Author: Frank De Stasi
The most confusing technical specification in a DC/DC converter is its quiescent current, or IQ. One reason for this is that each vendor uses different terminology and definitions to specify the same thing—at least for those who are not familiar with the detailed operation of switching regulators.
In the first part of these two series, I will focus on the current required to input the power supply into the buck regulator input voltage (VIN) pin. When studying the data sheet (you must always read the data sheet!), it is best to pay attention to the conditions of the input current, not to be confused by technical terms. Let's take a look at the three most important supply currents that ordinary users will be interested in.
“Shutdown current†usually refers to the supply current measured when the regulator is turned off. In these cases, the nominal input voltage is present, and the regulator's output is 0V, regardless of the voltage required to enable the pin to turn off the converter. This may seem a bit strange, and current is still needed when the regulator is turned off, and in fact, many converters draw only a small amount of leakage current when turned off.
However, some regulators require monitoring the input voltage or precision enable input in shutdown mode. These features require a limited amount of bias current to power the internal circuitry - this parameter becomes important when using a battery to power the regulator (for example, in automotive or portable applications). This shutdown current will be the minimum current drawn from the input supply.
A more confusing term is the "non-switching" current. This is the input supply current when the regulator is enabled but does not produce an output voltage. The non-switched current is typically measured in an open loop where the voltage on the feedback pin is high enough to prevent the regulator from switching and thus not producing an output voltage.
You might ask, "Why should I care about the regulator being turned on without generating a current that is drawn at a useful output?" Simply put, you shouldn't care about this because the regulator won't be in this situation. use. However, there are at least two good reasons to make this specification appear in the data sheet. First, when the regulator is in a known state, the non-switching current can measure the health of the internal circuit well and can be easily measured in an integrated circuit (IC) production environment. Second (and more importantly), this current is part of the overall supply current used by the regulator during no-load operation.
A more useful supply current specification is the operating current required for no load. In this case, the converter regulates the output voltage, but the load current is zero. Many systems require a stable voltage in standby mode, and in this case, you need to know the amount of current required to input the power supply. Currently, most regulators specify this current value in the datasheet. However, carefully confirm the operating conditions to ensure that the current values ​​listed are actually no-load operating currents.
In the second part of this series, I will focus on the no-load operating current, including its definition.
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