Datasheet

SHORT-CIRCUIT

clf

clr

OUT

CURRENT LIMIT

SHORT-CIRCUIT

REMOVED

100 mV

LM21215A

SNOSB87B –MARCH 2011–REVISED MARCH 2013

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CURRENT LIMIT

The LM21215A-1 has current limit protection to avoid dangerous current levels on the power FETs and inductor.

A current limit condition is met when the current through the high side FET exceeds the rising current limit level

CLR

). The control circuitry will respond to this event by turning off the high side FET and turning on the low side

FET. This forces a negative voltage on the inductor, thereby causing the inductor current to decrease. The high

side FET will not conduct again until the lower current limit level (I

CLF

) is sensed on the low side FET. At this

point, the device will resume normal switching.

A current limit condition will cause the internal soft-start voltage to ramp downward. After the internal soft-start

ramps below the Feedback (FB) pin voltage, (nominally 0.6 V), FB will begin to ramp downward, as well. This

voltage foldback will limit the power consumption in the device, thereby protecting the device from continuously

supplying power to the load under a condition that does not fall within the device SOA. After the current limit

condition is cleared, the internal soft-start voltage will ramp up again. Figure 26 shows current limit behavior with

, V

OUT

and V

SHORT-CIRCUIT PROTECTION

In the unfortunate event that the output is shorted with a low impedance to ground, the LM21215A-1 will limit the

current into the short by resetting the device. A short-circuit condition is sensed by a current-limit condition

coinciding with a voltage on the FB pin that is lower than 100 mV. When this condition occurs, the device will

begin its reset sequence, turning off both power FETs and discharging the soft-start capacitor after t

RESETSS

(nominally 110 µs). The device will then attempt to restart. If the short-circuit condition still exists, it will reset

again, and repeat until the short-circuit is cleared. The reset prevents excess current flowing through the FETs in

a highly inefficient manner, potentially causing thermal damage to the device or the bus supply.

Figure 26. Current Limit Conditions

THERMAL PROTECTION

Internal thermal shutdown circuitry is provided to protect the integrated circuit in the event that the maximum

junction temperature is exceeded. When activated, typically at 165°C, the LM21215A-1 tri-states the power FETs

and resets soft start. After the junction cools to approximately 155°C, the device starts up using the normal start

up routine. This feature is provided to prevent catastrophic failures from accidental device overheating. Note that

thermal limit will not stop the die from operating above the specified operating maximum temperature,125°C. The

die should be kept under 125°C to ensure correct operation.

Product Folder Links: LM21215A