LTC3613 datasheet(12/36 Pages) LINER

12 / 36 page

LTC3613

3613fa

OPERATION

When the RUN pin is pulled low to disable the controller or

when INTVCC drops below its undervoltage lockout thresh-

old of 3.65V, the TRACK/SS pin is pulled low internally.

Light Load Current Operation

When the DC load current is less than 1/2 of the peak-

to-peak inductor current ripple, the inductor current can

drop to zero or become negative. If the MODE/PLLIN pin

is connected to SGND, the LTC3613 will transition into

discontinuous mode operation (also called pulse-skipping

mode), where a current reversal comparator, IREV, detects

and prevents negative inductor current by shutting off the

bottom MOSFET, MB. In this mode, both switches remain

off with the output capacitor supplying the load current.

As the output capacitor discharges and the output volt-

age droops lower, the EA will eventually move the ITH

voltage above the zero current level to initiate another

switching cycle.

If the MODE/PLLIN pin is tied to INTVCC or an external

clock is applied to MODE/PLLIN, the LTC3613 will be forced

to operate in continuous mode (forced continuous mode)

and not transition into discontinuous mode. In this case

the current reversal comparator, IREV, is disabled, allowing

the inductor current to become negative and thus maintain

constant frequency operation.

Frequency Selection and External Clock

Synchronization

The steady-state switching frequency of the LTC3613 is

set by an internal oscillator. The frequency of this internal

oscillator can be programmed from 200kHz to 1MHz by

connecting a resistor from the RT pin to SGND. The RT

pin is forced to 1.2V internally. A phase-locked loop (PLL)

system synchronizes the turn-on of the switching cycle to

this internal oscillator when no external clock is provided.

For applications with stringent frequency or interfer-

ence requirements, an external clock source connected

to the MODE/PLLIN pin can be used to synchronize the

switching cycle turn-on to the rising edge of the clock.

The LTC3613 operates in forced continuous mode when

it is synchronized to the external clock. The external clock

frequency has to be within ±30% of the internal oscillator

frequency for successful synchronization and the clock

input levels should be greater than 2V for HI and less

than 0.5V for LO. The MODE/PLLIN pin has an internal

600kΩ pull-down resistor.

Power Good and Fault Protection

The power good pin, PGOOD, is connected internally to an

open-drain N-channel MOSFET. An external pull-up resistor

to a voltage supply of up to 6V (or INTVCC) completes the

power good detection scheme. Overvoltage and undervolt-

age comparators OV and UV turn on the MOSFET and pull

the PGOOD pin low when the differential feedback voltage

is outside a ±7.5% window of the 0.6V reference voltage.

The PGOOD pin is also pulled low when the LTC3613 is

in the soft-start or tracking phase, when in undervoltage

lockout, or when the RUN pin is low (shut down).

When the differential feedback voltage is within the ±7.5%

requirement, the open-drain NMOS is turned off and the

pin is pulled up by an external resistor. There is an internal

delay of 10μs before the PGOOD pin will indicate power

good once the differential feedback voltage is within the

±7.5% window. When the feedback voltage goes out of

the ±7.5% window, there is an internal 20μs delay before

PGOOD is pulled low. In an overvoltage condition, MT is

turned off and MB is turned on immediately without any

delay and held on until the overvoltage condition clears.

Foldback current limiting is provided if the output is shorted

to ground. As the differential feedback voltage drops, the

current threshold voltage on the ITH pin is pulled down

and clamped to 1.2V. This reduces the inductor valley

current level to one-fourth of its maximum value as the

differential feedback approaches 0V. Foldback current

limiting is disabled at start-up.

LTC3613 Datasheet(PDF) 12 Page - Linear Technology

LTC3613 Datasheet(HTML) 12 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

Datasheet Download

Link URL