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LTM4634

4634f

For more information www.linear.com/LTM4634

applicaTions inForMaTion

reference voltage and then proceed up to INTVCC. After the

0.8V ramp, the TK/SS pin will no longer be in control, and

the internal voltage reference will control output regula-

tion from the feedback divider. Foldback current limit is

disabled during this sequence of turn-on during tracking

or soft-starting. The TK/SS pins are pulled low when the

RUN pin is below 1.15V or INTVCC drops below 3.5V. The

total soft-start time can be calculated as:

tSS =

0.8V •CSS

1.5µA









Regardless of the mode selected by the MODE/PLLIN pin,

the regulator channels will always start in pulse-skipping

mode up to TK/SS = 0.64V. Between TK/SS = 0.64V and

0.74V, it will operate in forced continuous mode and revert

totheselectedmodeonceTK/SS>0.74V.Theoutputripple

is minimized during the 100mV forced continuous mode

window ensuring a clean PGOOD signal.

When the channel is configured to track another supply,

the feedback voltage of the other supply is duplicated by

a resistor divider and applied to the TK/SS pin. Therefore,

the voltage ramp rate on this pin is determined by the ramp

rate of the other supply’s voltage. Note that the small soft-

startcapacitorchargingcurrentisalwaysflowing,produc-

ing a small offset error. To minimize this error, select the

tracking resistive divider value to be small enough to make

thiserrornegligible.Inordertotrackdownanotherchannel

orsupplyafterthesoft-startphaseexpires,theLTM4634is

forced into continuous mode of operation as soon as VFB

is below the undervoltage threshold of 0.74V regardless of

the setting of the MODE/PLLIN pin. However, the LTM4634

should always be set in forced continuous mode tracking

down when there is no load. After TK/SS drops below 0.1V,

its channel will operate in discontinuous mode.

The master’s TK/SS pin slew rate is directly equal to the

master’s output slew rate in Volts/Time. The equation:

RTB =









•60.4k

where MR is the master’s output slew rate and SR is the

slave’s output slew rate in Volts/Time. When coincident

tracking is desired, then MR and SR are equal, thus RTB

is equal the 60.4k. RTA is derived from equation:

RTA =

0.8V

VFB

60.4k

VFB

RFB

–

VTRACK

RTB

where VFB is the feedback voltage reference of the regula-

tor, and VTRACK is 0.8V. Since RTB is equal to the 60.4k top

feedback resistor of the slave regulator in equal slew rate

or coincident tracking, then RTA is equal to RFB with VFB =

VTRACK.ThereforeRTB=60.4k,andRTA=60.4kinFigure3.

In ratiometric tracking, a different slew rate maybe desired

for the slave regulator. RTB can be solved for when SR is

slower than MR. Make sure that the slave supply slew

rate is chosen to be fast enough so that the slave output

voltage will reach it final value before the master output.

Power Good

The PGOOD12 pin is an open-drain pin that can be used

to monitor valid output voltage regulation for VOUT1 and

VOUT2, and PGOOD3 for monitoring VOUT3. These pins

monitor a ±7.5% window around the 0.8V feedback volt-

age on either VFB1,2,3 from the output regulation point. A

resistor can be pulled up to a particular supply voltage

no greater than 6V maximum for monitoring. Any of the

PGOOD pins are pulled low when the RUN pin of the cor-

responding channel is pulled low.

Overcurrent and Overvoltage Protection

Each of the regulator channels senses the peak inductor

current on a cycle-by-cycle basis in current mode opera-

tion. When current limit is reached the output voltage will

begin to fall and the internal current limit threshold will

begin fold back as the output voltage falls below 50% of

its value. Foldback current limit is disabled during start-

up or track-up. Under short-circuit condition at low duty

cycle operation, each of the regulator channels will begin

to skip cycles to limit the short-circuit current.

Overvoltage protection is implemented by monitoring

each one of the regulator’s VFB pins. When the VFB volt-

age exceeds ~7.5% of the 0.8V reference value, then an

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