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LTM4651 Datasheet(PDF) 18 Page - Linear Technology

Part # LTM4651
Description  EN55022B Compliant 58V, 24W Inverting-Output DC/DC 關Module Regulator
Download  34 Pages
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Manufacturer  LINER [Linear Technology]
Direct Link  http://www.linear.com
Logo LINER - Linear Technology

LTM4651 Datasheet(HTML) 18 Page - Linear Technology

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LTM4651
18
4651f
For more information www.linear.com/LTM4651
Hot-Plugging Safely
The small size, robustness and low impedance of ceramic
capacitors make them an attractive option for the input
bypasscapacitors(CDandCINH)oftheLTM4651.However,
these capacitors can cause problems if the LTM4651 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 LTM4651
can ring to twice the nominal input voltage, possibly ex-
ceeding the LTM4651’s rating and damaging the part. If
the input supply is poorly controlled or the user will be
plugging the LTM4651 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 LTM4651. 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 LTM4651; 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 LTM4651’s internal MOSFETs.
INTVCC is regulated at 3.3V above VOUT–. In this manner,
the LTM4651’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+|VOUT– |–3.3V)
(17)
The LDO draws current off of EXTVCCinsteadofSVINwhen
EXTVCCistiedtoavoltagehigherthan3.2VaboveVOUT–and
SVIN is 5V above VOUT–. For output voltages at or below
–4V, this pin can be connected to PGND through an RC-
filter. When the internal LDO derives power from EXTVCC
instead of SVIN, the internal LDO power dissipation is:
PLDO_LOSS(EXTVCC) =20mA •(|VOUT– |–3V)
(18)
TherecommendedvalueoftheresistorbetweenPGNDand
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 above EXTVCC— forward-
biasing the VOUT–-to-EXTVCC body diode. This resistor
limits the magnitude of current flow into EXTVCC. Bypass
EXTVCC to VOUT– with 1μF of X5R (or better) MLCC.
Multiphase Operation
Multiple LTM4651 devices can be paralleled for higher
output current applications. For lowest input and output
voltage and current ripples, it is advisable to synchronize
paralleled LTM4651s to an external clock (within ±40%
of the target switching frequency set by fSET—see Test
Circuit 1). See Figure 34 for an example of a synchroniz-
ing circuit.
LTM4651modulescanbeparalleledwithoutsynchronizing
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 LTM4651 device is an inherently current mode con-
trolled device, so parallel modules will have good current
sharing’s shown in Figure 35. This helps balance the
thermals on the design.
To parallel LTM4651s, connect the respective COMPa,
ISETa, and GNDSNS pins of each LTM4651 together to
sharethecurrentevenly.Inaddition,tietherespectiveRUN
pins of paralleled LTM4651 devices together, to ensure
proper start-up and shutdown behavior. Figure 34 shows
a schematic of LTM4651 devices operating in parallel.
Note that for parallel applications, EQ. 8 becomes:
RSET =
–VOUT–
50µA •N
(19)
APPLICATIONS INFORMATION


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