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LM3100-2006 View Datasheet(PDF) - National ->Texas Instruments

Part NameDescriptionManufacturer
LM3100(2006) SIMPLE SWITCHER® Synchronous 1MHz 1.5A Step-Down Voltage Regulator National-Semiconductor
National ->Texas Instruments National-Semiconductor
LM3100 Datasheet PDF : 15 Pages
First Prev 11 12 13 14 15
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 VIN and the pull-up resistor.
20174704
FIGURE 1. Shutdown Implementation
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 (IOCL) equal to 1.9A +I/2.
FIGURE 2. Inductor Current - Current Limit Operation
20174705
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