LTC4002 datasheet(9/24 Pages) LINER

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LTC3783

3783f

OPERATIO

Main Control Loop

The LTC3783 is a constant frequency, current mode

controller for PWM LED as well as DC/DC boost, SEPIC

and flyback converter applications. In constant current

LED applications, the LTC3783 provides an especially

wide PWM dimming range due to its unique switching

scheme, which allows PWM pulse widths as short as

several converter switching periods.

For voltage feedback circuit operation (defined by VFBP ≤

1.23V), please refer to the Block Diagram of the IC and the

Typical Application on the first page of this data sheet. In

normal operation with PWMIN high, the power MOSFET is

turned on (GATE goes high) when the oscillator sets the

PWM latch, and is turned off when the ITRIP current

comparator resets the latch. Based on the error voltage

represented by (VFBP – VFBN), the error amplifier output

signal at the ITH pin sets the ITRIP current comparator

input threshold. When the load current increases, a fall in

the FBN voltage relative to the reference voltage at FBP

causes the ITH pin to rise, causing the ITRIP current com-

parator to trip at a higher peak inductor current value. The

average inductor current will therefore rise until it equals

the load current, thereby maintaining output regulation.

When PWMIN goes low, PWMOUT goes low, the ITH

switch opens and GATE switching is disabled. Lowering

PWMOUT and disabling GATE causes the output capacitor

COUT to hold the output voltage constant in the absence of

load current. Opening the ITH switch stores the correct

load current value on the ITH capacitor CITH. As a result,

when PWMIN goes high again, both ITH and VOUT are

instantly at the appropriate levels.

In voltage feedback operation, an overvoltage compara-

tor, OV, senses when the OV/FB pin exceeds the reference

voltage by 7% and provides a reset pulse to the main RS

latch. Because this RS latch is reset-dominant, the power

MOSFET is actively held off for the duration of an output

overvoltage condition.

For constant current/constant voltage regulation opera-

tion (defined by VFBP > 2.5V), please refer to the Block

Diagram of the IC and Figure 11. Loop operation is similar

to the voltage feedback, except FBP and FBN now sense

the voltage across sense resistor RL in series with the load.

The ITH pin now represents the error from the desired

differential set voltage, from 10mV to 100mV, for ILIM

values of 0.123V to 1.23V. That is, with VILIM = 1.23V, the

loop will regulate such that VFBP – VFBN = 100mV; lower

values of ILIM attenuate the difference proportionally.

PWMIN is still functional as above, but will only work

properly if load current can be disconnected by the

PWMOUT signal.

In constant current/constant voltage operation, the OV/FB

pin becomes a voltage feedback pin, which causes the

loop to regulate such that VOV/FB = 1.23V, provided the

above current-sense voltage is not reached. In this way,

the loop regulates either voltage or current, whichever

parameter hits its preset limit first.

The nominal operating frequency of the LTC3783 is pro-

grammed using a resistor from the FREQ pin to ground

and can be controlled over a 20kHz to 1MHz range. In

addition, the internal oscillator can be synchronized to an

external clock applied to the SYNC pin and can be locked

to a frequency between 100% and 130% of its nominal

value. When the SYNC pin is left open, it is pulled low by

an internal 100k resistor. With no load, or an extremely

light one, the controller will skip pulses in order to main-

tain regulation and prevent excessive output ripple.

The RUN pin controls whether the IC is enabled or is in a

low current shutdown state. A micropower 1.248V refer-

ence and RUN comparator allow the user to program the

supply voltage at which the IC turns on and off (the RUN

comparator has 100mV of hysteresis for noise immunity).

With the RUN pin below 1.248V, the chip is off and the

input supply current is typically only 20

µA.

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