LP3218-04

Jun.-2015

www.lowpowersemi.com

Page 6 of 8

Preliminary Datasheet

LP3218

Dropout Operation

When the battery input voltage decreases near the value of

the output voltage, the LP3218 allows the main switch to

remain on for more than one switching cycle and increases

the duty cycle until it reaches 100%. The duty cycle D of a

step-down converter is defined as:

frequency. The output voltage then is the input voltage minus

the voltage drop across the main switch and the inductor. At

MOSFET increases, and the efficiency of the converter

decreases. Caution must be exercised to ensure the heat

dissipated does not exceed the maxi-mum junction

temperature of the IC.

Setting the Output Voltage

The LP3218 can be externally programmed. Resistors R1 and

R2 program the output to regulate at a voltage higher than

0.6V. To limit the bias current required for the external

feedback resistor string while maintaining good noise

immunity, the minimum suggested value for R1 is 50kΩ.

Although a larger value will further reduce quiescent current, it

will also increase the impedance of the feedback node,

making it more sensitive to external noise and interference.

The LP3218, combined with an external feed forward

capacitor (C3), delivers enhanced transient response for

extreme pulsed load applications. The addition of the feed

forward capacitor typically requires a larger output capacitor

C2 for stability. The external resistor sets the output voltage

according to the following equation:

Inductor Selection

For most designs, the LP3218 operates with inductor values

of 1μH to 4.7μH. Low inductance values are physically smaller

but require faster switching, which results in some efficiency

loss. The inductor value can be derived from the following

equation:

lower ripple current and small value inductors result in high

ripple currents. Choose inductor ripple current approximately

60% of the maximum load current 2A, or

For output voltages above 2.0V, when light-load efficiency is

important, the minimum recommended inductor is 2.2μH.

Manufacturer’s specifications list both the inductor DC current

rating, which is a thermal limitation, and the peak current

rating, which is determined by the saturation characteristics.

The inductor should not show any appreciable saturation

under normal load conditions. Some inductors may meet the

peak and average current ratings yet result in excessive

losses due to a high DCR.

Always consider the losses associated with the DCR and its

effect on the total converter efficiency when selecting an

inductor. For optimum voltage-positioning load transients,

choose an inductor with DC series resistance in the 20mΩ to

100mΩ range. For higher efficiency at heavy loads (above

200mA), or minimal load regulation (but some transient

overshoot), the resistance should be kept below 100mΩ.

The DC current rating of the inductor should be at least equal

to the maximum load current plus half the ripple current to

prevent core saturation (2A + 600mA).