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LM22670QMRE-ADJ/NOPB Datasheet(PDF) 11 Page - Texas Instruments

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No. de pieza LM22670QMRE-ADJ/NOPB
Descripción Electrónicos  42-V, 3-A SIMPLE SWITCHER, Step-Down Voltage Regulator With Features
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Fabricante Electrónico  TI1 [Texas Instruments]
Página de inicio  http://www.ti.com
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LM22670QMRE-ADJ/NOPB Datasheet(HTML) 11 Page - Texas Instruments

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LM22670
LM22670
RT/SYNC
RT/SYNC
Vout
1 k:
1 k:
2N7002
EN
EN
ENABLE
LM22670, LM22670-Q1
www.ti.com
SNVS584P – SEPTEMBER 2008 – REVISED NOVEMBER 2014
Feature Description (continued)
7.3.4 Self-Synchronization
It is possible to synchronize multiple LM22670 regulators together to share the same switching frequency. This
can be done by tying the RT/SYNC pins together through a MOSFET and connecting a 1 k
Ω resistor to ground
at each pin. Figure 13 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.
Figure 13. Self-Synchronization Set up
7.3.5 Boot-Strap Supply
The LM22670 device incorporates a floating high-side gate driver to control the power MOSFET. The supply or
this driver is the external boot-strap capacitor connected between the BOOT pin and SW. A good quality 10 nF
ceramic capacitor must be connected to these pins with short, wide PCB traces. One reason the regulator
imposes a minimum off-time is to ensure that this capacitor recharges every switching cycle. A minimum load of
about 5 mA is required to fully recharge the boot-strap capacitor in the minimum off-time. Some of this load can
be provided by the output voltage divider, if used.
7.3.6 Internal Loop Compensation
The LM22670 device has internal loop compensation designed to provide a stable regulator over a wide range of
external power stage components.
The internal compensation of the -ADJ option is optimized for output voltages below 5 V. If an output voltage of
5 V or greater is needed, the -5.0 option with an external resistor divider can be used.
Ensuring stability of a design with a specific power stage (inductor and output capacitor) can be tricky. The
LM22670 stability can be verified using the WEBENCH Designer online circuit simulation tool. A quick start
spreadsheet can also be downloaded from the online product folder.
The complete transfer function for the regulator loop is found by combining the compensation and power stage
transfer functions. The LM22670 has internal type III loop compensation, as detailed in Figure 14. This is the
approximate "straight line" function from the FB pin to the input of the PWM modulator. The power stage transfer
function consists of a dc gain and a second order pole created by the inductor and output capacitor(s). Due to
the input voltage feedforward employed in the LM22670, the power stage dc gain is fixed at 20 dB. The second
order pole is characterized by its resonant frequency and its quality factor (Q). For a first pass design, the
product of inductance and output capacitance should conform to Equation 3.
(3)
Copyright © 2008–2014, Texas Instruments Incorporated
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Product Folder Links: LM22670 LM22670-Q1


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