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LTC4012-3

Theoutputcapacitorshownacrossthebatteryandground

must also absorb PWM output ripple current. The general

formula for this capacitor current is:

L f

RMS

BAT

CLP

PWM









0 29

. •

• –

•

For example, IRMS = 0.22A with:

VBAT = 12.6V

VCLP = 19V

L1 = 10µH

fPWM = 550kHz

High capacity ceramic capacitors (20µF or more) available

from a variety of manufacturers can be used for input/out-

put capacitors. Other alternatives include OS-CON and

POSCAP capacitors from Sanyo.

Low ESR solid tantalum capacitors have high ripple cur-

rent rating in a relatively small surface mount package,

but exercise caution when using tantalum for input or

output bulk capacitors. High input surge current can be

created when the adapter is hot-plugged to the charger

or when a battery is connected to the charger. Solid tan-

talum capacitors have a known failure mechanism when

subjected to very high surge currents. Select tantalum

capacitors that have high surge current ratings or have

been surge tested.

EMI considerations usually make it desirable to minimize

ripple current in battery leads. Adding Ferrite beads or

inductors can increase battery impedance at the nominal

550kHzswitchingfrequency.Switchingripplecurrentsplits

between the battery and the output capacitor in inverse

relation to capacitor ESR and the battery impedance. If

the ESR of the output capacitor is 0.2

Ω and the battery

impedance is raised to 4

Ω with a ferrite bead, only 5%

of the current ripple will flow to the battery.

Inductor Selection

Higher switching frequency generally results in lower ef-

ficiencybecauseofMOSFETgatechargelosses,butitallows

smaller inductor and capacitor values to be used. A primary

effect of the inductor value L1 is the amplitude of ripple

current created. The inductor ripple current ∆IL decreases

with higher inductance and PWM operating frequency:

∆I

L f

BAT

CLP

PWM













• –

•

Accepting larger values of ∆IL allows the use of low in-

ductance, but results in higher output voltage ripple and

greater core losses. Lower charge currents generally call

for larger inductor values.

The LTC4012-3 limits maximum instantaneous peak in-

ductor current during every PWM cycle. To avoid unstable

switch waveforms, the ripple current must satisfy:

∆I

SENSE

MAX













2 150

•

–

so choose:

CLP

PWM

SENSE

MAX

0 125

150









•

–

For C-grade parts, a reasonable starting point for setting

ripple current is ∆IL = 0.4 • IMAX. For I-grade parts, use

∆IL =0.2•IMAXonlyiftheICwillactuallybeusedtocharge

batteries over the wider I-grade temperature range. The

voltage compliance of internal LTC4012-3 circuits also

imposes limits on ripple current. Select RIN (in Figure 1)

to avoid average current errors in high ripple designs. The

following equation can be used for guidance:

µA

SENSE

•

∆

≤

applicaTions inForMaTion

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