MAX1513 datasheet(18/28 Pages) MAXIM

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TFT-LCD Power-Supply Controllers

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taken from an appropriate curve in the Typical

Operating Characteristics, and an estimate of LIR

based on the above paragraphs:

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 the

following equation:

The expected efficiency at that operating point (

ηMIN)

can be 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

MAX1513/MAX1514s’ current limit (ILIM) should exceed

IPEAK, and the inductor’s DC current rating should

exceed IIN(DC, MAX).

Considering the typical operating circuit, the maximum

load current (IMAIN(MAX)) is 400mA for IMAIN directly

and 30mA for REG G to provide VGAMMA. The one-

stage negative charge pump provides 30mA to REG N

for VGOFF, and the one-stage positive charge pump

provides 20mA to REG P for VGON. Altogether, the

effective maximum output current (IMAIN(EFF)) is 500mA

with a 15V output and a typical 5V input voltage. The

switching frequency is set to 1.5MHz. Choosing an

LIR of 0.6 and estimating efficiency of 85% at this

operating point:

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

estimating efficiency of 80% at that operating point:

The ripple current and the peak current are:

The inductor DCR should be low enough for reasonable

efficiency. As a rule of thumb, do not allow the voltage

drop across the inductor DCR to exceed a few percent

of the input voltage at IPEAK.

Many notebook panel designs have height constraints

on the components. If a thin inductor with the required

current rating is not available, use two thin inductors in

series or parallel.

Current-Sense Network Selection

After selecting the inductor, use the following steps to

design the current-sense network for lossless current

sensing.

1) Calculate the RC time constant of the sense network

using the typical inductance and typical DCR:

2) Determine the component values of the sense net-

work. Select CS, and then calculate RS using:

3) Calculate the worst-case high sense voltage over

temperature using the maximum DCR value (RL(MAX))

found in the inductor technical specifications:

where IPEAK is the peak inductor current calculated in

the Inductor Selection section, TC is the temperature

coefficient of copper (0.5%/°C) and

∆T is the difference

between the specified temperature for RL(MAX) and the

maximum expected inductor temperature.

4) Compare the calculated sense voltage with the mini-

mum value of the current-limit threshold in the Electrical

Characteristics (100mV). If the sense voltage is between

80mV and 100mV, use the current-sense configuration

in Figure 8 with the calculated CS and RS above.

SENSE

PEAK

L MAX

()

=×

()

∆

= τ

()

RL TYP

MHz

RIPPLE

PEAK

()

××

≈

IN DC MAX

)

≈

MHz

= ⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝⎜

⎞

⎠⎟

≈

085

RIPPLE

IN MIN

MAIN

IN MIN

MAIN

OSC

PEAK

IN DC MAX

RIPPLE

()

)

()

××

IN DC MAX

MAIN EFF

MAIN

IN MIN

MIN

)

()

×η

LIR

MAIN

MAIN EFF

OSC

TYP

()

⎛

⎝⎜

⎞

⎠⎟

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝⎜

⎞

⎠⎟

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