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4012f

LTC4012/

LTC4012-1/LTC4012-2

The output capacitor shown across the battery and ground

must also absorb PWM output ripple current. The general

formula for this capacitor current is:

RMS

BAT

CLP

PWM

⎛

⎝⎜

⎞

⎠⎟

029

.•

•

–

•

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

550KHz switching frequency. Switching ripple current splits

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 efficiency because of MOSFET gate charge

losses, but it allows 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 ΔIL decreases with

higher inductance and PWM operating frequency:

ΔI

BAT

CLP

PWM

⎛

⎝

⎜

⎞

⎠

⎟

•–

•

Accepting larger values of ΔIL 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 limits maximum instantaneous peak inductor

current during every PWM cycle. To avoid unstable switch

waveforms, the ripple current must satisfy:

ΔI

SENSE

MAX

⎛

⎝

⎜

⎞

⎠

⎟

150

•–

so choose:

CLP

PWM

SENSE

MAX

0 125

150

⎛

⎝⎜

⎞

⎠⎟

.•

•–

A reasonable starting point for setting ripple current

is ΔIL = 0.4 • IMAX. The voltage compliance of internal

LTC4012 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

••

ΔΔ

≤≤

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