TIME (200 ns/DIV)

200 mA/DIV

2V/DIV

10 mV/DIV

AC Coupled

LM3674

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SNVS405F – DECEMBER 2005 – REVISED MAY 2013

Internal Synchronous Rectification

While in PWM mode, the LM3674 uses an internal NFET as a synchronous rectifier to reduce rectifier forward

voltage drop and associated power loss. Synchronous rectification provides a significant improvement in

efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier

diode.

Current Limiting

A current limit feature allows the LM3674 to protect itself and external components during overload conditions.

PWM mode implements current limiting using an internal comparator that trips at 1020 mA (typ). If the output is

shorted to ground the device enters a timed current limit mode where the NFET is turned on for a longer duration

until the inductor current falls below a low threshold, ensuring inductor current has more time to decay, thereby

preventing runaway.

SOFT-START

The LM3674 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

2.7V. Soft start is implemented by increasing switch current limit in steps of 70mA, 140mA, 280mA, and 1020mA

(typ. 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 10µF output capacitor and 300mA load current is 350µs and with 10mA

load current is 240µs.

LDO - LOW DROP OUT OPERATION

The LM3674-ADJ can operate at 100% duty cycle (no switching, PMOS switch completely on) for low drop out

support of the output voltage. In this way the output voltage will be controlled down to the lowest possible input

voltage. When the device operates near 100% duty cycle, the output voltage supply ripple is slightly higher,

approximately 25mV.

The minimum input voltage needed to support the output voltage is:

(3)

Load current

Drain to source resistance of PFET switch in the triode region

Inductor resistance

APPLICATION INFORMATION

OUTPUT VOLTAGE SELECTION FOR ADJUSTABLE (LM3674-ADJ)

200 k

Ω to keep the current drawn through this network small but large enough that it is not susceptible to noise.

The output voltage formula is:

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Product Folder Links: LM3674