LTC3109_15 Datasheet, PDF (16/24 Pages) Linear Technology – Auto-Polarity, Ultralow Voltage Step-Up Converter and Power Manager

LTC3109
Applications Information
Note that there must be enough energy available from the
input voltage source for V_OUT to recharge the capacitor
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_OUT), so be sure that the holdup capacitor has a work-
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_OUT . The current path from VSTORE to
V_OUT is limited to about 26mA max.
The VSTORE capacitor can be sized using the following
formula:
( ) _{CSTORE ≥}
7µA +I_Q +I_LDO + ₍I_PULSE • t_PULSE • f₎
5.25 – V_OUT
• t_STORE
where 7µA is the quiescent current of the LTC3109, I_Q is
the load on V_OUT in between pulses, I_LDO is the load on
the LDO between pulses, I_PULSE is the total load during the
pulse, t_PULSE is the duration of the pulse, f is the frequency
of the pulses, t_STORE is the total storage time required
and V_OUT is the output voltage required. Note that for a
programmed output voltage of 5V, the VSTORE capacitor
cannot provide any beneficial storage time to V_OUT .
To minimize losses and capacitor charge time, all capaci-
tors used for V_OUT and VSTORE should be low leakage.
See Table 6 for recommended storage capacitors.
Table 6. Recommended Storage Capacitors
VENDOR
PART NUMBER/SERIES
AVX
www.avx.com
BestCap Series
TAJ and TPS Series Tantalum
Cap-XX
www.cap-xx.com
GZ Series
Cooper/Bussman
KR Series
www.bussmann.com/3/PowerStor.html P Series
Vishay/Sprague
www.vishay.com/capacitors
Tantamount 592D
595D Tantalum
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_IN, the primary of the transformers
and the SW, V_IN and GND pins of the LTC3109 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 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
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For more information www.linear.com/LTC3109
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