LTC1558-5 Linear Technology, LTC1558-5 Datasheet - Page 14

• Manufacturer Part Number: LTC1558-5
• Description: Backup Battery Controller with Programmable Output
• Manufacturer: Linear Technology

APPLICATIONS
LTC1558-3.3/LTC1558-5
Table 2. Recommended Inductors
MANUFACTURER
Sumida
Sumida
Capacitor Selection
The LTC1558 requires a minimum V
to ensure that the boost converter can regulate the output
at 20mA load. The capacitor’s ESR should be small (< 0.2 )
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 V
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.
14
CDRH73/74
CD54-220
NUMBER
U
PART
INFORMATION
U
TYP INDUCTOR
22 H 20%
22 H 20%
VALUE
W
BAK
MAX
capacitor of 44 F
, I
RIPPLE(RMS)
DCR ( )
U
0.2/0.11
0.18
,
Table 3. Button/Cylindrical NiCd 1.2V Cells
MANUFACTURER
SAFT
(Memory Backup)
SAFT
(Standard)
Sanyo
(Standard)
Panasonic
(Standard)
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 47 F bypass capacitor
across the battery to ensure that the boost converter can
deliver the maximum output power regardless of the NiCd
internal resistance.
N-200AAA
N-50AAA
N-110AA
N-270AA
N-120TA
NUMBER
N-150N
N-500A
P-11AA
GB170
GB280
VB10E
VB22E
VB30E
VB60E
PART
GB60
CAPACITY
(mAhr)
100
220
300
600
120
170
280
130
170
220
305
500
110
60
55
R
SERIES
0.038
0.022
0.017
0.014
0.055
0.034
0.027
0.021
0.015
0.03
0.09
0.08
1.1
0.4
0.4
( )