ILR =

L

(VIN - VOUT) x ton

fSW(MAX) =

VOUT

VIN(MAX) x 100 ns

12

LM3103

SNVS523G – SEPTEMBER 2007 – REVISED JANUARY 2018

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Feature Description (continued)

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:

(5)

The LM3103 can be remotely shut down by pulling the voltage of the EN pin below 1.6 V. 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 19. Shutdown Implementation

7.2.7 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 0.9 A, 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 0.9 A (and the voltage

at the FB pin is below 0.6 V). The inductor current is monitored during the on-time of the synchronous MOSFET.

As long as the inductor current exceeds 0.9 A, 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 20 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 20), the next on-time will not initiate until that the current drops below 0.9 A (assume the voltage

at the FB pin is lower than 0.6 V). During each on-time the current ramps up an amount equal to:

(6)

During current limit, the LM3103 operates in a constant current mode with an average output current IOUT(CL)

equal to 0.9 A + ILR / 2.