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LTC4100

4100fb

APPLICATIONS INFORMATION

IMAX is the full-scale charge current. Chose the lowest

IMAX value that is still above your expected battery charge

current as requested over the SMBus. If you deviate

from the resistance values shown in Table 9, it will lead

to charge current gain errors. The requested current and

the actual charge current applied to the battery will not

be the same.

Table 9. Recommended Resistor Values

IMAX (A)

RSENSE (Ω) 1%

RSENSE (W)

RILIM (Ω) 1%

1.023

0.100

0.25

2.046

0.05

0.25

10k

3.068

0.025

0.5

33k

4.092

0.025

0.5

Open

Warning

DO NOT CHANGE THE VALUE OF RILIM DURING OPERA-

TION. The value must remain ﬁxed and track the RSENSE

value at all times. Changing the current setting can result

in currents that greatly exceed the requested value and

potentially damage the battery or overload the wall adapter

if no input current limiting is provided.

Inductor Selection

Higher operating frequencies allow the use of smaller

inductor and capacitor values. A higher frequency gener-

ally results in lower efﬁciency because of MOSFET gate

charge losses. In addition, the effect of inductor value

on ripple current and low current operation must also be

considered. The inductor ripple current

ΔILdecreaseswith

higher frequency and increases with higher VIN.

ΔI

OUT

= ()( )

−

⎛

⎝⎜

⎞

⎠⎟

Accepting larger values of

ΔIL allows the use of low

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is

ΔIL = 0.4(IMAX). Remember the

maximum

ΔIL occurs at the maximum input voltage. The

inductor value also has an effect on low current operation.

The transition to low current operation begins when the

inductor current reaches zero while the bottom MOSFET

is on. Lower inductor values (higher

ΔIL) will cause this

to occur at higher load currents, which can cause a dip in

efﬁciency in the upper range of low current operation. In

practice 10μH is the lowest value recommended for use.

Table 10. Recommended Inductor Values

Maximum Average

Current (A)

Input Voltage (V)

Minimum Inductor Value

(μH)

≤20

40 ± 20%

>20

56 ± 20%

≤20

20 ± 20%

>20

30 ± 20%

≤20

15 ± 20%

>20

20 ± 20%

≤20

10 ± 20%

>20

15 ± 20%

Charger Switching Power MOSFET

and Diode Selection

Two external power MOSFETs must be selected for use

with the charger: a P-channel MOSFET for the top (main)

switch and an N-channel MOSFET for the bottom (syn-

chronous) switch.

The peak-to-peak gate drive levels are set internally. This

voltage is typically 6V. Consequently, logic-level threshold

MOSFETs must be used. Pay close attention to the BVDSS

speciﬁcation for the MOSFETs as well; many of the logic

level MOSFETs are limited to 30V or less.

Selection criteria for the power MOSFETs include the

on-resistance RDS(ON), total gate capacitance QG, reverse

transfer capacitance CRSS, input voltage and maximum

output current. The charger is operating in continuous

mode so the duty cycles for the top and bottom MOSFETs

are given by:

Main Switch Duty Cycle = VOUT/VIN

Synchronous Switch Duty Cycle = (VIN – VOUT)/VIN

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