LM3673_15 datasheet(11/28 Pages) TI

部品番号	LM3673

部品情報	2-MHz, 350-mA Step-Down DC-DC Converter

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VOUT

TIME (4 Ps/DIV)

200 mA/DIV

IL

VSW

2V/DIV

20 mV/DIV

AC Coupled

VIN = 3.6V

VOUT = 1.5V

IOUT = 20 mA

LM3673

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SNVS434L – JULY 2006 – REVISED OCTOBER 2015

Feature Description (continued)

8.3.4 Current Limiting

A current limit feature allows the LM3673 to protect itself and external components during overload conditions.

PWM mode implements current limiting using an internal comparator that trips at 750 mA (typical). 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. This allows the inductor current more time to decay, thereby

preventing runaway.

8.3.5 Soft Start

The LM3673 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 2.7

V. Soft start is implemented by increasing switch current limit in steps of 70 mA, 140 mA, 280 mA, and 750 mA

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

start-up time with a 10-µF output capacitor and 150-mA load is 280 µs; with a 5-mA load start-up time is 240 µs.

8.3.6 Low Drop Out Operation (LDO)

The LM3673-ADJ can operate at 100% duty cycle (no switching; PMOS switch completely on) for LDO support of

the output voltage. In this way the output voltage is controlled down to the lowest possible input voltage. When

the device operates near 100% duty cycle, output voltage ripple is approximately 25 mV.

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

VIN, MIN = ILOAD × (RDSON, PFET + RINDUCTOR) + VOUT

where

•

ILOAD: Load current

•

RDSON, PFET: Drain-to-source resistance of PFET switch in the triode region

•

RINDUCTOR: Inductor resistance

(1)

8.4 Device Functional Modes

8.4.1 PFM Operation

At very light load, the converter enters PFM mode and operates with reduced switching frequency and supply

current to maintain high efficiency.

The part automatically transitions into PFM mode when either of two conditions occurs for a duration of 32 or

more clock cycles:

•

The NFET current reaches zero.

•

The peak PMOS switch current drops below the IMODE level (typically IMODE < 30 mA + VIN / 42 Ω ).

Figure 15. Typical PFM Operation

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