LT3575
7
3575f
OPERATION
The LT3575 is a current mode switching regulator IC
designed specifi cally for the isolated fl yback 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 maintain regulation. Historically,
this has been done with optoisolators or extra transformer
windings. Optoisolator circuits waste output power and
the extra components increase the cost and physical size
of the power supply. Optoisolators can also exhibit trouble
due to limited dynamic response, nonlinearity, unit-to-unit
variation and aging over life. Circuits employing extra
transformer windings also exhibit defi ciencies. Using an
extra winding adds to the transformer’s physical size and
cost, and dynamic response is often mediocre.
The LT3575 derives its information about the isolated
output voltage by examining the primary side fl yback
pulse waveform. In this manner, no optoisolator nor extra
transformer winding is required for regulation. The output
voltage is easily programmed with two resistors. Since this
IC operates in boundary control mode, the output voltage is
calculated from the switch pin when the secondary current
is almost zero. This method improves load regulation
without external 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: internal bias regulator,
oscillator, logic, current amplifi er and comparator, driver,
and output switch. The novel sections include a special
fl yback error amplifi er and a temperature compensation
circuit. In addition, the logic system contains additional
logic for boundary mode operation, and the sampling
error amplifi er.
The LT3575 features a boundary mode control method,
where the part operates at the boundary between continuous
conduction mode and discontinuous conduction mode. The
V
C
pin controls the current level just as it does in normal
current mode operation, but instead of turning the switch
on at the start of the oscillator period, the part detects
when the secondary side winding current is zero.
Boundary Mode Operation
Boundary mode is a variable frequency, current-mode
switching scheme. The switch turns on and the inductor
current increases until a V
C
pin controlled current limit. The
voltage on the SW pin rises to the output voltage divided
by the secondary-to-primary transformer turns ratio plus
the input voltage. When the secondary current through
the diode falls to zero, the SW pin voltage falls below V
IN
.
A discontinuous conduction mode (DCM) comparator
detects this event and turns the switch back on.
Boundary mode returns the secondary current to zero
every cycle, so the 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 no subharmonic oscillation.
At low output currents the LT3575 delays turning on the
switch, and thus operates in discontinuous mode. Unlike
a traditional fl yback converter, the switch has to turn on
to update the output voltage information. Below 0.6V on
the V
C
pin, the current comparator level decreases to
its minimum value, and the internal oscillator frequency
decreases in frequency. With the decrease of the internal
oscillator, the part starts to operate in DCM. The output
current is able to decrease while still allowing a minimum
switch off-time for the error amp sampling circuitry. The
typical minimum internal oscillator frequency with V
C
equal to 0V is 40kHz.
