LM27402_1

LM27402 Current Limit Circuits LM27402 Current Limit Application Circuits National Semiconductor Application Note 2060 Daniel Tennant June 10, 2010 Introduction Sensing the inductor DCR voltage can be an accurate and lossless technique of obtaining current information in DC/DC regulators. The absence of a series current sense resistor permits high current and high efficiency designs at a lower cost and higher density. Inductor DCR sensing also allows continuous monitoring of the inductor current as opposed to MOSFET RDS(ON) sensing which usually samples the voltage information across a FET during a small interval of time. Unlike MOSFETs, inductors can be purchased with low tolerance DCR specifications thus increasing the overall current sense accuracy. The LM27402 is equipped with a low offset current sense comparator to handle inductor DCR current sense applications. A +10 µA current in series with a resistor provides a voltage offset compared to the DCR voltage to set the current limit level. This application note will walk through the process of choosing the DCR current sense circuit components to optimize the current limit performance of the LM27402 DC/DC buck regulator controller. The derivation for matching time constants is as follows: DCR Current Sense Topology Figure 1 shows a typical topology for sensing the DCR voltage in a buck regulator. 30124301 FIGURE 1. DCR Current Sense Topology Components RS and CS create an RC filter. The time constant of the RC filter should match the time constant related to the inductor and its DCR to accurately reproduce the DCR voltage across CS. Given the following circuit: If the time constants do not match, the voltage VS will either lead or lag VDCR. Particularly: If τRC > τL, VS will lag VDCR by a factor of τRC/τL at any point in time