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LTC3646/LTC3646-1

36461fc

For more information www.linear.com/LTC3646

OPERATION

TheLTC3646andLTC3646-1arecurrentmode,monolithic,

synchronous, step-down regulators capable of 40V input

operation,andextremelyhighstepdownratioswhilemain-

taining constant frequency. (Both will be referred to as the

LTC3646 except as specifically noted.) Part operation is

enabled by raising the voltage of the RUN pin above 1.21V.

Main Control Loop

In normal operation a switching cycle is initiated by a

signal from the inductor valley current comparator (ICMP

in the Block Diagram). The top power MOSFET is turned

on, and a timer is simultaneously started in the on-time

controller. The on-time controller computes the correct on

time (subject to tON(MIN)) based on the desired switching

frequencyfO,andstep-downratioVON/VIN,accordingtothe

formula shown in the Block Diagram. In a typical applica-

tion,theVONpinshouldbeconnectedtotheoutputvoltage,

VOUT. When the timer expires, the top power MOSFET is

turned off and the bottom power MOSFET is turned on

until the current comparator (ICMP) trips, restarting the

timer and initiating the next cycle. The inductor current

is monitored by sensing the voltage drop across the SW

and PGND nodes of the bottom power MOSFET. The volt-

age at the ITH node sets the ICMP comparator threshold

corresponding to the inductor valley current. The error

amplifier (EA) adjusts the ITH voltage by comparing an

internal 0.6V reference voltage to the feedback signal,

VFB, derived from the output voltage. If, for example, the

load current increases, the output voltage will decrease

relative to the 0.6V reference. The ITH voltage will rise

until the average inductor current increases to match the

load current.

At low load currents, the inductor current can drop to

zero or become negative. If the LTC3646 is configured for

Burst Mode operation, this inductor current condition is

detected by the current reversal comparator (IREV) which

then shuts off the bottom power MOSFET and places the

part into a low quiescent current sleep state, resulting in

discontinuous operation and increased efficiency at low

load currents. Both power MOSFETs will remain off with

the part in sleep and the output capacitor supplying the

loadcurrentuntiltheITHvoltagerisessufficientlytoinitiate

anothercycle.Discontinuousoperationisdisabledbytying

the MODE/SYNC pin to ground, placing the LTC3646 into

forced continuous mode. During forced continuous mode

operation, synchronous operation occurs regardless of

the output load current, and the inductor current trough

levels are allowed to become negative.

The operating frequency is determined by the value of the

RRT resistor, which programs the current for the internal

oscillator.Aninternalphase-lockedloopadjuststheswitch-

ing regulator on-time so that the switching frequency

matches the programmed frequency, subject to tON and

tOFFtimeconstraintsshownintheElectricalCharacteristics

table. Alternatively, the RT pin can be connected to the

INTVCC pin which causes the internal oscillator to run at

the default frequency of 2.25MHz.

A clock signal can be applied to the MODE/SYNC pin to

synchronizetheswitchingfrequencytoanexternalsource.

When operating in this configuration, connect a resistor to

the RT pin with a value corresponding to the applied clock

frequency. With an external clock supplied to the MODE/

SYNC pin, the part will operate in forced continuous mode.

Power Good Status Output

The PGOOD open-drain output will be pulled low if the

regulator output exits the VPGOOD window around the

regulation point. This condition is released once regula-

tion within the specified window is achieved. To prevent

unwanted PGOOD glitches during transients or dynamic

VOUT changes, the LTC3646 PGOOD falling edge includes

a filter time of approximately 70 clock cycles.

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