LP3318-00

May.-2018

www.lowpowersemi.com

Page 4 of 6

Preliminary Datasheet

LP3318

Operation Information

The LP3318 uses a boost regulator architecture to

regulate voltage at the feedback pin. At the start of

each oscillator cycle the MOSFET is turned on

through the control circuitry. When this voltage

equals The output voltage of the error amplifier the

power MOSFET is turned off. The voltage at the

output of the error amplifier is an amplified version of

the difference between the 1.23V bandgap reference

voltage and the feedback voltage. In this way the

peak current level keeps the output in regulation. If

the feedback voltage starts to drop, the output of the

error amplifier increases. These results in more

current to flow through the power MOSFET, thus

increasing the power delivered to the output.

TheLP3318 has internal soft start to limit the amount

of input current at startup and to also limit the amount

of overshoot on the output.

Current Limitation

The internal power-MOS switch current is monitored

cycle-by-cycle and is limited to the value not exceed

1.5A (Typ.). When the switch current reaches the

limited value, the internal power-MOS is turned off

immediately until the next cycle.

Setting the Output Voltage

Set the output voltage by selecting the resistive

voltage divider ratio. The voltage divider drops the

output voltage to the 1.23V feedback voltage. Use a

100K resistor for R2 of the voltage divider. Determine

the high-side resistor R1 by the equation:

(

)

Inductor Selection

For a better efficiency in high switching frequency

converter, the inductor selection has to use a proper

core material such as ferrite core to reduce the core

loss and choose low ESR wire to reduce copper loss.

The most important point is to prevent the core

saturated when handling the maximum peak current.

Using a shielded inductor can minimize radiated

noise in sensitive applications. The maximum peak

inductor current is the maximum input current plus

the half of inductor ripple current. The calculated

peak current has to be smaller than the current

limitation in the electrical characteristics. A typical

setting of the inductor ripple current is 20% to 40% of

the maximum input current. If the selection is 40%,

the maximum peak inductor current is

(

)

(

)

(

(

)

(

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)

The minimum inductance value is derived from the

following equation:

(

)

(

(

(

)

Depending on the application, the recommended

inductor value is between 4.7μH to 10μH.

Diode Selection

To achieve high efficiency, Shottky diode is good

choice for low forward drop voltage and fast

switching time. The output diode rating should be

able to handle the maximum output voltage, average

power dissipation and the pulsating diode peak

current.