LM1771

SNVS446D – JUNE 2006 – REVISED JANUARY 2016

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Therefore, if the input supply is extremely slow rising such that at the end of soft-start the input voltage is still

near the UVLO threshold, a timing option must be chosen to ensure that maximum duty-cycle permits the output

to meet the minimum condition. TI recommends using the 2000-ns option (LM1771U) in conditions where the

output voltage is 2.5 V or greater to avoid false latch-offs when there is concern regarding the input supply slew

rate.

NOTE

As soon as soft-start terminates the short-circuit protection is enabled. This means that if

the output voltage does not reach at least 68% of its final value the part latches off.

In some situations, the internal soft-start routine can create a slight overshoot on the output voltage. If this must

be avoided, the use of a feedforward capacitor as detailed in the Feedforward Capacitor section.

7.3.5 Jitter

The LM1771 uses an adaptive ON-time control scheme that relies on the output voltage ripple to provide a

consistent switching frequency. Under certain conditions, excessive noise can couple onto the feedback pin

causing the switch node to appear to have a slight amount of jitter. This is not indicative of an unstable design.

The output voltage still regulates to the exact same value. Careful component selection and layout must minimize

any external influence.

In addition to any external noise that can add to the jitter seen on the switch node, the LM1771 always has a

slight amount of switch jitter. This is because the LM1771 makes a small alteration in the reference voltage every

128 cycles to improve its accuracy and long-term performance. This has the effect of causing a change in the

switching frequency at that instant. When viewed on an oscilloscope this can be seen as a jitter in the switch

node. The change in feedback voltage or output voltage, however, is almost indistinguishable.

7.4 Device Functional Modes

The LM1771 has essentially one normal operational mode: in normal operation, the part operates in full

synchronous mode to regulate the FB input to 0.8 V (typical) after soft-start period is over. The EN pin allows the

user to shut down the part. When the part is enabled, the IC enters soft-start for 1 ms to 1.8 ms depending on

the variant of the IC. See Soft-Start for more detail on the soft-start pattern.

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