LTC3552 Datasheet, PDF (14/24 Pages) Linear Technology – Standalone Linear Li-Ion Battery Charger and Dual Synchronous Buck Converter

LTC3552
APPLICATIO S I FOR ATIO
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 (t_TERM) 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 (V_RECHRG), 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 ΔI_L, which decreases with higher inductance and
increases with higher V_CC or V_OUT:
∆I_L
=
V_OUT
f_O • L
⎛
⎝⎜ ^1−
V_OUT
V_CC
⎞
⎠⎟
Accepting larger values of ΔI_L 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
ΔI_L = 0.3 • I_OUT(MAX),
where I_OUT(MAX) is 800mA for regulator 1 and 400mA for
regulator 2. The largest ripple current ΔI_L 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:
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 DCR
MAX DC
SIZE
(µH) (Ω MAX) CURRENT (A) W × L × H (mm)
Sumida
2.2
0.075
1.20
3.8 × 3.8 × 1.8
CDRH3D16 3.3
0.110
1.10
4.7
0.162
0.90
Sumida
1.5
CDRH2D11 2.2
0.068
0.170
0.900
0.780
3.2 × 3.2 × 1.2
Sumida
2.2
CMD4D11 3.3
0.116
0.174
0.950
0.770
4.4 × 5.8 × 1.2
Murata
1.0
0.060
1.00
2.5 × 3.2 × 2.0
LQH32CN 2.2
0.097
0.79
Toko
D312F
2.2
0.060
1.08
2.5 × 3.2 × 2.0
3.3
0.260
0.92
Murata
ELT5KT
3.3
0.17
4.7
0.20
1.00
4.5 × 5.4 × 1.2
0.95
L
=
V_OUT
f_O • ∆I_L
⎛
⎝⎜ ^1−
V_OUT
V_CC(MAX)
⎞
⎠⎟
3552f
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