LM22680

LM22680

RT/SYNC

RT/SYNC

1 k:

1 k:

2N7002

EN

EN

ENABLE

LM22680, LM22680-Q1

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SNVS595L – SEPTEMBER 2008 – REVISED NOVEMBER 2014

Feature Description (continued)

7.3.4 Self Synchronization

It is possible to synchronize multiple LM22680 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 14 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 14. Self Synchronizing Setup

7.3.5 Boot-Strap Supply

The LM22680 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.

7.3.6 Internal Loop Compensation

The LM22680 has internal loop compensation designed to provide a stable regulator over a wide range of

external power stage components.

Ensuring stability of a design with a specific power stage (inductor and output capacitor) can be tricky. The

LM22680 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 LM22680 has internal type III loop compensation, as detailed in Figure 15. 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 LM22680, the power stage dc gain is fixed at 20dB. 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 4.

(4)

Alternatively, this pole should be placed between 1.5 kHz and 15 kHz and is given by Equation 5.

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Product Folder Links: LM22680 LM22680-Q1