MAX1997 datasheet(25/31 Pages) MAXIM

25 / 31 page

Quintuple/Triple-Output TFT LCD Power Supplies

with Fault Protection and VCOM Buffer

______________________________________________________________________________________

After including the effects of resistor tolerance, com-

parator offset, and input voltage variation, the minimum

input overcurrent threshold equation is:

where VIN(MIN) is the minimum expected value of the

input voltage,

ε is the tolerance of the resistors, and 5mV

is the worst-case input offset voltage of the comparator.

To simplify the equation, define a constant (k) as follows:

The minimum threshold equation becomes:

Solving for R4/R5 yields:

The R4/R5 ratio guarantees the required minimum level

for IL(MAX). The typical overcurrent threshold is given by:

The following example shows how to apply the above

equations in the design. If 1% resistors are used, then

ε = 0.01. To set VOCP to be around 75% of VIN, select

R2 = 51.1k

Ω and R3 = 150kΩ. Assume that the mini-

mum input voltage is 2.7V and the typical input voltage

is 3.3V, the average inductor current at maximum load

is 1.25A, and the maximum RDS(ON) of P1 is 100m

Ω:

If R5 =150k

Ω, then R4 = 39.2kΩ. The typical overcur-

rent threshold is:

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. The number of positive charge-pump

stages is given by:

where NPOS is the number of positive charge-pump

stages, VG_ON is the positive linear-regulator (REG P)

output, VMAIN is the main step-up regulator output, VD

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

and VDROPOUT is the dropout margin for the linear reg-

ulator. Use VDROPOUT = 2V.

The number of negative charge-pump stages is given by:

where NNEG is the number of negative charge-pump

stages, VG_OFF is the negative linear-regulator (REG N)

output, VMAIN is the main step-up regulator output, VD

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

and VDROPOUT is the dropout margin for the linear reg-

ulator. Use VDROPOUT = 2V.

The above equations are derived based on the

assumption that the first stage of the positive charge

pump is connected to VMAIN and the first stage of the

negative charge pump is connected to ground.

Sometimes fractional stages are more desirable for bet-

ter efficiency. This can be done by connecting the first

stage to VIN or another available supply. If the first

charge-pump stage is powered from VIN, then the

above equations become:

V+ V

- V

-V+ V

+ V

POS

G_ON

DROPOUT

MAIN

NEG

G_OFF

DROPOUT

MAIN

-V+ V

NEG

G_OFF

DROPOUT

MAIN

V+ V

- V

POS

G_ON

DROPOUT

MAIN

3.3V

0.047

(39.2k +1

)

(51.1k +1

)

TH_TYP =×











 =

Ω

ΩΩ

Ω

ΩΩ

Ω

150

415

2.7V - 1.25A 0.1

2.7V

=×

+×













0 9802

150

0 9802 51 1

0 005

0 2637

Ω

ΩΩ

1 - 0.01

1 + 0.01

0 9802

(R4 +R5)

(R2+R3)

TH_TYP

IN(TYP)

DS(TYP)

=×













- I

IN(MIN)

L(MAX)

DS(MAX)

IN(MIN)

=×

+×













IN(MIN)

L(MAX)

DS(MAX)

×+

××

+×

(

)

= +

V- I

IN(MIN)

L(MAX)

DS(MAX)

×+

=×

×+

()

(

)

()

εε

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