LM22670

LM22670

RT/SYNC

RT/SYNC

1 k:

1 k:

2N7002

EN

EN

ENABLE

LM22670

SNVS584O – SEPTEMBER 2008 – REVISED MARCH 2013

www.ti.com

Figure 19. Self Synchronization Set up

Boot-Strap Supply

The LM22670 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 LM22670 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 LM22670 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

LM22670 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.

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 20. 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 D.C. 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 D.C. 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 the following equation:

(8)

Alternatively, this pole should be placed between 1.5kHz and 15kHz and is given by the equation shown below:

(9)

The Q factor depends on the parasitic resistance of the power stage components and is not typically in the

control of the designer. Of course, loop compensation is only one consideration when selecting power stage

components; see Application Information for more details.

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