LTC3552-1 Linear Technology, LTC3552-1 Datasheet - Page 13

• Manufacturer Part Number: LTC3552-1
• Description: Standalone Linear Li-Ion Battery Charger and Dual Synchronous Buck Converter
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
The charger constantly monitors the BAT pin voltage in
standby mode. If this voltage drops below the 4.1V re-
charge threshold (V
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
increases with higher V
Accepting larger values of ΔI
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
where I
regulator 2. The largest ripple current ΔI
maximum input voltage. To guarantee that the ripple cur-
rent stays below a specifi ed 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 begins
when the peak inductor current falls below a level set by
the burst clamp. Lower inductor values result in higher rip-
ple current which causes this to occur at lower load cur-
rents. This causes a dip in effi ciency in the upper range of
low current operation. In Burst Mode operation, lower induc-
tance 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
ΔI
L
∆ =
I
=
L
L
= 0.3 • I
f
OUT(MAX)
O
V
V
OUT
•
f
O
L
OUT
∆
, which decreases with higher inductance and
•
I
L
L
⎛
⎜
⎝
⎛
⎝ ⎜
OUT(MAX)
1
1
is 800mA for regulator 1 and 400mA for
−
−
V
V
U
RECHRG
V
CC MAX
OUT
V
CC
OUT
(
,
CC
⎞
⎠ ⎟
U
or V
), another charge cycle begins
)
⎞
⎟
⎠
OUT
L
allows the use of low
:
W
L
occurs at the
U
with similar electrical characteristics. The choice of which
style inductor to use often depends more on the price vs
size requirements and any radiated fi eld/EMI requirements
than on what the LTC3552-1 requires to operate. Table 1
shows some typical surface mount inductors that work
well in LTC3552-1 applications.
Input Capacitor (C
In continuous mode, the input current of the converter is a
square wave with a duty cycle of approximately V
To prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
where the maximum average output current I
the peak current minus half the peak-to-peak ripple cur-
rent, I
V
is commonly used to design because even signifi cant
deviations do not offer much relief. Note that capacitor
manufacturer’s ripple current ratings are often based
on only 2000 hours life-time. This makes it advisable to
further derate the capacitor, or choose a capacitor rated at
a higher temperature than required. Several capacitors may
also be paralleled to meet the size or height requirements
of the design. An additional 0.1µF to 1µF ceramic capacitor
is also recommended on V
when not using an all ceramic capacitor solution.
Table 1. Representative Surface Mount Inductors
PART
NUMBER
Sumida
CDRH3D16
Sumida
CDRH2D11
Sumida
CMD4D11
Murata
LQH32CN
Toko
D312F
Murata
ELT5KT
CC
I
RMS
= 2V
MAX
≈
OUT
I
MAX
= I
, where I
VALUE
(µH)
LIM
2.2
3.3
4.7
1.5
2.2
2.2
3.3
1.0
2.2
2.2
3.3
3.3
4.7
V
– ΔI
OUT
IN
(Ω MAX)
RMS
) Selection
L
0.075
0.110
0.162
0.068
0.170
0.116
0.174
0.060
0.097
0.060
0.260
(
DCR
0.17
0.20
/2.This formula has a maximum at
V
V
CC
CC
= I
CC
−
OUT
for high frequency decoupling,
V
CURRENT (A)
OUT
/2. This simple worst-case
MAX DC
0.900
0.780
0.950
0.770
1.20
1.10
0.90
1.00
0.79
1.08
0.92
1.00
0.95
)
LTC3552-1
W × L × H (mm)
3.8 × 3.8 × 1.8
3.2 × 3.2 × 1.2
4.4 × 5.8 × 1.2
2.5 × 3.2 × 2.0
2.5 × 3.2 × 2.0
4.5 × 5.4 × 1.2
MAX
SIZE
OUT
13
equals
/V
35521f
CC
.