LTC3459 LINER [Linear Technology], LTC3459 Datasheet - Page 10

• Manufacturer Part Number: LTC3459
• Description: 10V Micropower Synchronous Boost Converter in ThinSOT
• Manufacturer: LINER [Linear Technology]

LTC3459
Charging a SuperCap
SuperCaps have become a popular alternative to NiCd
batteries as backup power sources in portable equipment.
Capacitance values of one Farad and higher are achievable
in small package sizes with leakage currents in the low
microamps. SuperCaps are typically charged at low cur-
rents for several minutes until they reach the required
back-up voltage.
The LTC3459 is designed to control peak inductor current
when V
current to be controlled during start-up in a boost applica-
tion, for example, or V
powered from a fresh battery. Peak current control makes
the LTC3459 an ideal candidate for charging a back-up
source such as a SuperCap. Figure 5 shows an application
where the LTC3459 is used to charge a two Farad, 5V
supercap from a 3.3V input. A NiCd battery could be
charged by the LTC3459 as well, but that application may
require additional circuitry for proper charge termination.
10
TYPICAL APPLICATIO S
IN
is greater than or less than V
OUT
®
to be regulated below V
U
3.3V
Figure 5. Charging a SuperCap from a 3.3V Source
+
OUT
C
L1: 33 H, 1.7 TAIYO YUDEN LB2016
OUT
. This allows
: MAXWELL TECHNOLOGIES ULTRACAP PC5-5, 2F, 5V
OFF ON
1 F
IN
when
L1
V
SHDN
IN
LTC3459
GND
SW
V
When V
synchronous P-channel MOSFET rectifier is connected to
V
is delivered to the load. While efficiency is compromised
in this mode of operation, current to the SuperCap is
controlled, preventing any damaging effects of inrush
current. Proper heat sinking of the SOT package is re-
quired in this application as the die may dissipate 100mW
to 200mW during initial charging. When V
than ~3.5V normal boost mode operation and efficiency
begin, with the P-channel MOSFET acting as a synchro-
nous switch. Average input current is a constant 50mA
during charging, where the current delivered to the
SuperCap varies somewhat with duty cycle. Once the
supercap is charged to 5V, the LTC3459 begins to regulate
and the input current is reduced to the amount required to
support the load and/or self discharge of the SuperCap.
SuperCap is a registered trademark of Baknor Industries.
OUT
FB
IN
and the SW pin rises a diode above V
1M
332k
OUT
is less than ~3.5V, the body of the internal
1 F
3459 F05
C
2F
OUT
V
5V
OUT
IN
OUT
when current
is greater
3459f