LTM4633 datasheet(15/32 Pages) LINER

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LTM4633

4633f

For more information www.linear.com/LTM4633

applicaTions inForMaTion

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

or supply after the soft-start phase expires, the LTM4633

is forced into continuous mode of operation as soon as

VFB is below the undervoltage threshold of 0.74V regard-

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

LTM4633 should always be set in force continuous mode

tracking down when there is no load. After TK/SS drops

below0.1V,itschannelwilloperateindiscontinuousmode.

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=60.4kinFigure2.

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 as 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 a 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 voltage

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

internal comparator monitor will turn off the top power

switch, and turn on the bottom power switch to protect the

load. If the top power switch faults as a short, then a fuse

or circuit breaker would be recommended to protect the

system. This is due to the top switch being shorted while

the bottom switch is turning on to protect the output from

over voltage. High currents will flow and could damage

the bottom switch.

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