LT3573EMSEPBF LINER [Linear Technology], LT3573EMSEPBF Datasheet - Page 7

• Manufacturer Part Number: LT3573EMSEPBF
• Description: Isolated Flyback Converter without an Opto-Coupler
• Manufacturer: LINER [Linear Technology]

operation
The LT3573 is a current mode switching regulator IC
designed specifically for the isolated flyback topology.
The special problem normally encountered in such cir‑
cuits 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 optoisola‑
tors or extra transformer windings. Optoisolator circuits
waste output power and the extra components increase
the cost and physical size of the power supply. Optoiso‑
lators 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 deficiencies. Using an extra winding adds to
the transformer’s physical size and cost, and dynamic
response is often mediocre.
The LT3573 derives its information about the isolated
output voltage by examining the primary side flyback
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 cur‑
rent 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 amplifier and comparator, driver,
and output switch. The novel sections include a special
flyback error amplifier and a temperature compensation
circuit. In addition, the logic system contains additional
logic for boundary mode operation, and the sampling
error amplifier.
The LT3573 features a boundary mode control method,
where the part operates at the boundary between continu‑
ous conduction mode and discontinuous conduction mode.
The V
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
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
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 LT3573 delays turning on the
switch, and thus operates in discontinuous mode. Unlike
a traditional flyback converter, the switch has to turn on
to update the output voltage information. Below 0.6V on
the V
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
equal to 0V is 40kHz.
C
C
pin controls the current level just as it does in normal
pin, the current comparator level decreases to
C
pin controlled current limit. The
LT3573
3573fb

IN
C
.