LTC3805-5
8
38055fe
OPERATION
The LTC3805-5 is a programmable-frequency current mode
controller for flyback, boost and SEPIC DC/DC converters.
The LTC3805-5 is designed so that none of its pins need
to come in contact with the input or output voltages of
the power supply circuit of which it is a part, allowing
the conversion of voltages well beyond the LTC3805-5’s
absolute maximum ratings.
Main Control Loop
Please refer to the Block Diagram of this data sheet and
the Typical Application shown on the front page. An ex-
ternal resistive voltage divider presents a fraction of the
output voltage to the FB pin. The divider is designed so
that when the output is at the desired voltage, the FB pin
voltage equals the 800mV internal reference voltage. If
the load current increases, the output voltage decreases
slightly, causing the FB pin voltage to fall below the 800mV
reference. The error amplifier responds by feeding current
into the I
TH
pin causing its voltage to rise. Conversely, if
the load current decreases, the FB voltage rises above the
800mV reference and the error amplifier sinks current away
from the I
TH
pin causing its voltage to fall.
The voltage at the I
TH
pin controls the pulse-width modula-
tor formed by the oscillator, current comparator and SR
latch. Specifically, the voltage at the I
TH
pin sets the cur-
rent comparator’s trip threshold. The current comparator’s
I
SENSE
input monitors the voltage across an external current
sense resistor in series with the source of the external
MOSFET. At the start of a cycle, the LTC3805-5’s oscillator
sets the SR latch and turns on the external power MOSFET.
The current through the external power MOSFET rises as
does the voltage on the I
SENSE
pin. The LTC3805-5’s cur-
rent comparator trips when the voltage on the I
SENSE
pin
exceeds a voltage proportional to the voltage on the I
TH
pin.
This resets the SR latch and turns off the external power
MOSFET. In this way, the peak current levels through the
external MOSFET and the flyback transformer’s primary
and secondary windings are controlled by the voltage on
the I
TH
pin. If the current comparator does not trip, the
LTC3805-5 automatically limits the duty cycle to 80%,
resets the SR latch, and turns off the external MOSFET.
The path from the FB pin, through the error amplifier,
current comparator and the SR latch implements the
closed-loop current mode control required to regulate the
output voltage against changes in input voltage or output
current. For example, if the load current increases, the
output voltage decreases slightly, and sensing this, the
error amplifier sources current from the I
TH
pin, raising
the current comparator threshold, thus increasing the peak
currents through the transformer primary and secondary.
This delivers more current to the load and restores the
output voltage to the desired level.
The I
TH
pin serves as the compensation point for the control
loop. Typically, an external series RC network is connected
from I
TH
to ground and is chosen for optimal response to
load and line transients. The impedance of this RC network
converts the output current of the error amplifier to the
I
TH
voltage which sets the current comparator threshold
and commands considerable influence over the dynamics
of the voltage regulation loop.
