LTM4611 datasheet(16/28 Pages) LINER

16 / 28 page

LTM4611

4611f

applicaTions inForMaTion

Decreasing the PLLFLTR/fSET RC time constant can be

accomplished, for example, by driving the PLLFLTR/fSET

pin with an external, lower impedance resistor divider

network from INTVCC and GND to PLLFLTR/fSET—in the

simplest of implementations, by shorting PLLFLTR/fSET

to INTVCC (thereby programming the switching frequency

to 780kHz, nominal), or by driving the PLLFLTR/fSET pin

from a low impedance voltage source.

When,inadditiontoneedingfasterturn-ontime,oneisalso

synchronizing to an external clock signal, one need bear in

mind: the PLL’s sink and source current is recommended

for not more than ±8µA loading, and the PLL will need

to successfully drive any external PLLFLTR/fSET network

impedance to achieve phase lock; and lastly, some phase

shiftinclocksynchronizationwilloccurasexternalloading

on PLLFLTR/fSET becomes heavier.

Tobeclear,usingaCSSvalueof10nF(orhigher)eliminates

the need for any of the above special considerations or

provisions.

Ratiometric tracking can be achieved by a few simple

calculations and the slew rate value applied to the master’s

TRACK/SSpin.Asmentionedabove,theTRACK/SSpinhas

a control range from 0V to 0.8V. The master’s TRACK/SS

pin slew rate is directly equal to the master’s output slew

rate in volts/time. The equation:

• 60.4k

= RTB

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 to 60.4k. RTA is derived from equation:

RTA=

0.8V

VFB

60.4k

VFB

RFB2

–

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 RFB2 with

VFB = VTRACK. Therefore RTB = 60.4k, and RTA = 121k in

Figure 4.

Inratiometrictracking,adifferentslewratemaybedesired

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

slower than MR. Make sure that the slave supply slew rate

ischosentobefastenoughsothattheslaveoutputvoltage

will reach its final value before the master output.

For example, MR = 1.5V/ms, and SR = 1.2V/ms. Then RTB

= 75k. Solve for RTA to equal to 87k.

Beware that without any kind of soft-start ramp up, it is

important to provide thorough input filter capacitance to

handle input surge currents at start-up, so as to avoid

excessive input line sag and power supply motor boating.

Leavingprovisionforatleastasoft-startcapacitorinone’s

application is strongly recommended.

Overcurrent and Overvoltage Protection

The LTM4611 has overcurrent protection (OCP) in a

short circuit. The internal current comparator threshold

folds back during a short to reduce the output current.

An overvoltage condition (OVP) above 7.5% of the regu-

lated output voltage will force the top MOSFET off and

the bottom MOSFET on until the condition is cleared. An

input electronic circuit breaker or fuse can be sized to be

tripped or cleared when the bottom MOSFET is turned on

toprotectagainsttheovervoltage.Foldbackcurrentlimiting

is disabled during soft-start or tracking start-up.

Run Enable

The RUN pin is used to enable the power module or

sequence the power module. The threshold is 1.22V. The

RUN pin must be used as an undervoltage lockout (UVLO)

function by connecting a resistor divider from the input

supply to the RUN pin:

VUVLO

1.22V

– 1

To achieve the lowest possible UVLO, 1.22V, leave R2

unpopulated. R1 can be 10k, or if R2 is unpopulated, R1

may be replaced with a hardwired connection from VIN

to RUN.

LTM4611 Datasheet(PDF) 16 Page - Linear Technology

LTM4611 Datasheet(HTML) 16 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

Datasheet Download

Link URL