Rainbow-electronics MAX8758 Manual de usuario descargar pdf (Pagina 16)

MAX8758

In Figure 1’s Typical Operating Circuit, the LCD’s gate-

on and gate-off voltages are generated from two unreg-

ulated charge pumps driven by the step-up regulator’s

LX node. The additional load on LX must therefore be

considered in the inductance calculation. The effective

maximum output current I

MAIN(EFF)

becomes the sum

of the maximum load current on the step-up regulator’s

output plus the contributions from the positive and neg-

ative charge pumps:

MAIN(EFF)

= I

MAIN(MAX)

+ n

NEG

x I

NEG

+ (n

POS

+ 1) x I

POS

where I

MAIN(MAX)

is the maximum output current, n

NEG

is the number of negative charge-pump stages, n

POS

the number of positive charge-pump stages, I

NEG

the negative charge-pump output current, and I

POS

the positive charge-pump output current, assuming the

pump source for I

POS

is V

MAIN

The required inductance can then be calculated as

follows:

where V

is the typical input voltage and η

TYP

is the

expected efficiency obtained from the appropriate

curve in the Typical Operating Characteristics.

Choose an available inductor value from an appropriate

inductor family. Calculate the maximum DC input cur-

rent at the minimum input voltage V

IN(MIN)

using con-

servation of energy and the expected efficiency at that

operating point (η

MIN

) taken from an appropriate curve

in the Typical Operating Characteristics:

Calculate the ripple current at that operating point and

the peak current required for the inductor:

The inductor’s saturation current rating and the guaran-

teed minimum value of the MAX8758’s LX current limit

LIM

) should exceed I

PEAK

and the inductor’s DC current

rating should exceed I

IN(DC,MAX)

. For good efficiency,

choose an inductor with less than 0.1Ω series resistance.

Considering the Typical Operating Circuit, the maxi-

mum load current (I

MAIN(MAX)

) is 300mA for the step-

up regulator, 20mA for the two-stage positive charge

pump, and 20mA for the one-stage negative charge

pump. Altogether, the effective maximum output cur-

rent, I

MAIN(EFF)

is 360mA with an 8.5V output and a

typical input voltage of 3.3V. The switching frequency is

set to 1.2MHz. Choosing an LIR of 0.4 and estimating

efficiency of 85% at this operating point:

Using the circuit’s minimum input voltage (3V) and esti-

mating efficiency of 80% at that operating point:

The ripple current and the peak current are:

The peak-inductor current does not exceed the guaran-

teed minimum value of the LX current limit in the

Electrical Characteristics table.

Step-Up Regulator Output Capacitor Selection

The total output voltage ripple has two components: the

capacitive ripple caused by the charging and discharg-

ing of the output capacitance, and the ohmic ripple due

to the capacitor’s equivalent series resistance (ESR):

RIPPLE

= V

RIPPLE(C)

+ V

ARIPPLE(ESR)

and

RIPPLE(ESR)

≈ I

PEAK

x R

ESR

where I

PEAK

is the peak inductor current (see the Step-

Up Regulator Inductor Selection section). For ceramic

capacitors, the output voltage ripple is typically dominat-

ed by V

RIPPLE(C)

. The voltage rating and temperature

characteristics of the output capacitor must also be con-

sidered.

RIPPLE C

MAIN

MAIN IN

MAIN SW

()

≈ ×













−

PEAK

.=+≈128

148

VVV

HVMHz

RIPPLE

(. )

. . .

××

≈

−3853

42 85 12

IN DC MAX(, )

. .

≈

036 85

308

128

A MHz

. .













−

























≈

85 33

036 12

085

PEAK IN DC MAX

RIPPLE

(, )

VVV

LV f

RIPPLE

IN MIN MAIN IN MIN

MAIN OSC

() ()

()

××

−

IN DCMAX

MAIN EFF MAIN

IN MIN MIN

(, )

()

× η

IfLIR

MAIN

MAIN IN

MAIN EFF OSC

TYP

()





































−

Step-Up Regulator with Switch Control and

Operational Amplifier for TFT LCD

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