LP28300 datasheet(8/10 Pages) POWER

8 / 10 page

Preliminary Datasheet

LP28300

LP28300 – 01 Version

1.1 Datasheet

Sep.-2010

www.lowpowersemi.com

Page 8 of 10

temperature rises to 50°C, the resistance of the

NTC will be approximately 4.2kΩ. With the 85µA

pull-up current source, the Hot temperature

voltage threshold is 360mV. For Cold temperature,

the voltage threshold is set at 2.4V which is equal

to 0°C (Rts 28kΩ) with 85µA of pull-up current.

If the temperature is outside the window, the

GATE pin will be pulled up to VCC and the timer

frozen while the output status at the STAT pin

remains the same. The charge cycle begins or

resumes once the temperature is within the

acceptable range. Short the TS pin to ground to

disable the temperature qualification feature.

However the user may modify these thresholds by

adding two external resistor. See figure 3.

Figure 2. Temperature Sensing Configuration

Figure 3. Temperature Sensing Thresholds

Input and Output Capacitors

Since the input capacitor is assumed to absorb all

input switching ripple current in the converter, it

must have an adequate ripple current rating.

Worst-case RMS ripple current is approximately

one-half of output charge current. Actual

capacitance value is not critical. Solid tantalum

capacitors have a high ripple current rating in a

relatively small surface mount package, but

caution must be used when tantalum capacitors

are used for input bypass. High input surge

currents can be created when the adapter is

hot-plugged to the charger and solid tantalum

capacitors have a known failure mechanism when

subjected to very high turn-on surge currents.

Selecting the highest possible voltage rating on

the capacitor will minimize problems. Consult

with the manufacturer before use. The selection

of output capacitor COUT is primarily determined

by the ESR required to minimize ripple voltage

and load step transients. The output ripple ∆VOUT

is approximately bounded by:



Since ∆IL increases with input voltage, the output

ripple is highest at maximum input voltage.

Typically, once the ESR requirement is satisfied,

the capacitance is adequate for filtering and has

the necessary RMS current rating. Switching

ripple current splits between the battery and the

output capacitor depending on the ESR of the

output capacitor and the battery impedance. EMI

considerations usually make it desirable to

minimize ripple current in the battery leads.

Ferrite beads or an inductor may be added to

increase battery impedance at the 500kHz

switching frequency. If the ESR of the output

capacitor is 0.2Ω and the battery impedance is

raised to 4Ω with a bead or inductor, only 5% of

the current ripple will flow in the battery.

Inductor Selection

A high (1.5MHz) operating frequency was chosen

for the buck switcher in order to minimize the size

of the inductor. However, take care to use inductors

with low core losses at this frequency. A good

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