6

LTC3803

3803i

OPERATIO

The LTC3803 is a constant frequency current mode con-

troller for flyback and DC/DC boost converter applications

in a tiny ThinSOT package. The LTC3803 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 LTC3803’s absolute maximum ratings.

Main Control Loop

Due to space limitations, the basics of current mode

DC/DC conversion will not be discussed here; instead, the

reader is referred to the detailed treatment in Application

Note 19, or in texts such as Abraham Pressman’s

Switch-

ing Power Supply Design.

Please refer to the Block Diagram and the Typical Applica-

tion on the front page of this data sheet. An external

resistive voltage divider presents a fraction of the output

voltage will equal the 800mV from the internal reference.

If the load current increases, the output voltage will

800mV. The error amplifier responds by feeding current

voltage will rise above 800mV and the error amplifier will

modulator formed by the oscillator, current comparator

sets the current comparator’s trip threshold. The current

comparator monitors the voltage across a current sense

resistor in series with the source terminal of the external

MOSFET. The LTC3803 turns on the external power

MOSFET when the internal free-running 200kHz oscillator

sets the RS latch. It turns off the MOSFET when the current

comparator resets the latch or when 80% duty cycle is

reached, whichever happens first. In this way, the peak

current levels through the flyback transformer’s primary

fier whenever the output voltage is below nominal, and

decreased whenever output voltage exceeds nominal, the

voltage regulation loop is closed. For example, whenever

the load current increases, output voltage will decrease

slightly, and sensing this, the error amplifier raises the

raising the current comparator threshold, thus increasing

the peak currents through the transformer primary and

secondary. This delivers more current to the load, bring-

ing the output voltage back up.

control loop. Typically, an external series RC network is

optimal response to load and line transients. The imped-

ance of this RC network converts the output

current of the

current comparator threshold and commands consider-

able influence over the dynamics of the voltage regulation

loop.

Start-Up/Shutdown

The LTC3803 has two shutdown mechanisms to disable

supply pin voltage, and a forced shutdown whenever

transitions into and out of shutdown according to the state

diagram (Figure 1).

LTC3803

SHUT DOWN

(NOMINALLY 0.28V)

3803 F01

(NOMINALLY 5.7V)

(NOMINALLY 8.7V)

LTC3803

ENABLED

Figure 1. Start-Up/Shutdown State Diagram