LTM4613 datasheet(14/30 Pages) LINER

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LTM4613

4613fd

For more information www.linear.com/LTM4613

APPLICATIONS INFORMATION

phase’sinductorripplecurrent∆ILis~5.0Afora36Vto12V

design. The duty cycle is about 0.33. The 2-phase curve

shows a ratio of ~0.33 for a duty cycle of 0.33. This 0.33

ratio of output ripple current to the inductor ripple current

∆IL at 5.0A equals 1.65A of output ripple current (∆IO).

The output voltage ripple has two components that are

related to the amount of bulk capacitance and effective

series resistance (ESR) of the output bulk capacitance.

The equation is:

∆VOUT(P−P) ≈

∆IO

8 • f •N•COUT

⎛

⎝

⎜

⎜⎜

⎞

⎠

⎟

⎟⎟ +

ESR•∆IO

where f is the frequency and N is the number of paralleled

phases. This calculation process can be easily accom-

plished by using LTpowerCAD™.

Fault Conditions: Current Limit and

Overcurrent Foldback

LTM4613 has a current mode controller, which inherently

limitsthecycle-by-cycleinductorcurrentnotonlyinsteady

state operation, but also in response to transients.

To further limit current in the event of an overload condi-

tion,theLTM4613providesfoldbackcurrentlimiting.Ifthe

output voltage falls by more than 50%, then the maximum

output current is progressively lowered to about one sixth

of its full current limit value.

Soft-Start and Tracking

The TRACK/SS pin provides a means to either soft-start

the regulator or track it to a different power supply. A

capacitor on this pin will program the ramp rate of the

output voltage. A 1.5µA current source will charge up the

external soft-start capacitor to 80% of the 0.6V internal

voltage reference plus or minus any margin delta. This will

control the ramp of the internal reference and the output

voltage. The total soft-start time can be calculated as:

tSOFTSTART ≅ 0.8 • 0.6V± VOUT(MARGIN)

(

)•

CSS

1.5µA

If the RUN pin falls below 1.5V, then the TRACK/SS pin

is reset to allow for proper soft-start control when the

regulator is enabled again. Current foldback and forced

continuous mode are disabled during the soft-start pro-

cess. The soft-start function can also be used to control

the output ramp rise time, so that another regulator can

be easily tracked to it.

Output Voltage Tracking

Output voltage tracking can be programmed externally

using the TRACK/SS pin. The output can be tracked up

and down with another regulator. Figure 5 shows an ex-

ample of coincident tracking where the master regulator’s

output is divided down with an external resistor divider

that is the same as the slave regulator’s feedback divider.

Ratiometric modes of tracking can be achieved by select-

ing different resistor values to change the output tracking

ratio. The master output must be greater than the slave

output for coincident tracking to work. Figure 6 shows the

coincident output tracking characteristics.

Figure 5. Coincident Tracking Schematic

Figure 6. Coincident Output Tracking Characteristics

PGOOD

RUN

COMP

INTVCC

DRVCC

TRACK/SS

fSET

VOUT

VFB

FCB

MARG0

MARG1

MPGM

TRACK

CONTROL

PLLIN

LTM4613

RFB

5.23k

51k

MASTER

OUTPUT

100k

COUT

SLAVE

OUTPUT

4613 F05

CIN

VIN

PGND

SGND

VIN

5.23k

10µF

×3

OUTPUT

VOLTAGE

TIME

4613 F06

MASTER OUTPUT

SLAVE OUTPUT

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