MAX1889 データシートの表示（PDF） - Maxim Integrated

部品番号

コンポーネント説明

メーカー

MAX1889

Triple-Output TFT LCD Power Supply with Fault Protection

Maxim Integrated 

Triple-Output TFT LCD Power Supply

with Fault Protection

If the junction temperature is +100°C, the maximum on-

state resistance overtemperature is:

[ ] RDS(MAX) = 70mΩ × 1+ 0.005 × (100 -25) = 100mΩ

For given R1 and R2 values, the ideal ratio of R3/R4

can be determined:

R3 = R1+ R2 × VIN -IPEAK(MAX) × RDS(MAX) -1

R4 R2

VIN

To consider the effect of resistor tolerance, comparator

offset, and input voltage variation, the minimum threshold

equation is:

R2 × (1+ ε)

VIN(MIN) × R1× (1- ε) + R2 × (1+ ε) + 5mV =

( ) R4 × (1- ε)

( ) ( ) VIN(MIN) -IL(MAX) × RDS(MAX)

×

R3 ×

1+ ε

+ R4 ×

1- ε

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

input voltage, ε is the tolerance of the resistors and the

5mV is the worst-case input offset voltage of the com-

parator. To simplify the equation, define a constant k as

follows:

k = 1- ε

1+ ε

The minimum threshold equation becomes:

VIN(MIN)

×

k

×

R2

R1+

R2

+

5mV

=

( ) VIN(MIN) -IL(MAX) × RDS(MAX)

× k × R4

R3 + k × R4

Solving for R3/R4 yields:





R3

R4

=

k

×







VIN(MIN) -IL(MAX) × RDS(MAX)

VIN(MIN)

×

R2

R2

+k×

R1

+

5mV

-1



The R3/R4 ratio guarantees the required minimum level

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

(( )) ITH_

TYP

=

VIN(TYP)

RDS(TYP)

 R2 ×

×

1-



R4

×

R3 + R4

R1+ R2







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

R1 = 51.1kΩ and R2 = 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 100mΩ:

k = 1- 0.01 = 0.9802

1+ 0.01





R3

R4

=

0.9802

×







2.7V

×

2.7V -1.25A × 0.1Ω

150kΩ

150kΩ + 0.9802 × 51.1kΩ

+

0.005V

-1



= 0.2637

If R4 =150kΩ, then R3 = 39.2kΩ. The typical overcurrent

threshold is:

ITH_ TYP

=

3.3V

0.047Ω

×



1-



150kΩ × (39.2kΩ +150kΩ)

150kΩ × (51.1kΩ +150kΩ)







= 4.15A

Charge Pumps

Selecting the Number of

Charge-Pump Stages

For highest efficiency, always choose the lowest num-

ber of charge-pump stages that meets the output

requirement.

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

NPOS

=

VPL

+ VDROPOUT - VMAIN

VMAIN - 2 × VD

where NPOS is the number of positive charge-pump

stages, VPL is the positive linear-regulator 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 regula-

tor. Use VDROPOUT = 2V.

20 ______________________________________________________________________________________

Share Link: