LT8315
9
8315fa
For more information www.linear.com/LT8315
OPERATION
Boundary Mode Operation
Boundary mode is a variable frequency, current-mode
switching scheme. The internal N-channel MOSFET turns
on and the inductor current increases until it reaches the
limit determined by the voltage on the VC pin and the sense
resistor’s value. After the internal MOSFET turns off, the
voltage on the tertiary winding rises to the output voltage
multiplied by the transformer tertiary-to-secondary turns
ratio. After the current through the output diode falls to
zero, the voltage on the tertiary winding falls. A boundary
mode detection comparator on the DCM pin detects the
negative dV/dt associated with the falling voltage and trig-
gers the sample-and-hold circuit to sample the FB voltage.
When the tertiary voltage reaches its minimum and stops
falling, the boundary mode comparator turns the internal
MOSFET back on for minimal switching energy loss.
Boundary mode operation returns the secondary current
to zero every cycle, so parasitic resistive voltage drops
do not cause load regulation errors. Boundary mode
also allows the use of a smaller transformer compared
to continuous conduction mode and does not exhibit
subharmonic oscillation.
Discontinuous Conduction Mode Operation
As the load gets lighter, the peak switch current decreases.
Maintaining boundary mode requires the switching fre-
quency to increase. An excessive switching frequency
increases switching and gate charge losses. To limit these
losses, the LT8315 features an internal oscillator which
limits the maximum switching frequency to 140kHz. Once
the switching frequency hits this limit, the part starts to
reduce its switching frequency and operates in discontinu-
ous conduction mode.
Low Ripple Burst Mode Operation
Unlike traditional flyback converters, the internal MOSFET
has to turn on and off to generate a flyback pulse in order
to update the sampled output voltage. The duration of a
well-formed flyback pulse must exceed the minimum-off
time for proper sampling. To this end, a minimum switch
turn-off current is necessary to ensure a flyback pulse of
sufficient duration.
The LT8315 is a high-voltage current-mode switching
regulator designed for the isolated flyback topology. The
special problem normally encountered in such circuits is
that information relating to the output voltage on the isolated
secondary side of the transformer must be communicated
to the primary side in order to achieve regulation. This is
often performed by opto-isolator circuits, which waste
output power, require extra components that increase the
cost and physical size of the power supply, and exhibit
trouble due to limited dynamic response, nonlinearity,
unit-to-unit variation, and aging over life.
The LT8315 does not need an opto-isolator because it
derives its information about the isolated output voltage
by examining the flyback pulse waveform appearing on a
tertiary winding on the transformer. The output voltage is
easily programmed with two resistors.
The LT8315 features a boundary mode control method (also
called critical conduction mode), where the part operates
at the boundary between continuous conduction mode and
discontinuous conduction mode. Due to boundary mode
operation, the output voltage can be determined from the
tertiary winding’s voltage when the secondary current is
almost zero. This method improves load regulation without
extra resistors and capacitors.
The Block Diagram shows an overall view of the system.
Many of the blocks are similar to those found in traditional
switching regulators, including current comparator, internal
reference, LDO, logic, timers, and an N-channel MOSFET.
The novel sections include a special sampling error ampli-
fier, a temperature compensation circuit, an output current
regulator
, and a depletion-mode startup FET.
Depletion Startup FET
The LT8315 features an internal depletion mode MOSFET.
At startup, this transistor charges the INTVCC capacitor
so that the LT8315 has power to begin switching. This
removes the need for an external bleeder resistor or other
components.