LTM4612 Ver la hoja de datos (PDF) - Linear Technology
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LTM4612 Datasheet PDF : 24 Pages
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LTM4612
APPLICATIONS INFORMATION
COMP Pin
The pin is the external compensation pin. The module
has already been internally compensated for most output
voltages. An Excel design tool from Linear Technology will
be provided for more control loop optimization.
FCB Pin
The FCB pin determines whether the bottom MOSFET
remains on when current reverses in the inductor. Tying
this pin above its 0.6V threshold enables discontinuous
operation where the bottom MOSFET turns off when in-
ductor current reverses. FCB pin below the 0.6V threshold
forces continuous synchronous operation, allowing current
to reverse at light loads and maintaining high frequency
operation.
PLLIN Pin
The power module has a phase-locked loop comprised
of an internal voltage controlled oscillator and a phase
detector. This allows the internal top MOSFET turn-on
to be locked to the rising edge of the external clock.
The frequency range is ±30% around the set operating
frequency. A pulse detection circuit is used to detect a
clock on the PLLIN pin to turn on the phase-locked loop.
The pulse width of the clock has to be at least 400ns, and
2V in amplitude. During the start-up of the regulator, the
phase-locked loop function is disabled.
Parallel Operation
The LTM4612 device is an inherently current mode con-
trolled device. This allows the paralleled modules to have
very good current sharing and balanced thermal on the
design. Figure 21 shows a schematic of the parallel design.
The voltage feedback equation changes with the variable
N as modules are paralleled. The equation:
VOUT
=
0.6V
100k
N
+
RFB
RFB
N is the number of paralleled modules.
Radiated EMI Noise
High radiated EMI noise is a disadvantage for switching
regulators by nature. Fast switching turn-on and turn-off
make the large di/dt change in the converters, which act
as the radiation sources in most systems. LTM4612 inte-
grates the feature to minimize the radiated EMI noise to
meet the most applications with low noise requirements.
An optimized gate driver for the MOSFET and a noise
cancellation network are installed inside the LTM4612
to achieve the low radiated EMI noise. Figure 8 shows a
typical example for the LTM4612 to meet the Class B of
CISPR 22 radiated emission limit.
90
INTVCC and DRVCC Connection
An internal low dropout regulator produces an internal
5V supply that powers the control circuitry and DRVCC
for driving the internal power MOSFETs. Therefore, if
the system does not have a 5V power rail, the LTM4612
can be directly powered by VIN. The gate driver current
through the LDO is about 20mA. The internal LDO power
dissipation can be calculated as:
PLDO_LOSS = 20mA • (VIN – 5V)
The LTM4612 also provides the external gate driver voltage
pin DRVCC. If there is a 5V rail in the system, it is recom-
mended to connect the DRVCC pin to the external 5V rail.
This is especially true for higher input voltages. Do not
apply more than 6V to the DRVCC pin.
14
70
50
CISPR22, CLASS B
30
10
0
0 100 200 300 400 500 600 700 800 900 1000
FREQUENCY (MHz)
4612 F08
Figure 8. Radiated Emission Scan with 24VIN to
12VOUT at 5A Measured in 10 Meter Chamber
Thermal Considerations and Output Current Derating
In different applications, LTM4612 operates in a variety
of thermal environments. The maximum output current is
limited by the environment thermal condition. Sufï¬cient
cooling should be provided to help ensure reliable opera-
4612f
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