MAX1518B Datasheet, PDF(18/25 Page) Maxim Integrated Products – TFT-LCD DC-DC Converter with Operational Amplifiers

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MAX1518B Datasheet, PDF (18/25 Pages) Maxim Integrated Products – TFT-LCD DC-DC Converter with Operational Amplifiers

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TFT-LCD DC-DC Converter with

Operational Amplifiers

Design Procedure

Main Step-Up Regulator

Inductor Selection

The minimum inductance value, peak current rating,

and series resistance are factors to consider when

selecting the inductor. These factors influence the con-

verterâs efficiency, maximum output load capability,

transient-response time, and output voltage ripple. Size

and cost are also important factors to consider.

The maximum output current, input voltage, output volt-

age, and switching frequency determine the inductor

value. Very high inductance values minimize the cur-

rent ripple and therefore reduce the peak current,

which decreases core losses in the inductor and con-

duction losses in the entire power path. However, large

inductor values also require more energy storage and

more turns of wire, which increases size and can

increase conduction losses in the inductor. Low induc-

tance values decrease the size but increase the current

ripple and peak current. Finding the best inductor

involves choosing the best compromise between circuit

efficiency, inductor size, and cost.

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

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 (Î·TYP) taken from

an appropriate curve in the Typical Operating

Characteristics section, and an estimate of LIR based

on the above discussion:

ï£«

ï£ï£¬

VIN

VMAIN

ï£¶

ï£¸ï£·

ï£«

ï£¬

ï£

VMAIN â

IMAIN(MAX)

VIN

Ã fOSC

ï£¶

ï£·

ï£¸

ï£«

ï£ï£¬

Î·TYP

LIR

ï£¶

ï£¸ï£·

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 (Î·MIN) taken from the appropriate curve

in the Typical Operating Characteristics:

IIN(DC,MAX)

IMAIN(MAX) Ã VMAIN

VIN(MIN) Ã Î·MIN

Calculate the ripple current at that operating point and

the peak current required for the inductor:

IRIPPLE

VIN(MIN) Ã (VMAIN â VIN(MIN))

L Ã VMAIN Ã fOSC

IPEAK

IIN(DC,MAX)

IRIPPLE

The inductorâs saturation current rating and the

MAX1518Bâs LX current limit (ILIM) should exceed IPEAK,

and the inductorâs DC current rating should exceed

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

less than 0.1â¦ series resistance.

Considering the Typical Operating Circuit, the maximum

load current (IMAIN(MAX)) is 500mA with a 13V output and

a typical input voltage of 5V. Choosing an LIR of 0.5 and

estimating efficiency of 85% at this operating point:

ï£«

ï£ï£¬

13V

ï£¶

ï£¸ï£·

2ï£«

ï£ï£¬

13V â 5V

0.5A Ã1.2MHz

ï£¶ï£«

ï£¸ï£· ï£ï£¬

0.85 ï£¶

0.5 ï£¸ï£·

3.3ÂµH

Using the circuitâs minimum input voltage (4.5V) and

estimating efficiency of 80% at that operating point:

IIN(DC,MAX)

0.5A Ã13V

4.5V Ã 0.8

â 1.8A

The ripple current and the peak current are:

IRIPPLE

4.5V Ã (13V â 4.5V)

3.3ÂµH Ã13V Ã1.2MHz

0.74A

IPEAK

= 1.8A

0.74A

â 2.2A

18 ______________________________________________________________________________________

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