15585 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'15585' PDF : 20 Pages View PDF
LTC1558-3.3/LTC1558-5
APPLICATIONS INFORMATION
Table 2. Recommended Inductors
MANUFACTURER
PART
NUMBER
TYP INDUCTOR
VALUE
Sumida
CD54-220
22µH ±20%
Sumida
CDRH73/74 22µH ±20%
DCR (Ω)
0.18
0.2/0.11
Capacitor Selection
The LTC1558 requires a minimum VBAK capacitor of 44µF
to ensure that the boost converter can regulate the output
at 20mA load. The capacitor’s ESR should be small (< 0.2Ω)
to minimize voltage spikes that might incorrectly trigger
the LTC1558’s internal FB comparators. Note that the
LTC1558 can usually share the output capacitor with the
system regulator. Thus its ratings like VMAX, IRIPPLE(RMS),
etc., will all have to meet the system regulator’s specifica-
tions as well.
Battery Selection
A primary application for the LTC1558 is a “bridging”
supply, only providing backup current while the main
system battery is being replaced. In these applications,
the LTC1558 works well with NiCd button cells or small
cylindrical cells, reducing system costs and board space.
It is optimized for use with up to 512mAhr battery
capacities.
The LTC1558 can work with standard or memory backup
specific NiCd cells. Memory backup cells can operate at
higher temperatures and have lower self discharge rates.
The LTC1558’s trickle charger is designed to accommo-
date both memory backup cells (with low self discharge)
and standard cells (with higher self discharge). Some
recommended manufacturers and part numbers are listed
in Table 3.
Table 3. Button/Cylindrical NiCd 1.2V Cells
MANUFACTURER
PART
NUMBER
CAPACITY
(mAhr)
SAFT
GB60
60
(Memory Backup)
GB170
170
GB280
280
SAFT
(Standard)
VB10E
100
VB22E
220
VB30E
300
VB60E
600
Sanyo
(Standard)
N-50AAA
55
N-110AA
120
N-120TA
130
N-150N
170
N-200AAA
220
N-270AA
305
N-500A
500
Panasonic
(Standard)
P-11AA
110
RSERIES (Ω)
1.1
0.4
0.4
0.038
0.022
0.017
0.014
0.055
0.03
0.034
0.027
0.021
0.015
0.09
0.08
The internal resistance of the backup cell increases power
dissipation as the boost converter draws current from it
during switching, degrading efficiency. Due to the fixed
inductor peak current architecture, the LTC1558’s boost
converter output power drops significantly when the NiCd
cell’s internal resistance increases at the end of its charge.
This is because the inductor charging time will increase
due to a larger R/L time constant, decreasing the switching
frequency. It is advisable, especially for batteries with high
internal resistance, to include a 47µF bypass capacitor
across the battery to ensure that the boost converter can
deliver the maximum output power regardless of the NiCd
internal resistance.
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