Datasheet

LTC4100
22
4100fc
For more information www.linear.com/LTC4100
applicaTions inForMaTion
I
MAX
is the full-scale charge current. Chose the lowest I
MAX
value that is still above your expected battery charge cur-
rent 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
I
MAX
(A) R
SENSE
(Ω) 1% R
SENSE
(W) R
ILIM
(Ω) 1%
1.023 0.100 0.25 0
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 R
ILIM
DURING OPERA-
TION. The value must remain fixed and track the R
SENSE
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 ∆I
L
decreases with
higher frequency and increases with higher V
IN
.
∆I
L
=
1
(f)(L)
V
OUT
1−
V
OUT
V
IN






Accepting larger values of ∆I
L
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 ∆I
L
= 0.4(I
MAX
). Remember the
maximum ∆I
L
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 ∆I
L
) 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)
1 ≤20 40 ± 20%
1 >20 56 ± 20%
2 ≤20 20 ± 20%
2 >20 30 ± 20%
3 ≤20 15 ± 20%
3 >20 20 ± 20%
4 ≤20 10 ± 20%
4 >20 15 ± 20%
Charger Switching Power MOSFET
and Diode Selection
Tw o
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 BV
DSS
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 R
DS(ON)
, total gate capacitance Q
G
, reverse
transfer capacitance C
RSS
, 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 = V
OUT
/V
IN
Synchronous Switch Duty Cycle = (V
IN
– V
OUT
)/V
IN