LTC3823IGN datasheet(10/26 Pages) LINER

10 / 26 page

LTC3823

3823fd

OPERATION

Main Control Loop

The LTC3823 is a current mode controller for DC/DC

step-down converters. In normal operation, the top

MOSFET is turned on for a ﬁxed interval determined by

a one-shot timer, OST. When the top MOSFET is turned

off, the bottom MOSFET is turned on until the current

comparator ICMP trips, restarting the one-shot timer and

initiating the next cycle. Inductor current is determined

by sensing the voltage between the SENSE– and SENSE+

pins using a sense resistor or the bottom MOSFET on-

resistance . The voltage on the ITH pin sets the comparator

threshold corresponding to inductor valley current. The

error ampliﬁer EA adjusts this voltage by comparing the

feedback signal, VFB, to an internal reference voltage. If

the load current increases, it causes a drop in the feedback

voltage relative to the reference. The ITH voltage then rises

until the average inductor current again matches the load

current.

At low load currents, the inductor current can drop to zero

and become negative. This is detected by current reversal

comparator IREV which then shuts off M2, resulting in

discontinuous operation. Both switches will remain off

with the output capacitor supplying the load current until

the ITH voltage rises above the zero current level (0.75V)

to initiate another cycle. Discontinuous mode operation

is disabled by comparator F when the FCB pin is brought

below 0.6V, forcing continuous synchronous operation.

The operating frequency is determined implicitly by the

top MOSFET on-time and the duty cycle required to main-

tain regulation. The one-shot timer generates an on time

that is proportional to the ideal duty cycle, thus holding

frequency approximately constant with changes in VIN.

The nominal frequency can be adjusted with an external

resistor, RON.

For applications with stringent constant frequency re-

quirements, the LTC3823 can be synchronized with an

external clock. By programming the nominal frequency

of the LTC3823 the same as the external clock frequency,

the LTC3823 behaves as a constant frequency part against

the load and supply variations.

Overvoltage and undervoltage comparators OV and UV

pull the PGOOD output low if the output feedback voltage

exits a ±10% window around the regulation point after the

internal 20μs power bad mask timer expires. Furthermore,

in an overvoltage condition, M1 is turned off and M2 is

turned on immediately and held on until the overvoltage

condition clears.

Foldback current limiting is provided if the output is shorted

to ground. As VFB drops, the buffered current threshold

voltage, ITHB, is pulled down and clamped to 0.9V. This

reduces the inductor valley current level to one-tenth of its

maximum value as VFB approaches 0V. Foldback current

limiting is disabled at start-up.

Pulling the RUN pin low forces the controller into its

shutdown state, turning off both M1 and M2. Forcing a

voltage above 1.5V will turn on the device.

INTVCC Power

Power for the top and bottom MOSFET drivers and most of

the internal controller circuitry is derived from the INTVCC

pin. The top MOSFET driver is powered from a ﬂoating

bootstrap capacitor, CB. This capacitor is recharged from

INTVCC through an external Schottky diode, DB, when the

top MOSFET is turned off. If the input voltage is low and

INTVCC drops below 3V, undervoltage lockout circuitry

prevents the power switches from turning on.

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