LTC3109
15
3109fb
For more information www.linear.com/LTC3109
applicaTions inForMaTion
Using a 1:100 primary-secondary ratio yields start-up
voltages as low as 30mV. Other factors that affect per
-
formance are the resistance of the transformer windings
and the inductance of the windings. Higher DC resistance
will result in lower efficiency and higher start-up volt
-
ages. The secondary winding inductance will determine
the resonant frequency of the oscillator, according to the
formula below.
Freq =
2 • π • L
SEC
• C
Hz
where L
SEC
is the inductance of one of the secondary
windings and C is the load capacitance on the second
-
ary winding. This is comprised of the input capacitance
at pin C2A or C2B, typically 70pF each, in parallel with
the transformer secondary winding’s shunt capacitance.
The recommended resonant frequency is in the range of
10kHz to 100kHz. Note that loading will also affect the
resonant frequency. See Table 5 for some recommended
transformers.
Table 5. Recommended Transformers
VENDOR
TYPICAL START-
UP VOLTAGE PART NUMBER
Coilcraft
www.coilcraft.com
25mV
35mV
85mV
LPR6235-752SML (1:100 ratio)
LPR6235-123QML (1:50 ratio)
LPR6235-253PML (1:20 ratio)
Würth
www.we-online
25mV
35mV
85mV
74488540070 (1:100 Ratio)
74488540120 (1:50 Ratio)
74488540250 (1:20 Ratio)
USING EXTERNAL CHARGE PUMP RECTIFIERS
The synchronous rectifiers in the LTC3109 have been
optimized for low frequency, low current operation, typical
of low input voltage applications. For applications where
the resonant oscillator frequency exceeds 100kHz, or a
transformer turns ratio of less than 1:20 is used, or the
C1A and C1B capacitor values are greater than 68nF, the
use of external charge pump rectifiers (1N4148 or 1N914
or equivalent) is recommended. See the Typical Application
circuits for an example. Avoid the use of Schottky recti
-
fiers, as their low forward voltage increases the minimum
start-up voltage.
C1 CAPACITOR
The charge pump capacitor that is connected from each
transformer’s secondary winding to the corresponding
C1A and C1B pins has an effect on converter input resis
-
tance and maximum output current capability. Generally
a minimum value of 1nF is recommended when operating
from very low input voltages using a transformer with
a ratio of 1:100. Capacitor values of 2.2nF to 10nF will
provide higher output current at higher input voltages,
however larger capacitor values can compromise perfor
-
mance when operating at low input voltage or with high
resistance sources. For higher input voltages and lower
turns ratios, the value of the C1 capacitor can be increased
for higher output current capability. Refer to the Typical
Applications examples for the recommended value for a
given turns ratio.
C2 CAPACITOR
The C2 capacitors connect pins C2A and C2B to their
respective transformer secondary windings. For most
applications a capacitor value of 470pF is recommended.
Smaller capacitor values tend to raise the minimum
start-up voltage, and larger capacitor values can lower
efficiency.
Note that the C1 and C2 capacitors must have a voltage
rating greater than the maximum input voltage times the
transformer turns ratio.
V
OUT
AND VSTORE CAPACITOR
For pulsed load applications, the V
OUT
capacitor should
be sized to provide the necessary current when the load
is pulsed on. The capacitor value required will be dictated
by the load current (I
LOAD
), the duration of the load pulse
(t
PULSE
), and the amount of V
OUT
voltage droop the ap-
plication can tolerate (∆V
OUT
). The capacitor must be
rated for whatever voltage has been selected for V
OUT
by
VS1 and VS2:
C
OUT
(µF) ≥
LOAD(mA)
PULSE(ms)
∆V
OUT
(V)
