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LM22671MRX-5.0 数据表(PDF) 11 Page - Texas Instruments

部件名 LM22671MRX-5.0
功能描述  LM22671/LM22671-Q1 42V, 500mA SIMPLE SWITCHER짰 Step-Down Voltage Regulator
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制造商  TI [Texas Instruments]
网页  http://www.ti.com
标志 TI - Texas Instruments

LM22671MRX-5.0 数据表(HTML) 11 Page - Texas Instruments

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LM22671
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SNVS589M – SEPTEMBER 2008 – REVISED APRIL 2013
The choice of switching frequency is governed by several considerations. As an example, lower frequencies may
be desirable to reduce switching losses or improve duty cycle limits. Higher frequencies, or a specific frequency,
may be desirable to avoid problems with EMI or reduce the physical size of external components. The flexibility
of increasing the switching frequency above 500 kHz can also be used to operate outside a critical signal
frequency band for a given application. Keep in mind that the values of inductor and output capacitor cannot be
reduced dramatically, by operating above 500 kHz. This is true because the design of the internal loop
compensation restricts the range of these components.
Frequency synchronization requires some care. First the external clock frequency must be greater than the
internal clock frequency, and less than 1 MHz. The maximum internal switching frequency is ensured in the
Electrical Characteristics table. Note that the frequency adjust feature and the synchronization feature can not be
used simultaneously. The synchronizing frequency must always be greater than the internal clock frequency.
Secondly, the RT/SYNC pin must see a valid high or low voltage, during start-up, in order for the regulator to go
into the synchronizing mode (see above). Also, the amplitude of the synchronizing pulses must comport with
VSYNC levels found in the Electrical Characteristics table. The regulator will synchronize on the rising edge of the
external clock. If the external clock is lost during normal operation, the regulator will revert to the 500 kHz (typ.)
internal clock.
If the frequency synchronization feature is used, current limit foldback is not operational; see the Current Limit
section for details.
Figure 18. Switching Frequency vs RT/SYNC Resistor
Self Synchronization
It is possible to synchronize multiple LM22671 regulators together to share the same switching frequency. This
can be done by tieing the RT/SYNC pins together through a MOSFET and connecting a 1 K
Ω resistor to ground
at each pin. Figure 19 shows this connection. The gate of the MOSFET should be connected to the regulator
with the highest output voltage. Also, the EN pins of both regulators should be tied to the common system
enable, in order to properly initialize both regulators. The operation is as follows: When the regulators are
enabled, the outputs are low and the MOSFET is off. The 1 k
Ω resistors pull the RT/SYNC pins low, thus
enabling the synchronization mode. These resistors are small enough to pull the RT/SYNC pin low, rather than
activate the frequency adjust mode. Once the output voltage of one of the regulators is sufficient to turn on the
MOSFET, the two RT/SYNC pins are tied together and the regulators will run in synchronized mode. The two
regulators will be clocked at the same frequency but slightly phase shifted according to the minimum off-time of
the regulator with the fastest internal oscillator. The slight phase shift helps to reduce stress on the input
capacitors of the regulator. It is important to choose a MOSFET with a low gate threshold voltage so that the
MOSFET will be fully enhanced. Also, a MOSFET with low inter-electrode capacitance is required. The 2N7002
is a good choice.
Copyright © 2008–2013, Texas Instruments Incorporated
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