MAX8798 datasheet(22/31 Pages) MAXIM

22 / 31 page

Internal-Switch Boost Regulator with

Integrated 3-Channel Scan Driver for TFT LCDs

______________________________________________________________________________________

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 converter’s effi-

ciency, maximum output-load capability, transient

response time, and output-voltage ripple. Physical size

and cost are also important factors to be considered.

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 that

decreases core losses in the inductor and I2R losses in

the entire power path. However, large inductor values

also require more energy storage and more turns of

wire that increase physical size and can increase I2R

losses in the inductor. Low inductance values decrease

the physical size, but increase the current ripple and

peak current. Finding the best inductor involves choos-

ing the best compromise between circuit efficiency,

inductor size, and cost.

The equations used here include a constant called LIR

that is the ratio of the inductor peak-to-peak ripple cur-

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

current. The best trade-off between inductor size and

circuit 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.

In Figure 2, the LCD’s gate-on and gate-off supply volt-

ages are generated from two unregulated charge

pumps driven by the step-up regulator’s LX node. The

additional load on LX must therefore be considered in

the inductance and current calculations. The effective

maximum output current, IMAIN(EFF) becomes the sum

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

output plus the contributions from the positive and neg-

ative charge pumps:

where IMAIN(MAX) is the maximum step-up output cur-

rent, nNEG is the number of negative charge-pump

stages, nPOS is the number of positive charge-pump

stages, INEG is the negative charge-pump output cur-

rent, and IPOS is the positive charge-pump output cur-

rent, assuming the initial pump source for IPOS is

VMAIN.

Calculate the approximate inductor value using the typ-

ical input voltage (VIN), the maximum output current

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

an appropriate curve in the

Typical Operating Char-

acteristics, 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 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:

Calculate the ripple current at that operating point and

the peak current required for the inductor:

The inductor’s saturation current rating and the

MAX8798’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 0.1

Ω series resistance.

Considering Figure 2, the maximum load current

(IMAIN(MAX)) is 300mA, with an 8V output and a typical

input voltage of 3.3V. The effective full-load step-up

current is:

ImA

MAIN EFF

()

=+ ×

+ + ×

300

2 20

2 1

400

PEAK

IN DCMAX

RIPPLE

)

RIPPLE

IN MIN

MAIN

IN MIN

MAIN

OSC

×−

()

××

()

IN DCMAX

MAIN EFF

MAIN

IN MIN

MIN

)

()

×η

LIR

MAIN

MAIN EFF

OSC

TYP

= ⎛

⎝⎜

⎞

⎠⎟

−

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

()

MAIN EFF

MAIN MAX

NEG

POS

()

(

)

()

+ ×

MAX8798 Datasheet(PDF) 22 Page - Maxim Integrated Products

MAX8798 Datasheet(HTML) 22 Page - Maxim Integrated Products

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Datasheet Download

Link URL