value of C10 is between

100pF and 1000pF.

When adding lag compensation, always check the loop

stability by monitoring the transient response to a

pulsed output load.

The circuit of Figure 1 works well without compensation.

The circuit of Figure 9 uses lag compensation to allow

higher value FB divider resistors, at the expense of

transient response speed, potentially requiring higher

value output capacitors (see Typical Operating

Characteristics). Using one of these two circuits is

recommended.

Using Compensation for Improved Soft-Start

The digital soft-start of the main step-up regulator limits

the average input current during startup. If even

smoother startup is needed, add a low-frequency lead

compensation network (Figure 9). The improved soft-

start is active only during startup when the output volt-

age rises. Positive changes in the output are

instantaneously coupled to the FB pin through D1 and

feed-forward capacitor C9. This arrangement generates

a smoothly rising output voltage. When the output volt-

age reaches regulation, capacitor C9 charges up

through R9 and diode D1 turns off. If desired, C9 and

R9 can be chosen also to provide some lead compen-

sation in normal operation. In most applications, lead

compensation is not needed, and can be disabled by

making R9 large. With R9 much greater than R7, the

pole and the zero in the compensation network are very

close to one another and cancel out after startup, elimi-

nating the effect of the lead compensation.

Input Capacitor

The input capacitor (CIN) reduces the current peaks

drawn from the input supply and reduces noise injec-

tion into the device. A 10µF ceramic capacitor is used

in the standard application 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 may be reduced below the values used

in the standard applications circuit. Ensure a low-noise

supply at the IN pin by using adequate CIN. Alter-

natively, greater voltage variation can be tolerated on

CIN if IN is decoupled from CIN using an RC lowpass fil-

ter (see R1, C1 in Figure 1).

Rectifier Diode

The MAX1997/MAX1998s’ high switching frequency

demands a high-speed rectifier. Schottky diodes are rec-

ommended for most applications because of their fast

recovery time and low forward voltage. In general, a 1A

Schottky diode complements the internal MOSFET well.

P_LAG

Z_LAG









210

π()

Z_LEAD

P_LEAD

+×









()

MAX1997 Datasheet(PDF) 23 Page - Maxim Integrated Products

MAX1997 Datasheet(HTML) 23 Page - Maxim Integrated Products

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Datasheet Download

Link URL