LT1105_15 Datasheet, PDF (18/32 Pages) Linear Technology – Offline Switching Regulator

LT1103/LT1105
APPLICATIONS INFORMATION
low resistance for best performance. This will help minimize
voltage spikes associated with the high dI/dt switch current.
Avoiding long wire runs to the ground pin minimizes load
regulation effects and inductive voltages created by the
high dI/dt switch current. Ground plane techniques should
also be used and will help keep EMI to a minimum.
Grounding techniques are illustrated in the Typical
Applications section.
Oscillator
The oscillator of the LT1103/LT1105 is a linear ramp type
powered from the internal 6V bias line. The charging
currents and voltage thresholds are generated internally
so that only one external capacitor is required to set the
frequency. The 150µA pull-up current, which is on all the
time, sets the preset maximum on-time of the switch and
the 450µA pull-down current which is turned on and off,
sets the dead time. The threshold voltages are typically 2V
and 4.5V, so for a 400pF capacitor the ramp-up time of the
voltage on the OSC pin is 6.67µs and the ramp-down time
is 3.3µs, resulting in an operating frequency of 100kHz.
Although the oscillator, as well as the rest of the switching
regulator, will function at higher frequencies, 200kHz is
the practical upper limit that will allow control range for
line and load regulation. The lowest operating frequency is
limited by the sampling error amplifier to about 10kHz.
The frequency temperature coefficient is typically –80ppm/
°C with a good low T.C. capacitor. This means that with a
low temperature coefficient capacitor, the temperature
coefficient of the currents and the temperature coefficient
of the thresholds sum to –80ppm/°C over the commercial
temperature range. Bowing in the temperature coefficient
of the currents affects the frequency about ±3% at the
extremes of the military temperature range. The capacitor
type chosen will have a direct effect on the frequency
tempco.
Maximum duty cycle is set internally by the pull-up and
pull-down currents, independent of frequency. It can be
adjusted externally by modifying the fixed pull-up current
with an additional resistor. In practice, one resistor from
the OSC pin to the 5V reference or to ground does the job.
Note that the capacitor value must change to maintain the
same frequency. For example, a 24k resistor from 5V to
OSC and a 440pF capacitor from OSC to ground will yield
100kHz with 50% maximum duty cycle. A 56k resistor and
a 280pF capacitor from OSC to ground will yield 100 kHz
with 80% maximum duty cycle.
The oscillator can be synchronized to an external clock by
coupling a sync pulse into the OSC pin. The width of this
pulse should be a minimum of 500ns. The oscillator can
only be synchronized up in frequency and the synchronizing
frequency must be greater than the maximum possible
unsynchronized frequency (for the chosen oscillator
capacitor value). The amplitude of the sync pulse must be
chosen so that the sum of the oscillator voltage amplitude
plus the sync pulse amplitude does not exceed the 6V bias
reference. Otherwise, the oscillator pull-up current source
will saturate and erroneous operation will result. If the
LT1103/LT1105 is positioned on the primary side of the
transformer and the external clock on the isolated secondary
output side, the sync signal must be coupled into the OSC
pin using a pulse transformer. The pulse transformer must
meet all safety/isolation requirements as it also crosses
the isolation boundary. An example of externally
synchronizing the oscillator is shown in the Typical
Applications section.
Gate Biasing (LT1103)
The LT1103 is designed to drive an external power MOSFET
in the common gate or cascode connection with the V_SW
pin. The advantage is that the switch current can be sensed
internally, eliminating a low value, power sense resistor.
The gate needs to be biased at a voltage high enough to
guarantee that the FET is saturated when the open-collector
source drive is on. This means 10V as specified in FET data
sheets, plus 1V for the typical switch saturation voltage,
plus a couple of volts for temperature variations and
processing tolerances. This leads to 15V for a practical
gate bias voltage.
Power MOSFETs are well suited to switching power supplies
because their high speed switching characteristics promote
high switching efficiency. To achieve high switching speed,
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