Datasheet

LM22679/LM22679Q

SNVS581K –FEBRUARY 2013–REVISED NOVEMBER 2012

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Figure 13. Current Limit vs IADJ Resistor

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 the following equation:

(6)

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

Boot-Strap Supply

The LM22679 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 LM22679 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 Compensation

The LM22679 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

LM22679 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: LM22679/LM22679Q