Datasheet

LTC3625/LTC3625-1
12
3625f
applicaTions inForMaTion
The buck and boost converters are designed to work with
inductors over a wide range of inductances. Choosing a
higher valued inductor will decrease operating frequen-
cies, while a lower valued inductor will increase frequency
but also increase peak current overshoot/undershoot. For
most applications a 3.3µH inductor is recommended. To
maximize efficiency, choose an inductor with a low DC
resistance. Choose an inductor with a DC current rating
at least as large as the maximum I
PEAK
the application will
see according to the specifications table to ensure that the
inductor does not saturate during normal operation. If the
single inductor application is used, make sure to size the
inductor for the higher of buck or boost peak currents.
Different core materials and shapes will change the size/cur-
rent and price/current relationship of an inductor. 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. Inductors that are very thin or
have a very small volume typically have much higher core
and DCR losses, and will not give the best efficiency. The
choice of which style inductor to use often depends more
on the price versus size, performance and any radiated
EMI requirements than on what the LTC3625/LTC3625-1
family requires to operate.
Table 2 shows several inductors that work well with the
LTC3625/LTC3625-1 regulators. These inductors offer a
good compromise in current rating, DCR and physical
size. Consult each manufacturer for detailed information
on their entire selection of inductors.
Supercapacitor Selection
The LTC3625/LTC3625-1 are designed to charge super-
capacitors of values greater than 0.1F per cell. In general,
lower capacitance cells have higher ESRs, therefore lower
charge currents should be used to help reduce sleep
modulation towards the end of a charge cycle. In general,
the ESR of a supercapacitor cell should not exceed:
ESR
mV
I
BUCK
≤
100
where 100mV is the sleep threshold hysteresis. Higher
capacitance cells typically have lower ESRs and can
therefore be charged with higher currents. Typically, the
LTC3625/LTC3625-1 are designed to charge supercapaci-
tors with values up to 100F, but higher capacitance cells
could be used at the expense of greater charge time.
T
able
3 shows several supercapacitors that work well with
the LTC3625/LTC3625-1.
Printed Circuit Board Layout Considerations
In order to be able to deliver maximum current under all
conditions, it is critical that the exposed pad on the backside
of the LTC3625/LTC3625-1 package be soldered to the PC
Table 2. Inductor Manufacturers
MANUFACTURER PART NUMBER INDUCTANCE (µH) CURRENT (A) DCR (mΩ) SIZE (mm)
Coiltronics DR73-3R3-R 3.3 3.0 20
7 × 7
Coilcraft MSS7341-332NL 3.3 3.2 20
7 × 7
Vishay IHLM2525CZER3R3M11 3.3 6.5 26
6.5 × 6.9
Sumida CDRH6D28P-3RON 3.0 3.0 24
7 × 7
TOKO B1077AS-3RON 3.0 3.3 30
7.6 × 7.6
Table 3. Supercapacitor Manufacturers
MANUFACTURER PART NUMBER VALUE (F) OPERATING VOLTAGE (V) MAXIMUM ESR (mΩ) SIZE (mm)
Cooper Bussmann B1860-2R5107-R 100 2.5 20
18 × 60
Illinois Capacitor 107DCN2R7M 100 2.7 10
22 × 45
NESS Capacitor ESHSR-0100C0002R7 100 2.7 9
22 × 45
Tecate TPLS-100//22 X 45F 100 2.7 9
22 × 45
Maxwell BCAP120P250 120 2.5 2.5
26 × 51