LM3102 datasheet(13/30 Pages) TI

13 / 30 page

ILR =

(VIN - VOUT) x ton

LM3102, LM3102-Q1

www.ti.com

SNVS515H – SEPTEMBER 2007 – REVISED JUNE 2015

Feature Description (continued)

7.3.6 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 2.7 A, the current limit comparator toggles, and as a result disabling the start of the next ON-time

period. The next switching cycle starts when the re-circulating current falls back below 2.7 A (and the voltage at

the FB pin is below 0.8V). The inductor current is monitored during the ON-time of the synchronous MOSFET. As

long as the inductor current exceeds 2.7 A, the main MOSFET will remain inhibited to achieve current limit. The

operating frequency is lower during current limit due 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 2.7 A (assume the

voltage at the FB pin is lower than 0.8 V). During each ON-time the current ramps up an amount equal to:

(4)

During current limit, the LM3102/-Q1 operates in a constant current mode with an average output current IOUT(CL)

equal to 2.7 A + ILR / 2.

Figure 20. Inductor Current - Current Limit Operation

7.3.7 N-Channel MOSFET and Driver

The LM3102/-Q1 integrates an N-channel main MOSFET and an associated floating high voltage main MOSFET

gate driver. The gate drive circuit works in conjunction with an external bootstrap capacitor CBST and an internal

high voltage diode. CBST connecting between the BST and SW pins powers the main MOSFET gate driver during

the main MOSFET ON-time. During each OFF-time, the voltage of the SW pin falls to approximately -1 V, and

CBST charges from VCC through the internal diode. The minimum OFF-time of 260 ns provides enough time for

charging CBST in each cycle.

7.3.8 Soft-Start

The soft-start feature allows the converter to gradually reach a steady-state operating point, thereby reducing

startup stresses and current surges. Upon turnon, after VCC reaches the undervoltage threshold, an 8-µA internal

current source charges up an external capacitor CSS connecting to the SS pin. The ramping voltage at the SS pin

(and the non-inverting input of the regulation comparator as well) ramps up the output voltage VOUT in a

controlled manner.

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LM3102 Datasheet(PDF) 13 Page - Texas Instruments