Rainbow-electronics MAX8715 Manual de usuario descargar pdf (Pagina 11)

MAX1790/MAX8715

Low-Noise Step-Up DC-DC Converters

______________________________________________________________________________________ 11

ratio of inductor resistance to other power path resis-

tances, the best LIR can shift up or down. If the induc-

tor resistance is relatively high, more ripple can be

accepted to reduce the number of turns required and

increase the wire diameter. If the inductor resistance is

relatively low, increasing inductance to lower the peak

current can decrease losses throughout the power

path. If extremely thin high-resistance inductors are

used, as is common for LCD-panel applications, the

best LIR can increase to between 0.5 and 1.0.

Once a physical inductor is chosen, higher and lower

values of the inductor should be evaluated for efficiency

improvements in typical operating regions.

Calculate the approximate inductor value using the typ-

ical input voltage (V

), the maximum output current

MAIN(MAX)

), the expected efficiency (η

TYP

) taken from

an appropriate curve in the Typical Operating

Characteristics, and an estimate of LIR based on the

above discussion:

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

MAX1790/MAX8715s’ LX current limit (I

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 application circuit in Figure 4, the maxi-

mum load current (I

MAIN(MAX)

) is 150mA with a 9V output

and a typical input voltage of 3.3V. Choosing an LIR of 0.5

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:

Diode Selection

The output diode should be rated to handle the output

voltage and the peak switch current. Make sure that the

diode’s peak current rating is at least I

and that its

breakdown voltage exceeds V

OUT

. Schottky diodes are

recommended.

Input and Output Capacitor Selection

Low-ESR capacitors are recommended for input

bypassing and output filtering. Low-ESR tantalum

capacitors are a good compromise between cost and

performance. Ceramic capacitors are also a good

choice. Avoid standard aluminum electrolytic capaci-

tors. A simple equation to estimate input and output-

capacitor values for a given voltage ripple is as follows:

where V

RIPPLE

is the peak-to-peak ripple voltage on the

capacitor.

Output Voltage

The MAX1790/MAX8715 operate with an adjustable

output from V

to 13V. Connect a resistor voltage-

divider to FB (see the Typical Operating Circuit) from

the output to GND. Select the resistor values as follows:

where V

, the boost-regulator feedback set point, is

1.24V. Since the input bias current into FB is typically 0,

OUT

12 1=−

⎛

⎝

⎜

⎞

⎠

⎟

0.5 L I

V V

RIPPLE OUT

≥

××

⎛

⎝

⎞

⎠

PEAK

=+ ≈06

025

0 725.

VVV

H V MHz

RIPPLE

×−

××

≈

393

68 9 12

025

()

IN DC MAX(, )

≈

015 9

308

A MHz

⎛

⎝

⎜

⎞

⎠

⎟

−

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟

≈

933

015 12

085

. µ

PEAK IN DC MAX

RIPPLE

(, )

VVV

LV f

RIPPLE

IN MIN MAIN IN MIN

MAIN OSC

×−

××

() ()

()

IN DC MAX

MAIN MAX MAIN

IN MIN MIN

(, )

()

×η

I f LIR

MAIN

MAIN IN

MAIN MAX OSC

TYP

⎛

⎝

⎜

⎞

⎠

⎟

−

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟

()

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Rainbow-electronics MAX8715 Manual de usuario Pagina 11

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