(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

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-

up an amount equal to:

(6)

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

with an average load current (I

∆I/2.

20174704

FIGURE 1. Shutdown Implementation

20174705

FIGURE 2. Inductor Current - Current Limit Operation

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