LM67660 datasheet(24/66 Pages) TI2

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Upon initiating soft start, the device takes the following actions:

• The internal reference to regulate the output voltage slowly ramps from zero. The net result is that the output

voltage takes tSS to reach 90% of the desired value.

• The operating mode sets to AUTO, activating diode emulation such that a prebiased start-up occurs without

pulling down on the output voltage.

• Hiccup-mode protection remains disabled for the duration of soft start; see Section 7.3.11.3.

Together, these actions provide a start-up profile with limited inrush current and allow high output capacitance

and high loading conditions such that the device can approach current limit during start-up without triggering

hiccup. See Figure 7-8.

VEN/UVLO

VOUT setpoint

VOUT

90% of VOUT

setpoint

tSS

Triggering event

AUTO-to-FPWM

transition occurs here (if

FPWM is enabled)

tEN

VEN/UVLO

VOUT setpoint

VOUT

90% of VOUT

setpoint

tSS

Triggering event

tEN

(a)

(b)

AUTO-to-FPWM

transition occurs here (if

FPWM is enabled)

After soft start initiates, the output voltage reaches 90% of the output setpoint after a time interval, tSS. The device disables FPWM and

hiccup during the soft-start interval, and subsequently enables these modes once the output voltage reaches regulation.

Figure 7-8. Output Voltage Soft Start: No Prebias (a), With Prebias (b)

7.3.10.1 Recovery From Dropout

Any time the output voltage falls more than a few percent (for example, when the input voltage falls below

the output voltage setpoint), the output voltage ramps up slowly during the recovery. Known as recovery from

dropout, this condition differs from soft start in three important ways:

• The reference voltage is set to approximately 1% above what is needed to achieve the output voltage

setpoint. The reference voltage is not started from zero.

• Hiccup is allowed only if output voltage is less than 40% of the setpoint. As described in Section 7.3.11.3, the

device inhibits hiccup when operating in dropout.

• FPWM mode is allowed during recovery from dropout. If the output voltage is suddenly pulled up by an

external supply, the converter can pull down on the output. Note that all the protections present during normal

operation are in place, protecting the device if the output gets shorted to a high voltage or ground.

Despite being called recovery from dropout, this feature is active whenever the output voltage drops to a few

percent lower than the output setpoint. This action primarily occurs under the following conditions:

• Dropout: When there is insufficient input voltage to maintain the desired output voltage

• Overcurrent: When there is an overcurrent event that is not severe enough to trigger hiccup

LM65680

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