LTC3109 Linear Technology, LTC3109 Datasheet - Page 16

• Manufacturer Part Number: LTC3109
• Description: Ultralow Voltage Step-Up Converter and Power Manager
• Manufacturer: Linear Technology

applicaTions inForMaTion
LTC3109
Note that there must be enough energy available from the
input voltage source for V
during the interval between load pulses (as discussed in
Design Example 1). Reducing the duty cycle of the load
pulse will allow operation with less input energy.
The VSTORE capacitor may be of very large value (thou-
sands of microfarads or even Farads), to provide energy
storage at times when the input voltage is lost. Note that
this capacitor can charge all the way to the VAUX clamp
voltage of 5.25V typical (regardless of the settings for
V
ing voltage rating of at least 5.5V at the temperature that
it will be used.
The VSTORE input is not designed to provide high pulse
load currents to V
V
The VSTORE capacitor can be sized using the following
formula:
where 7µA is the quiescent current of the LTC3109, I
the load on V
the LDO between pulses, I
pulse, t
of the pulses, t
and V
programmed output voltage of 5V, the VSTORE capacitor
cannot provide any beneficial storage time to V
To minimize losses and capacitor charge time, all capaci-
tors used for V
See Table 6 for recommended storage capacitors.
Table 6. 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

C
OUT
OUT
STORE
), so be sure that the holdup capacitor has a work-
is limited to about 26mA max.
OUT
PULSE
≥
(
is the output voltage required. Note that for a
7
µA I
is the duration of the pulse, f is the frequency
OUT
OUT
+ +
STORE
Q
OUT
in between pulses, I
and VSTORE should be low leakage.
I
. The current path from VSTORE to
LDO
is the total storage time required
PULSE
+
OUT
5
(
I
.2 2 5 – V
PULSE
to recharge the capacitor
is the total load during the
PART NUMBER/SERIES
BestCap Series
TAJ and TPS Series Tantalum
GZ Series
KR Series
P Series
Tantamount 592D
595D Tantalum
OUT
•
t
PULSE
LDO
is the load on
•
f
)
)
OUT
•
t
STORE
.
Q
is
Note that storage capacitors requiring voltage balancing
resistors are not recommended due to the steady-state
current draw of the 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.
Due to the very low input voltages the circuit operates from,
the connections to V
and the SW, V
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 efficiency and increase start-up voltage and capaci-
tor charge time.
Also, due to the low charge currents available at the out-
puts of the LTC3109, any sources of leakage current on
the output voltage pins must be minimized. An example
board layout is shown in Figure 4.
Figure 4. Example Component Placement for 2-Layer PC Board
(QFN Package). Note That VSTORE and VOUT Capacitor Sizes
are Application Dependent
IN
and GND pins of the LTC3109 should be
IN
, the primary of the transformers
www.DataSheet4U.com
3109f