LM3103

SNVS523E – SEPTEMBER 2007 – REVISED NOVEMBER 2009

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SIMPLIFIED FUNCTIONAL BLOCK DIAGRAM

Functional Description

The LM3103 Step Down Switching Regulator features all required functions to implement a cost effective,

efficient buck power converter which is capable of supplying 0.75A to loads. It contains dual N-Channel main and

synchronous MOSFETs. The Constant ON-Time (COT) regulation scheme requires no loop compensation,

results in a fast load transient response and simple circuit implementation. The regulator can function properly

even with an all ceramic output capacitor network, and does not rely on the output capacitor’s ESR for stability.

The operating frequency remains constant with line variations due to the inverse relationship between the input

voltage and the on-time. The valley current limit detection circuit, with a limit set internally at 0.9A, inhibits the

main MOSFET until the inductor current level subsides.

The LM3103 can be applied in numerous applications and can operate efficiently for inputs as high as 42V.

drive under-voltage lock-out. The LM3103 is available in the thermally enhanced HTSSOP-16 package.

COT Control Circuit Overview

COT control is based on a comparator and a one-shot on-timer, with the output voltage feedback (feeding to the

FB pin) compared with a 0.6V internal reference. If the voltage of the FB pin is below the reference, the main

upon which the on-time varies inversely. Following the on-time, the main MOSFET remains off for a minimum of

240 ns. Then, if the voltage of the FB pin is below the reference, the main MOSFET is turned on again for

another on-time period. The switching will continue to achieve regulation.

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