Datasheet

LTC3459
9
3459fc
Very low operating quiescent current and synchronous
operation allow for greater than 85% conversion effi ciency
in many applications. Lower output voltages will result in
lower effi ciencies since the N- and P-channel R
DS(ON)
s
will increase. The switching frequency and output power
capability of the LTC3459 are also dependant on input and
output voltages.
Charging a SuperCap
®
SuperCaps have become a popular alternative to NiCd
batteries as back-up power sources in portable equip-
ment. 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 currents for several minutes until they reach the
required back-up voltage.
TYPICAL APPLICATIONS
5V from Li-Ion Input
The LTC3459 is designed to control peak inductor current
when V
IN
is greater than or less than V
OUT
. This allows
current to be controlled during start-up in a boost applica-
tion, for example, or V
OUT
to be regulated below V
IN
when
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 Su-
perCap 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.
When V
OUT
is less than ~3.5V, the body of the internal
synchronous P-channel MOSFET rectifi er is connected to
V
IN
, and the SW pin rises a diode above V
IN
when current
is delivered to the load. While effi ciency is compromised
in this mode of operation, current to the SuperCap is
10V from 3.3V or 5V Input
SW
15
μH*
V
IN
1M
V
OUT
5V
4.7
μF
47pF
1μF
LTC3459
SHDN
V
OUT
FB
Li-Ion
BATTERY
V
IN
2.5V TO 4.2V
OFF ON
GND
332k
3459 TA04a
*COILCRAFT DO3314
+
I
LOAD
(mA)
60
EFFICIENCY (%)
70
80
90
100
0.01 1 10 100
3459 TA04b
50
0.1
V
OUT
= 5V
V
IN
= 4.2V
V
IN
= 2.5V
SW
33
μH*
V
IN
2M
V
OUT
10V
V
IN
3.3V TO 5V
4.7
μF
47pF
1μF
LTC3459
SHDN
V
OUT
FBOFF ON
GND
280k
3459 TA05a
*COILCRAFT DO3314
I
LOAD
(mA)
60
EFFICIENCY (%)
70
80
90
100
0.01 1 10 100
3459 TA05b
50
0.1
V
OUT
= 10V
V
IN
= 3.3V
V
IN
= 5V