LTM8045 datasheet(30/54 Pages) AD

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LTM4655

Rev. 0

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can be connected to VOUTn+ through an RC-filter. When

the internal LDO derives power from EXTVCCn instead of

SVINn, the internal LDO power dissipation is shown in

Equation 17.

PLDO_LOSSn(EXTVCC) = 20mA •(VOUTn − VOUTn− −3V) (17)

The recommended value of the resistor between VOUTn+

and EXTVCCn is roughly (VOUTn+ – VOUTn–) • 4Ω/V. This

resistor, REXTVCCn, must be rated to continually dissipate

(0.02A)² • REXTVCCn. The primary purpose of this resistor

is to prevent EXTVCCn overstress under a fault condition.

For example, when an inductive short-circuit is applied

to the module’s output, VOUT+ may be briefly dragged

below VOUTn–—forward biasing the VOUTn–-to-EXTVCCn

body diode. This resistor limits the magnitude of current

flow in EXTVCCn. If the application requires a low resistive

path to EXTVCCn, apply a protective Schottky diode across

EXTVCCn and VOUTn–; see Figure 52. Bypass EXTVCCn to

VOUTn– with 1μF of X5R (or better) MLCC.

Multiphase Operation

Multiple LTM4655 channels and modules can be paralleled

for higher output current applications. For lowest input

and output voltage and current ripples, it is advisable to

synchronize paralleled LTM4655s to a clock (within ±40%

of the target switching frequency set by fSETn.

LTM4655 channels and modules can be paralleled with-

out synchronizing circuits: just be aware that some beat-

frequency ripple will be present in the output voltage and

reflected input current by virtue of the fact that such mod-

ules are not operating at identical, synchronized switching

frequencies.

The LTM4655 device is an inherently current mode con-

trolled device, so parallel channels and modules will have

good current sharing as shown in Figure 45 and Figure 47.

To parallel LTM4655 channels and/or modules, con-

nect the respective COMP

na, ISETna, and VOSNSn+ pins

of each LTM4655 together to share the current evenly.

In addition, tie the respective RUN

n pins of paralleled

LTM4655 channels and/or modules together, to ensure

proper start-up and shutdown behavior.

Note that for parallel applications, VOUT can be set by a single,

common resistor on the ISET

na net (see Equation 18).

RISETn =

VOUTn+ − VOUTn−

50µA •N

(18)

where N is the number of LTM4655 channels in parallel

configuration.

Depending on the duty cycle of operation, the output

voltage ripple achieved by paralleled, synchronized

LTM4655 modules may be considerably smaller than

what is yielded by a single-phase solution. Application

Note 77 provides a detailed explanation of multiphase

operation (relevant to parallel LTM4655 applications)

pertaining to noise reduction and output and input ripple

current cancellation. Regardless of ripple current cancel-

lation, it remains important for the output capacitance of

paralleled LTM4655 applications to be designed for loop

stability and transient response. LTpowerCAD is available

for such analysis.

Figure 5 illustrates the RMS ripple current reduction as a

function of the number of interleaved (paralleled and syn-

chronized) LTM4655 modules—derived from Application

Note 77.

APPLICATIONS INFORMATION

Figure 5. Normalized Input RMS Ripple Current vs Duty Cycle for

One to Six LTM4655 Channels (Phases)

0.75 0.8

4655 F05

0.7

0.65

0.6

0.55

0.5

0.45

0.4

0.35

0.3

0.25

0.2

0.15

0.1

0.85 0.9

DUTY CYCLE (–VOUT– / VIN – VOUT–)

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

1-PHASE

2-PHASE

3-PHASE

4-PHASE

6-PHASE

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