LTM4658

12

Rev. 0

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When configured for master/slave operation, the slave

phases operate in forced continuous modes.

LTM4658

FREQ

FB

4658 F02

R4

R3

AGND

Figure 2. Phase Programming

The LTM4658 device is an inherently current mode con-

trolled device, so parallel modules will have very good

current sharing. This will balance the thermals on the

design. Tie RUN, SSTT, FB and COMP pins of each paral-

leling channel together. Figure 17 through Figure 19 show

examples of parallel operation and pin connections.

INPUT RMS Ripple Current Cancellation

Application Note 77 provides a detailed explanation of

multiphase operation. The input RMS ripple current can-

cellation mathematical derivations are presented, and a

graph is displayed representing the RMS ripple current

reductionasafunctionofthenumberofinterleavedphases.

Figure 3 shows this graph.

0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9

0.60

0.55

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0

4658 F03

6-PHASE

4-PHASE

3-PHASE

2-PHASE

1-PHASE

Figure 3. Input RMS Current Ratios to DC Load

Current as a Function of Duty Cycle

Soft-Start /Tracking/Temperature Monitor

The LTM4658 allows the user to program its output volt-

age ramp rate by means of the SSTT pin.

An internal 10μA pulls up the SSTT pin. Putting an external

capacitor on SSTT enables soft-starting the output to pre-

vent current surge on the input supply and output voltage

overshoot. During the soft-start ramp, the output volt-

age will proportionally track the SSTT pin voltage. When

the soft-start is complete, the pin will servo to a voltage

proportional to the LTM4658 junction temperature. See

Figure 4 showing the SSTT pin operating range.

0

DIE TEMP

(°C)

FB

(V)

150

0.1

125

75

0.6

0.3

100

50

0.5

25

0.4

0.2

0

0.5

1.5

0.6

1.6

0.3

1.3

4658 F04

1.7

0.4

1.4

0.2

1.2

0.1

SSTT (V)

TEMP

MONITOR

~4mV/°C

SSTT PIN VOLTAGE

OPERATING RANGE

SOFT-START

AND TRACKING

Figure 4. Soft-Start and Temperature Monitor Operation

The soft-start time is calculated with Equation 5.

500mV

10µA

(5)

For output tracking applications, SSTT can be externally

driven by another voltage source. From 0V to 0.5V, the

SSTT voltage will override the internal 0.5V reference

input to the error amplifier, thus regulating the FB pin

voltage to that of SSTT pin. When SSTT is above 0.5V,

tracking is disabled and the feedback voltage will regulate

to the internal reference voltage.

An active pull-down circuit is connected to the SSTT pin

to discharge the external soft-start capacitor in the case

of fault conditions. The ramp will restart when the fault is

cleared. Fault conditions that clear the soft-start capacitor

too low or thermal shutdown.

APPLICATIONS INFORMATION