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LM3103 Просмотр технического описания (PDF) - Texas Instruments

Номер в каталогеLM3103 TI
Texas Instruments TI
Компоненты ОписаниеSIMPLE SWITCHER® Synchronous 1MHz 0.75A Step-Down Voltage Regulator
LM3103 Datasheet PDF : 22 Pages
1 2 3 4 5 6 7 8 9 10 Next Last
The inverse relationship of ton and VIN gives a nearly constant frequency as VIN is varied. RON should be selected
such that the on-time at maximum VIN is greater than 100 ns. The on-timer has a limiter to ensure a minimum of
100 ns for ton. This limits the maximum operating frequency, which is governed by the following equation:
fSW(MAX) = VIN(MAX) x 100 ns
The LM3103 can be remotely shut down by pulling the voltage of the EN pin below 1.6V. In this shutdown mode,
the SS pin is internally grounded, the on-timer is disabled, and bias currents are reduced. Releasing the EN pin
allows normal operation to resume because the EN pin is internally pulled up.
Figure 20. Shutdown Implementation
Current Limit
Current limit detection is carried out during the off-time by monitoring the re-circulating current through the
synchronous MOSFET. Referring to the Functional Block Diagram, when the main MOSFET is turned off, the
inductor current flows through the load, the PGND pin and the internal synchronous MOSFET. If this current
exceeds 0.9A, the current limit comparator toggles, and as a result the start of the next on-time period is
disabled. The next switching cycle starts when the re-circulating current falls back below 0.9A (and the voltage at
the FB pin is below 0.6V). The inductor current is monitored during the on-time of the synchronous MOSFET. As
long as the inductor current exceeds 0.9A, the main MOSFET will remain inhibited to achieve current limit. The
operating frequency is lower during current limit owing to a longer off-time.
Figure 21 illustrates an inductor current waveform. On average, the output current IOUT is the same as the
inductor current IL, which is the average of the rippled inductor current. In case of current limit (the current limit
portion of Figure 21), the next on-time will not initiate until that the current drops below 0.9A (assume the voltage
at the FB pin is lower than 0.6V). During each on-time the current ramps up an amount equal to:
(VIN - VOUT) x ton
During current limit, the LM3103 operates in a constant current mode with an average output current IOUT(CL)
equal to 0.9A + ILR / 2.
Figure 21. Inductor Current - Current Limit Operation
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