LTC3805IDD-5-TRPBF datasheet(8/20 Pages) LINER

Part #	LTC3805IDD-5-TRPBF

Description	Adjustable Frequency Current Mode Flyback/ Boost/SEPIC DC/DC Controller

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Manufacturer	LINER [Linear Technology]

Direct Link	http://www.linear.com

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LTC3805-5

8

38055fa

OPERATION

The LTC3805-5 is a programmable-frequency current mode

controller for ﬂyback, 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 ampliﬁer responds by feeding current

into the ITH 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 ITH pin causing its voltage to fall.

The voltage at the ITH pin controls the pulse-width modula-

tor formed by the oscillator, current comparator and SR

latch. Speciﬁcally, the voltage at the ITH pin sets the current

comparator’s trip threshold. The current comparator’s

ISENSE input monitors the voltage across an external cur-

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

does the voltage on the ISENSE pin. The LTC3805-5’s cur-

rent comparator trips when the voltage on the ISENSE pin

exceeds a voltage proportional to the voltage on the ITH 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 ITH 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 ITH 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 ITHpinservesasthecompensationpointforthecontrol

loop. Typically, an external series RC network is connected

from ITH 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

ITH voltage which sets the current comparator threshold

and commands considerable inﬂuence over the dynamics

of the voltage regulation loop.

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LTC3805IDD-5-TRPBF Datasheet(PDF) 8 Page - Linear Technology

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