LTC3803-5

7

38035fd

OPERATION

The LTC3803-5 is a constant frequency current mode

controller for flyback, SEPIC and DC/DC boost converter

applications in a tiny ThinSOT package. The LTC3803-5 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 LTC3803-5’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

Switching

Power Supply Design.

Please refer to the Block Diagram and the Typical Ap-

plication 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 de-

800mV. The error amplifier responds by feeding current

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 LTC3803-5 turns on the external power

MOSFET when the internal free-running 200kHz oscillator

sets the RS latch. It turns off the MOSFET when the cur-

rent 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, bringing

the output voltage back up.

the control loop. Typically, an external series RC network

optimal response to load and line transients. The impedance

of this RC network converts the output

current of the error

comparator threshold and commands considerable influ-

ence over the dynamics of the voltage regulation loop.

Start-Up/Shutdown

The LTC3803-5 has two shutdown mechanisms to disable

supply pin voltage, and a forced shutdown whenever ex-

transitions into and out of shutdown according to the state

diagram (Figure 1).

Figure 1. Start-Up/Shutdown State Diagram

LTC3803-5

SHUT DOWN

(NOMINALLY 0.28V)

38035 F01

(NOMINALLY 4V)

(NOMINALLY 4.8V)

LTC3803-5

ENABLED