LM3686 datasheet(16/27 Pages) TI

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LM3686

SNVS520E – AUGUST 2008 – REVISED MAY 2013

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Soft Start

The DC-DC converter 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_DCDC goes from logic low to logic high after

VBATT reaches 2.7V. Soft start is implemented by increasing switch current limit in steps of 200 mA, 400 mA, 600

mA and 1220 mA (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 10 µF output capacitor and 200 mA load is 350

µs and with 1 mA load is 200 µs.

LINEAR REGULATOR OPERATION (LILO)

In the typical post regulation application the power input voltage VIN_LILO for the linear regulator is generated by

the DC-DC converter. Using a buck converter to reduce the battery voltage to a lower input voltage for the linear

regulator translates to higher efficiency and lower power dissipation.

It's also possible to operate the linear regulator independent of the DC-DC converter output voltage either from

VIN_LDO/VBATT or from a different source (VIN_LILO) - (IOUT_LILO = 50 mA max in independent mode).

An input capacitor of 1 µF at VIN_LILO is needed to be added if no other filter or bypass capacitor is present in the

VIN_LILO path.

Startup Mode

If VIN_LILO > VOUT_LILO(NOM) + 250 mV the main regulator is active, offering a rated output current of 350 mA and

supplied by VIN_LILO. (Large NMOS)

If VIN_LILO < VOUT_LILO(NOM) + 150 mV the startup LILO is active, providing a reduced rated output current of 50 mA

typical, supplied by VBATT. (Small NMOS)

Figure 23. Startup Sequence, VEN_DCDC = VEN_LILO = V EN_LDO= VBATT

Current Limiting (LDO and LILO)

The LM3686 incorporates also a current limit for the LDO and LILO to protect itself and external components

during overload conditions at their outputs. In the event of a peak over-current condition at VOUT_LDO or VOUT_LILO

the output current through the NFET pass device will be limited.

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