LM22679, LM22679-Q1

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SNVS581L – FEBRUARY 2013 – REVISED NOVEMBER 2014

7.3 Feature Description

7.3.1 UVLO

The LM22679 also incorporates an input undervoltage lock-out (UVLO) feature. This prevents the regulator from

turning on when the input voltage is not great enough to properly bias the internal circuitry. The rising threshold is

4.3 V (typ) while the falling threshold is 3.9 V (typ).

7.3.2 Soft-Start

The soft-start feature allows the regulator to gradually reach steady-state operation, thus reducing start-up

stresses. The internal soft-start feature brings the output voltage up in about 500 µs. This time can be extended

by using an external capacitor connected to the SS pin. Values in the range of 100 nF to 1 µF are recommended.

The approximate soft-start time can be estimated from Equation 1.

SS

(1)

Soft-start is reset any time the part is shut down or a thermal overload event occurs.

7.3.3 Boot-Strap Supply

The LM22679 device 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.

7.3.4 Internal Compensation

The LM22679 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

LM22679 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 LM22679 has internal type III loop compensation, as detailed in Internal Loop

Compensation section. 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 LM22679, 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 2.

(2)

Alternatively, this pole should be placed between 1.5 kHz and 15 kHz and is given by Equation 3.

(3)

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 and Implementation for more details).

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Product Folder Links: LM22679 LM22679-Q1