LTC3108 Linear Technology, LTC3108 Datasheet - Page 14

• Manufacturer Part Number: LTC3108
• Description: Ultralow Voltage Step-Up Converter and Power Manager
• Manufacturer: Linear Technology

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LTC3108
APPLICATIONS INFORMATION
To minimize losses and capacitor charge time, all capaci-
tors used for V
See Table 5 for recommended storage capacitors.
Table 5. Recommended Storage Capacitors
VENDOR
AVX
www.avx.com
Cap-XX
www.cap-xx.com
Cooper/Bussman
www.bussmann.com/3/PowerStor.html
Vishay/Sprague
www.vishay.com/capacitors
Storage capacitors requiring voltage balancing are not
recommended due to the current draw of the balancing
resistors.
PCB Layout Guidelines
Due to the rather low switching frequency of the resonant
converter and the low power levels involved, PCB layout
is not as critical as with many other DC/DC converters.
There are, however, a number of things to consider.
14
OUT
and VSTORE should be low leakage.
Figure 3. Example Component Placement for Two-Layer PC Board (DFN Package)
V
V
OUT2
PART NUMBER/SERIES
BestCap Series
TAJ and TPS Series Tantalum
GZ Series
KR Series
P Series
Tantamount 592D
595D Tantalum
150CRZ/153CRV Aluminum
013 RLC (Low Leakage)
VIAS TO GROUND PLANE
OUT
VLDO
PGOOD
GND
VSTORE
V
VAUX
VLDO
V
OUT2
PGD
OUT
1
2
3
4
5
6
Due to the very low input voltage the circuit may operate
from, the connections to V
former and the SW and GND pins of the LTC3108 should
be designed to minimize voltage drop from stray resistance
and able to carry currents as high as 500mA. Any small
voltage drop in the primary winding conduction path will
lower effi ciency and increase capacitor charge time.
Also, due to the low charge currents available at the out-
puts of the LTC3108, any sources of leakage current on
the output voltage pins must be minimized. An example
board layout is shown in Figure 3.
Design Example 1
This design example will explain how to calculate the
necessary storage capacitor value for V
applications, such as a wireless sensor/transmitter. In these
types of applications, the load is very small for a major-
ity of the time (while the circuitry is in a low power sleep
state), with bursts of load current occurring periodically
during a transmit burst. The storage capacitor on V
supports the load during the transmit burst, and the long
sleep time between bursts allows the LTC3108 to recharge
3108 FO3
12
11
10
9
8
7
SW
C2
C1
V
VS1
VS2
OUT2_EN
V
IN
IN
, the primary of the trans-
OUT
in pulsed load
3108p
OUT