LM3100 datasheet(11/15 Pages) NSC

11 / 15 page

ON-Time Timer, Shutdown (Continued)

(5)

The LM3100 can be remotely shut down by taking the EN pin

below 1.1V. Refer to Figure 1. In this mode the SS pin is

internally grounded, the on-timer is disabled, and bias cur-

rents are reduced. Releasing the EN pin allows normal

operation to resume. The voltage at the EN pin is between

1.5V and 3.0V, depending on V

IN and the pull-up resistor.

Current Limit

Current limit detection occurs during the off-time by monitor-

ing the re-circulating current through the low-side synchro-

nous switch. Referring to Functional Block Diagram, when

the buck switch is turned off, inductor current flows through

the load, into PGND, and through the internal low-side syn-

chronous switch. If that current exceeds 1.9A the current

limit comparator toggles, forcing a delay to the start of the

next on-time period. The next cycle starts when the re-

circulating current falls back below 1.9A and the voltage at

FB is below 0.8V. The inductor current is monitored during

the low-side switch on-time. As long as the overload condi-

tion persists and the inductor current exceeds 1.9A, the

high-side switch will remain inhibited. The operating fre-

quency is lower during an over-current due to longer than

normal off-times.

Figure 2 illustrates the inductor current waveform. During

normal operation the load current is low, the average of the

ripple waveform. When an overload occurs the current ratch-

ets up until it exceeds 1.9A. During the Current Limited

portion of Figure 2, the current ramps down to 1.9A during

each off-time, initiating the next on-time (assuming the volt-

age at FB is <0.8V). During each on-time the current ramps

up an amount equal to:

(6)

During this time the LM3100 is in a constant current mode,

with an average load current (I

OCL) equal to 1.9A +

∆I/2.

20174704

FIGURE 1. Shutdown Implementation

20174705

FIGURE 2. Inductor Current - Current Limit Operation

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