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LM3679

www.ti.com

SNVS510A – SEPTEMBER 2007 – REVISED FEBRUARY 2008

SHUTDOWN MODE

Setting the EN input pin low (<0.4V) places the LM3679 in shutdown mode. During shutdown the PFET switch,

NFET switch, reference, control and bias circuitry of the LM3679 are turned off. Setting EN high (>1.0V) enables

normal operation. It is recommended to set EN pin low to turn off the LM3679 during system power up and

undervoltage conditions when the supply is less than 2.5V. Do not leave the EN pin floating.

SOFT START

The LM3679 has a soft-start circuit that limits in-rush current during start-up. During start-up the switch current

limit is increased in steps. Soft start is activated only if EN goes from logic low to logic high after Vin reaches

2.5V. Soft start is implemented by increasing switch current limit in steps of 200mA, 400mA, 600mA and 920mA

(typical switch current limit). The start-up time thereby depends on the output capacitor and load current

demanded at start-up. Typical start-up times with a 10µF output capacitor and 350mA load is 300 µs and with

1mA load is 200µs.

APPLICATION INFORMATION

INDUCTOR SELECTION

There are two main considerations when choosing an inductor; the inductor should not saturate, and the inductor

current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current

rating specifications are followed by different manufacturers so attention must be given to details. Saturation

current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of

application should be requested form the manufacturer. The minimum value of inductance to guarantee good

performance is 0.5µH for a 300mA application and 0.7µH for a 350mA application. Shielded inductors

radiate less noise and should be preferred.

There are two methods to choose the inductor saturation current rating.

Method 1:

The saturation current is greater than the sum of the maximum load current and the worst case average to peak

inductor current. This can be written as

(1)

•

•

•

•

L : min inductor value including worst case tolerances (30% drop can be considered for method 1)

•

f : minimum switching frequency (2.5MHz)

•

Method 2:

A more conservative and recommended approach is to choose an inductor that has saturation current rating

greater than the max current limit of 1075mA.

A 1.0 µH inductor with a saturation current rating of at least 1075 mA is recommended for most applications. The

inductor’s resistance should be less than 0.200

Ω for good efficiency. Table 2 lists suggested inductors and

suppliers. For low-cost applications, an unshielded bobbin inductor could be considered. For noise critical

applications, a toroidal or shielded-bobbin inductor should be used. A good practice is to lay out the board with

overlapping footprints of both types for design flexibility. This allows substitution of a low-noise shielded inductor

in the event that noise from low-cost bobbin models is unacceptable.

Copyright © 2007–2008, Texas Instruments Incorporated

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