MAX8727(2004) Ver la hoja de datos (PDF) - Maxim Integrated

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componentes Descripción

Fabricante

MAX8727
(Rev.:2004)

TFT-LCD Step-Up DC-DC Converter

Maxim Integrated 

TFT-LCD Step-Up DC-DC Converter

The equations used here include a constant LIR, which

is the ratio of the inductor peak-to-peak ripple current

to the average DC inductor current at the full load cur-

rent. The best trade-off between inductor size and cir-

cuit efficiency for step-up regulators generally has an

LIR between 0.3 and 0.5. However, depending on the

AC characteristics of the inductor core material and the

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 efficien-

cy improvements in typical operating regions.

Calculate the approximate inductor value using the typ-

ical input voltage (VIN), the maximum output current

(IMAIN(MAX)), the expected efficiency (ηTYP) taken from

an appropriate curve in the Typical Operating

Characteristics, and an estimate of LIR based on the

above discussion:

L

=

⎛

⎜

⎝

VIN

VMAIN

⎞

⎟

⎠

2

⎛

⎝⎜⎜

VMAIN − VIN

IMAIN(MAX) × fOSC

⎞

⎠⎟⎟

⎛

⎜

ηTYP

⎞

⎟

⎝ LIR ⎠

Choose an available inductor value from an appropriate

inductor family. Calculate the maximum DC input cur-

rent at the minimum input voltage VIN(MIN) using con-

servation of energy and the expected efficiency at that

operating point (ηMIN) taken from an appropriate curve

in the Typical Operating Characteristics:

IIN(DC, MAX)

=

IMAIN(MAX) × VMAIN

VIN(MIN) × ηMIN

Calculate the ripple current at that operating point and

the peak current required for the inductor:

IRIPPLE = VIN(MIN) × (VMAIN − VIN(MIN) )

L × VMAIN × fOSC

IPEAK = IIN(DC, MAX) + IRIPPLE

2

The inductor’s saturation current rating and the

MAX8727’s LX current limit (ILIM) should exceed IPEAK,

and the inductor’s DC current rating should exceed

IIN(DC,MAX). For good efficiency, choose an inductor

with less than 0.1Ω series resistance.

Considering the typical operating circuit, the maximum

load current (IMAIN(MAX)) is 600mA with a 15V output and

a typical input voltage of 5V. Choosing an LIR of 0.35 and

estimating efficiency of 85% at this operating point:

L

=

⎛ 5V ⎞ 2 ⎛ 15V − 5V ⎞ ⎛ 0.85⎞

⎝⎜ 15V ⎠⎟ ⎝⎜ 0.6A × 1.2MHz ⎠⎟ ⎝⎜ 0.35⎠⎟

≈

3.6µH

Using the circuit’s minimum input voltage (4.5V) and

estimating efficiency of 85% at that operating point:

IIN(DC, MAX)

=

0.6A × 15V

4.5V × 0.85

≈

2.35A

The ripple current and the peak current are:

IRIPPLE

=

4.5V × (15V − 4.5V)

3.6µH × 15V × 1.2MHz

≈ 0.73A

IPEAK

= 2.35A +

0.73A

2

≈ 2.70A

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):

VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR)

VRIPPLE(C)

≈

IMAIN

⎛

⎜

VMAIN

−

VIN

⎞

⎟,

and

COUT ⎝ VMAIN fOSC ⎠

VRIPPLE(ESR) ≈ IPEAK RESR(COUT)

where IPEAK is the peak inductor current (see the

Inductor Selection section). For ceramic capacitors,

the output voltage ripple is typically dominated by

VRIPPLE(C). The voltage rating and temperature charac-

teristics of the output capacitor must also be considered.

_______________________________________________________________________________________ 9

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