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LTM4653

Rev 0

For more information www.analog.com

APPLICATIONS INFORMATION

Hot-Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypasscapacitors(CDandCINH)oftheLTM4653.However,

these capacitors can cause problems if the LTM4653 is

plugged into a live supply (see Linear Technology Ap-

plication Note 88 for a complete discussion). The low

loss ceramic capacitor combined with stray inductance

in series with the power source forms an under damped

tank circuit, and the voltage at the VIN pin of the LTM4653

can ring to twice the nominal input voltage, possibly ex-

ceeding the LTM4653’s rating and damaging the part. If

the input supply is poorly controlled or the user will be

plugging the LTM4653 into an energized supply, the input

network should be designed to prevent this overshoot by

introducing a damping element into the path of current

flow. This is often done by adding an inexpensive elec-

trolytic bulk capacitor (CINL) across the input terminals

of the LTM4653. The selection criteria for CINL calls for:

an ESR high enough to damp the ringing; a capacitance

value several times larger than CINH. CINL does not need

to be located physically close to the LTM4653; it should

be located close to the application board’s input connec-

tor, instead.

INTVCC and EXTVCC Connection

When RUN is logic high, an internal low dropout regula-

tor regulates an internal supply, INTVCC, that powers the

control circuitry for driving LTM4653’s internal MOSFETs.

INTVCCisregulatedat3.3V.Inthismanner,theLTM4653’s

INTVCC is directly powered from SVIN, by default. The gate

driver current through the LDO is about 20mA for a typical

1MHz application. The internal LDO power dissipation can

be calculated as:

PLDO_LOSS(INTVCC) = 20mA •(SVIN – 3V)

The LDO draws current off of EXTVCC instead of SVIN

when EXTVCC is higher than 3.2V and SVIN is above 5V.

For output voltages of 4V and higher, EXTVCC can be con-

nected to VOUTthroughanRC-filter.WhentheinternalLDO

derives power from EXTVCC instead of SVIN, the internal

LDO power dissipation is:

PLDO_LOSS(EXTVCC) = 20mA •(VOUT − 3V)

The recommended value of the resistor between VOUT and

EXTVCC is roughly VOUT • 4Ω/V. This resistor, REXTVCC,

must be rated to continually dissipate (0.02A)² • REXTVCC.

The primary purpose of this resistor is to prevent EXTVCC

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 PGND—forward bias-

ing the PGND-to-EXTVCC body diode. This resistor limits

the magnitude of current flow in EXTVCC. Bypass EXTVCC

with 1μF of X5R (or better) MLCC.

Multiphase Operation

Multiple LTM4653 devices can be paralleled for higher

output current applications. For lowest input and output

voltage and current ripples, it is advisable to synchronize

paralleled LTM4653s to an external clock (within ±40%

of the target switching frequency set by fSET—see Test

Circuit 1). See Figure 32 for an example of a synchroniz-

ing circuit.

LTM4653modulescanbeparalleledwithoutsynchronizing

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 modules are not operating

at identical, synchronized switching frequencies.

The LTM4653 device is an inherently current mode con-

trolled device, so parallel modules will have good current

sharing’s shown in Figure 33. This helps balance the

thermals on the design.

To parallel LTM4653s, connect the respective COMPa,

ISETa, and VOSNS pins of each LTM4653 together to share

the current evenly. In addition, tie the respective RUN

pins of paralleled LTM4653 devices together, to ensure

proper start-up and shutdown behavior. Figure 32 shows

a schematic of LTM4653 devices operating in parallel.

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

single, common resistor on the ISETa net:

RISET =

VOUT

50µA •N

where N is the number of LTM4653 modules in parallel

configuration.

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