LTC3805-5 Linear Technology, LTC3805-5 Datasheet - Page 8

• Manufacturer Part Number: LTC3805-5
• Description: Adjustable Frequency Current Mode Flyback DC/DC Controller
• Manufacturer: Linear Technology

www.datasheet4u.com
LTC3805-5
OPERATION
The LTC3805-5 is a programmable-frequency current mode
controller for fl yback, boost and SEPIC DC/DC convert-
ers. 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 amplifi er responds by feeding current
into the I
the load current decreases, the FB voltage rises above the
800mV reference and the error amplifi er sinks current away
from the I
The voltage at the I
tor formed by the oscillator, current comparator and SR
latch. Specifi cally, the voltage at the I
comparator’s trip threshold. The current comparator’s
I
rent 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
8
SENSE
input monitors the voltage across an external cur-
TH
TH
pin causing its voltage to rise. Conversely, if
pin causing its voltage to fall.
TH
pin controls the pulse-width modula-
TH
pin sets the current
does the voltage on the I
rent comparator trips when the voltage on the I
exceeds a voltage proportional to the voltage on the I
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 fl yback transformer’s primary
and secondary windings are controlled by the voltage on
the I
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 amplifi er,
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 amplifi er sources current from the I
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
loop. Typically, an external series RC network is connected
from I
load and line transients. The impedance of this RC network
converts the output current of the error amplifi er to the
I
and commands considerable infl uence over the dynamics
of the voltage regulation loop.
TH
voltage which sets the current comparator threshold
TH
TH
TH
pin serves as the compensation point for the control
pin. If the current comparator does not trip, the
to ground and is chosen for optimal response to
SENSE
pin. The LTC3805-5’s cur-
TH
pin, raising
SENSE
TH
38055fa
pin.
pin