Datasheet

LM22672 LM22672

RT/SYNC

out

1 k

2N7002

ENABLE

1 k

RT/SYNC

LM22672

SNVS588L –SEPTEMBER 2008–REVISED APRIL 2013

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Self Synchronization

It is possible to synchronize multiple LM22672 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 KΩ 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 kΩ 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 19. Self Synchronizing Setup

Boot-Strap Supply

The LM22672 incorporates a floating high-side gate driver to control the power MOSFET. The supply for 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.

Thermal Protection

Internal thermal shutdown circuitry protects the LM22672 should the maximum junction temperature be

exceeded. This protection is activated at about 150°C, with the result that the regulator will shutdown until the

temperature drops below about 135°C.

Internal Loop Compensation

The LM22672 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 5V. If an output voltage of 5V 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

LM22672 stability can be verified using the WEBENCH® Designer online circuit simulation tool at www.ti.com. A

quick start spreadsheet can also be downloaded from the online product folder.

Product Folder Links: LM22672