LP3219-04

Apr.-2016

www.lowpowersemi.com

Page 9 of 12

Preliminary Datasheet

LP3219

Dropout Operation

When the battery input voltage decreases near the value of

the output voltage, the LP3219 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:

oscillator frequency. The output voltage then is the input

voltage minus the voltage drop across the main switch and

P-channel 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.

Maximum Load Current

The LP3219 will operate with an input supply voltage as low

as 2.5V, however, the maximum load current decreases at

lower input voltages due to a large IR drop on the main switch

and synchronous rectifier. The slope compensation signal

reduces the peak inductor current as a function of the duty

cycle to prevent sub-harmonic oscillations at duty cycles

greater than 50%. Conversely, the current limit increases as

the duty cycle decreases.

Setting the Output Voltage

The LP3219 can be externally programmed. Resistors R1 and

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 LP3219 delivers enhanced transient response for

extreme pulsed load applications. The addition of the feed

output voltage according to the following equation:

Inductor Selection

For most designs, the LP3219 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:

Where ΔIL is inductor ripple current. Large value inductors

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 1μH.

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+1.2A).