LTC3108-1_12 Datasheet, PDF (15/24 Pages) Linear Technology – Ultralow Voltage Step-Up Converter

LTC3108-1
APPLICATIONS INFORMATION
C1 Capacitor
The charge pump capacitor that is connected from the
transformer’s secondary winding to the C1 pin has an ef-
fect 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 Applications
schematic examples for the recommended value for a
given turns ratio.
Squegging
Certain types of oscillators, including transformer-coupled
oscillators such as the resonant oscillator of the LTC3108-1,
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 LTC3108-1, 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 LTC3108-1.
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 LTC3108-1
(connected to the C1 pin) are 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 cur-
rent capability from the LTC3108. However, due to the
resulting increase in rectifier currents and resonant oscil-
lator frequency in these applications, the use of external
charge pump rectifiers is recommended for optimal
performance.
In applications where the step-up ratio is 1:20 or less, and
the C1 capacitor is 10nF or greater, the C1 pin should be
grounded and two external rectifiers (such as 1N4148 or
1N914 diodes) should be used. These are available as
dual diodes in a single package. Avoid the use of Schottky
rectifiers, as their lower forward-voltage drop increases
the minimum startup voltage. See the Typical Applications
schematics for an example.
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, the duration of the load pulse, and
the amount of voltage droop the circuit can tolerate. The
capacitor must be rated for whatever voltage has been
selected for V_OUT by VS1 and VS2.
C_OUT (μF)
≥
I_LOAD(mA) • t_PULSE(ms)
ΔV_OUT (V)
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 (to be discussed
in the next example). 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 holdup
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