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LTC3552

3552f

The inductor value will also have an effect on Burst Mode

operation. The transition from low current operation be-

gins when the peak inductor current falls below a level

set by the burst clamp. Lower inductor values result in

higher ripple current which causes this to occur at lower

load currents. This causes a dip in efﬁciency in the upper

range of low current operation. In Burst Mode operation,

lower inductance values will cause the burst frequency

to increase.

Inductor Core Selection

Different core materials and shapes will change the

size/current and price/current relationship of an induc-

tor. Toroid or shielded pot cores in ferrite or permalloy

materials are small and do not radiate much energy, but

generally cost more than powdered iron core inductors

with similar electrical characteristics. The choice of which

style inductor to use often depends more on the price vs

size requirements and any radiated ﬁeld/EMI requirements

than on what the LTC3552 requires to operate. Table 1

shows some typical surface mount inductors that work

well in LTC3552 applications.

Table 1. Representative Surface Mount Inductors

PART

NUMBER

VALUE

(µH)

DCR

(Ω MAX)

MAX DC

CURRENT (A)

SIZE

W × L × H (mm)

Sumida

CDRH3D16

2.2

3.3

4.7

0.075

0.110

0.162

1.20

1.10

0.90

3.8 × 3.8 × 1.8

Sumida

CDRH2D11

1.5

2.2

0.068

0.170

0.900

0.780

3.2 × 3.2 × 1.2

Sumida

CMD4D11

2.2

3.3

0.116

0.174

0.950

0.770

4.4 × 5.8 × 1.2

Murata

LQH32CN

1.0

2.2

0.060

0.097

1.00

0.79

2.5 × 3.2 × 2.0

Toko

D312F

2.2

3.3

0.060

0.260

1.08

0.92

2.5 × 3.2 × 2.0

Murata

ELT5KT

3.3

4.7

0.17

0.20

1.00

0.95

4.5 × 5.4 × 1.2

When charging, transient loads on the BAT pin can cause

the ITERM pin to fall below 100mV for short periods of

time before the DC charge current has dropped to 10%

of the programmed value. The 1ms ﬁlter time (tTERM) on

the termination comparator ensures that transient loads

of this nature do not result in premature charge cycle

termination. Once the average charge current drops be-

low the programmed termination threshold, the charger

terminates the charge cycle and stops providing current

out of the BAT pin. In this state, any load on the BAT pin

must be supplied by the battery.

The charger constantly monitors the BAT pin voltage in

standby mode. If this voltage drops below the 4.1V re-

charge threshold (VRECHRG), another charge cycle begins

and charge current is once again supplied to the battery.

To manually restart a charge cycle when in standby mode,

the input voltage must be removed and reapplied, or the

charger must be shut down and restarted using the

⎯E⎯N pin.

Switching Regulator Inductor Selection

The inductor value has a direct effect on inductor ripple

current ΔIL, which decreases with higher inductance and

increases with higher VCC or VOUT:

∆=

−

⎛

⎝⎜

⎞

⎠⎟

OUT

•

Accepting larger values of ΔIL allows the use of low

inductances, but results in higher output ripple voltage,

greater core losses, and lower output current capability.

A reasonable starting point for setting ripple current is

ΔIL = 0.3 • IOUT(MAX),

where IOUT(MAX) is 800mA for regulator 1 and 400mA for

regulator 2. The largest ripple current ΔIL occurs at the

maximum input voltage. To guarantee that the ripple cur-

rent stays below a speciﬁed maximum, the inductor value

should be chosen according to the following equation:

OUT

CC MAX

=−

⎛

⎝

⎜

⎞

⎠

⎟

•

()

∆

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