LT1930 datasheet(9/32 Pages) LINER

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LTC1871X

1871xf

For more information www.linear.com/LTC1871X

OPERATION

The nominal operating frequency of the LTC1871X is pro-

grammed using a resistor from the FREQ pin to ground

and can be controlled over a 65kHz to 900kHz range. In

addition, the internal oscillator can be synchronized to

an external clock applied to the MODE/SYNC pin and can

be locked to a frequency between 100% and 130% of its

nominal value. When the MODE/SYNC pin is left open, it

is pulled low by an internal 50k resistor and Burst Mode

operation is enabled. If this pin is taken above 2V or an

external clock is applied, Burst Mode operation is disabled

and the IC operates in continuous mode. With no load (or

an extremely light load), the controller will skip pulses

in order to maintain regulation and prevent excessive

output ripple.

TheRUNpincontrolswhethertheICisenabledorisinalow

current shutdown state. A micropower 1.248V reference

and comparator C2 allow the user to program the supply

voltage at which the IC turns on and off (comparator C2

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 10µA.

An overvoltage comparator OV senses when the FB pin

exceeds the reference voltage by 6.5% 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.

The LTC1871X can be used either by sensing the voltage

drop across the power MOSFET or by connecting the

SENSE pin to a conventional shunt resistor in the source

of the power MOSFET, as shown in Figure 2. Sensing the

voltage across the power MOSFET maximizes converter

efficiency and minimizes the component count, but limits

theoutputvoltagetothemaximumratingforthispin(36V).

By connecting the SENSE pin to a resistor in the source

of the power MOSFET, the user is able to program output

voltages significantly greater than 36V.

Programming the Operating Mode

For applications where maximizing the efficiency at very

light loads (e.g., <100µA) is a high priority, the current

in the output divider could be decreased to a few micro-

amps and Burst Mode operation should be applied (i.e.,

the MODE/SYNC pin should be connected to ground).

In applications where fixed frequency operation is more

critical than low current efficiency, or where the lowest

outputrippleisdesired,pulse-skipmodeoperationshould

be used and the MODE/SYNC pin should be connected

to the INTVCC pin. This allows discontinuous conduction

mode (DCM) operation down to near the limit defined

by the chip’s minimum on-time (about 175ns). Below

this output current level, the converter will begin to skip

cycles in order to maintain output regulation. Figures 3

and 4 show the light load switching waveforms for Burst

Mode and pulse-skip mode operation for the converter

in Figure 1.

Burst Mode Operation

Burst Mode operation is selected by leaving the MODE/

SYNC pin unconnected or by connecting it to ground. In

normaloperation,therangeontheITHpincorrespondingto

no load to full load is 0.30V to 1.2V. In Burst Mode opera-

tion, if the error amplifier EA drives the ITH voltage below

0.525V, the buffered ITH input to the current comparator

C1 will be clamped at 0.525V (which corresponds to 25%

of maximum load current). The inductor current peak is

then held at approximately 30mV divided by the power

Figure 2. Using the SENSE Pin On the LTC1871

COUT

VSW

2a. SENSE Pin Connection for

Maximum Efﬁciency (VSW < 36V)

VOUT

VIN

GND

COUT

1871x F02

2b. SENSE Pin Connection for Precise

Control of Peak Current or for VSW > 36V

VOUT

VIN

GND

GATE

GND

VIN

SENSE

GATE

GND

VIN

SENSE

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