LT3574IMS#PBF Linear Technology, LT3574IMS#PBF Datasheet - Page 7
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LT3574IMS#PBF
Manufacturer Part Number
LT3574IMS#PBF
Description
IC PWM FLYBCK ISO CM 16MSOP
Manufacturer
Linear Technology
Type
Flyback, Isolated, PWM - Current Moder
Datasheet
1.LT3574EMSPBF.pdf
(24 pages)
Specifications of LT3574IMS#PBF
Internal Switch(s)
Yes
Synchronous Rectifier
No
Number Of Outputs
1
Voltage - Output
Adj to 60V
Current - Output
650mA
Frequency - Switching
40kHz ~ 1MHz
Voltage - Input
3 ~ 40 V
Operating Temperature
-40°C ~ 125°C
Mounting Type
*
Package / Case
16-MSOP
Voltage - Supply
3 V ~ 40 V
Frequency-max
1MHz
Duty Cycle
52%
Pwm Type
Current Mode
Buck
No
Boost
No
Flyback
Yes
Inverting
No
Doubler
No
Divider
No
Cuk
No
Isolated
Yes
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Available stocks
Company
Part Number
Manufacturer
Quantity
Price
operation
The LT3574 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 opto-isola-
tors or extra transformer windings. Opto-isolator circuits
waste output power and the extra components increase
the cost and physical size of the power supply. Opto-
isolators 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 LT3574 derives its information about the isolated
output voltage by examining the primary side flyback
pulse waveform. In this manner, no opto-isolator 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 LT3574 features a boundary mode control method,
where the part operates at the boundary between continu-
ous conduction mode and discontinuous conduction mode.
The VC 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 VC 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
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 LT3574 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 VC 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 VC
equal to 0V is 40kHz.
LT3574
3574f
IN
.