LTC3350_15 Datasheet, PDF (26/46 Pages) Linear Technology – High Current Supercapacitor Backup Controller and System Monitor

LTC3350
Applications Information
maximum power transfer rule to maximize the utilization
ratio. The minimum voltage in this case is:
V_CELL(MIN) =
4R_SC •P_BACKUP
nη
where η is the efficiency of the boost converter
(~90% to 96%). For the backup equation, γ_MAX and γ_MIN,
substitute P_BACKUP/η for P_BACKUP. In this case the energy
needed for backup is governed by the following equation:
P_BACKUP
η
t_BACKUP
≤
21 ^nCSC
• _VC2ELL(MAX) •



α_B
+
2
α_B
– ^{1– αB}
2
ln


1+
1–
α_B
α_B




7. If a suitable capacitor is not available, iterate by choosing
more capacitance, a higher cell voltage, more capacitors
in the stack and/or a lower utilization ratio.
8. Make sure to take into account the lifetime degrada-
tion of ESR and capacitance, as well as the maximum
discharge current rating of the supercapacitor. A list of
supercapacitor suppliers is provided in Table 2.
Table 2. Supercapacitor Suppliers
AVX
www.avx.com
Bussman
www.cooperbussman.com
CAP-XX
www.cap-xx.com
Illinois Capacitor
www.illcap.com
Maxwell
www.maxwell.com
NESS CAP
www.nesscap.com
Tecate Group
www.tecategroup.com
Once a capacitance is found using the above equation the
maximum ESR allowed needs to be checked:
( ) _RSC
≤
η
1– α_B nV_C2ELL(MAX)
4P_BACKUP
Capacitor Selection Procedure
1. Determine backup requirements P_BACKUP and t_BACKUP.
2. Determine maximum cell voltage that provides accept-
able capacitor lifetime.
3. Choose number of capacitors in the stack.
Inductor Selection
The switching frequency and inductor selection are in-
terrelated. Higher switching frequencies allow the use
of smaller inductor and capacitor values, but generally
results in lower efficiency due to MOSFET switching and
gate charge losses. In addition, the effect of inductor value
on ripple current must also be considered. The inductor
ripple current decreases with higher inductance or higher
frequency and increases with higher V_IN. Accepting larger
values of ripple current allows the use of low inductances
but results in higher output voltage ripple and greater
core losses.
4. Choose a desired utilization ratio, α_B, for the superca- _{For the LTC3350, the best overall performance will be}
pacitor (e.g., 80%).
attained if the inductor is chosen to be:
5. Solve for capacitance, C_SC:
C_SC
≥
2P_BACKUP • t_BACKUP
nη_{VC2 ELL(MAX )}
•
( ) ^

α_B
+
 ²
α_B
– ^{1– αB}
2

ln_{}
1+ α_B
1– α_B
 ^–1



L = ^VIN(MAX)
I_CHG(MAX) • f_SW
for V_IN(MAX) ≤ 2V_CAP and:
L
=


1–
V_CAP
V_{IN(MAX )}


0.25
V_CAP
• I_{CHG(MAX )}
•
f_SW
6. Find supercapacitor with sufficient capacitance C_SC and
minimum R_SC:
( ) _RSC
≤
η
1– α_B nV_C2ELL(MAX)
4P_BACKUP
for V_IN(MAX) ≥ 2V_CAP, where V_CAP is the final supercapaci-
tor stack voltage, V_IN(MAX) is the maximum input voltage,
I_CHG(MAX) is the maximum regulated charge current, and
f_SW is the switching frequency. Using these equations, the
inductor ripple will be at most 25% of I_CHG(MAX).
3350fb
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