Low-side current sensing

What is not clock instability If, for example, you have a Buck with 100% duty cycle, you may find that under transient conditions, the switch will stay completely ON for several cycles. But that is normal. Similarly, on an IC with low-side current sensing, pulses may be omitted entirely under sudden transients. That too is normal. So don t forget to interpret the scope waveforms you see, with due regard to the part s architecture and the applied conditions, not just the topology. 

Answer One of the reasons for limiting Dmax to less than 100% is specific to synchronous buck regulators (Figure 4-3) — when it utilizes a technique called low-side current sensing. ... 

In low-side current sensing, to save the expense of a separate low-resistance sense resistor, the Rds of the low-side mosfet (the one across the optional diode in Figure 4-3) is often used for sensing the current. The voltage drop across this mosfet is measured, and so if we know its Rds, the current through it is also known by Ohm s law. It becomes obvious that in fact for any low-side current sense technique, we need to turn the high-side mosfet OFF, and thereby force the inductor current into the freewheeling path, so we can measure the current therein. That means we need to set the maximum duty cycle to less than 100%. 

For such reasons, low-side current sensing is becoming increasingly popular. Sometimes, a current sense resistor may be placed in the freewheeling path for the purpose. However, since low-resistance resistors are expensive, the forward drop across the low-side mosfet is often used for the purpose. 

All current-carrying traces should be as wide and as short as possible. One-point grounding practices between the input, output, and low-level grounds should be done at the ground side of the current sense resistor. 

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