LTC3107
17
3107f
For more information www.linear.com/LTC3107
applicaTions inForMaTion
of the windings. Higher DC resistance will result in lower
efficiency. The secondary winding inductance will deter-
mine the
resonant frequency of the oscillator, according
to the formula:
FREQ =
2• π • L•C
Hz
Where L is the inductance of the transformer secondary
winding and C is the load capacitance on the secondary
winding. This is comprised of the input capacitance at pin
C2, typically 30pF, in parallel with the transformer sec
-
ondary winding’s shunt capacitance.
The recommended
resonant frequency is in the range of 10kHz to 100kHz.
Note that the loading also plays a role in the effective load
capacitance, and will therefore have an effect on the fre
-
quency. See Tab
le 3 for some recommended transformers.
Squegging
Certain types of oscillators, including transformer coupled
oscillators such as the resonant oscillator of the LTC3107,
can exhibit a phenomenon called squegging. This term
refers to a condition that can occur which blocks or stops
the oscillation for a period of time much longer than the
period of oscillation, resulting in bursts of oscillation. An
example of this is the Blocking Oscillator, which is designed
to squegg to produce bursts of oscillation. Squegging is also
encountered in RF oscillators and regenerative receivers.
In the case of the LTC3107, squegging can occur when a
charge builds up on the C2 gate coupling capacitor, such
that the DC bias point shifts and oscillation is extinguished
for a
certain period of time,
until the charge on the capacitor
bleeds off, allowing oscillation to resume. It is difficult to
predict when and if squegging will occur in a given ap
-
plication. While squegging is not harmful, it reduces the
average output current capability of the LTC3107.
Squegging can easily be avoided by the addition of a
bleeder resistor in parallel with the coupling capacitor on
the C2 pin. Resistor values in the range of 100k to 1MΩ
are sufficient to eliminate squegging without having any
negative impact on performance. For the 330pF capacitor
used for C2 in most applications, a 499k bleeder resistor
is recommended. See the Typical Applications schematics
for an example.
Using External Charge Pump Rectifiers
The synchronous charge pump rectifiers in the LTC3107
(connected to the C1 pin) are low current and optimized
for operation from very low input voltage sources, using
typical transformer step-up ratios between 1:100 and
1:50, and typical C1 charge pump capacitor values less
than 10nF.
Operation from higher input voltage sources (typically
250mV or greater, under load), allows the use of lower
transformer step-up ratios (such as 1:20 and 1:10) and
larger C1 capacitor values to provide higher output current
capability
from the LTC3107. However, due to the result-
ing increase in rectifier currents and resonant oscillator
frequency in these
applications, the use of external charge
pump rectifiers is recommended for optimal performance
in these applications.
Table 3. Recommended Transformers
VENDOR PART NUMBER
Coilcraft
www.coilcraft.com
LPR6235-752SML (1:100 Ratio)
LPR6235-123QML (1:50 Ratio)
LPR6235-253PML (1:20 Ratio)
Würth
www.we-online
74488540070 (1:100 Ratio)
74488540120 (1:50 Ratio)
74488540250 (1:20 Ratio)
C1 Capacitor
The charge pump capacitor that is connected from the
transformer’s secondary winding to the C1 pin has an
effect on converter input resistance and maximum output
current capability. Generally a minimum value of 1nF is
recommended when operating from very low input volt
-
ages using a transformer with a ratio of 1:100. Too large
a capacitor value can compromise performance 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 Application
schematic examples for the recommended value for a
given turns ratio.
