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To Be Low Iq or Not To Be Low Iq? 

 著者: Kieran McDonald - 2014-10-21 10:59:16.0

To be low Iq or not to be low Iq, that is the question…  Doubtful use of Shakespeare, twisted to title an engineering topic, aside, quiescent current (Iq) is a big question for many automotive systems.  How low is low?  How does one achieve it?  And what are the trade-offs?  Very often the story starts with an overall system Iq target set by the automotive Own Equipment Manufacturer (OEM), which is then divided amongst the key system blocks; generally the remainder is what the power supply is required to comply with.  The secondary, point-of-load (POL), regulators can often be disabled to consume as little as 1 µA each.   However, the primary power supply, which sources from the battery, is often a major consumer of quiescent current. 

Fig. 1 – Typical Automotive System Power Supply Block Diagram

 

Where the load current is high or power dissipation needs to be minimised then a switched mode power supply (SMPS) needs to be considered.  As discussed in my previous blog, one way of achieving this is by use of techniques such as pulse frequency modulation (PFM) or burst-mode, but another means might be the use of an integrated hybrid topology where a linear regulator and an SMPS are used in parallel.  

When the system is awake then the SMPS is enabled and the linear regulator disabled, and when the system is in its quiescent state then the SMPS is disabled and the linear regulator enabled.  The linear regulator draws a low Iq.  Such an approach can deliver an Iq as low as 25 µA (typ).  The switching function (between SMPS and linear modes) can be automated by monitoring the load current, such as with the ON Semiconductor NCV891330 low-Iq dual-mode step-down regulator, or manual where the system controls the switching, such as with the NCV97310 multi-output power management unit.  The output capacitance in both cases is relied upon to deliver smooth switching between modes, without glitches.  One limitation of such approaches though is input voltage compliance.  

Fig. 2 – Hybrid Linear/SMPS Converter

 

If the SMPS has a PMOS high-side switch, a boot-strap is not required and 100 % duty cycle is possible, then there is just the voltage drop across the PMOS itself to be considered (input filter aside).  If the SMPS has an NMOS high-side switch then there will be a maximum duty cycle, set to allow the NMOS boot-strap to replenish itself per cycle.  This maximum duty cycle limits the minimum input voltage.  For example the NCV891330 with a 5.0 V output and 0.5 A load is capable of input voltages as low as 5.34 V.  Any input voltage lower than this requires a pre-booster, a boost controller between the battery and the primary power supply, such as the NCV8877 start stop non-synchronous boost controller.The NCV8877, for example, will add a further 12 µA (typ) to the Iq total when inactive.     

 

Fig. 3 - Typical Automotive System Power Supply Block Diagram with Pre-Booster

 

If the load current is low or efficiency is a lessor concern then a linear regulator can be considered.  With this approach then quiescent currents as low as 21 µA (typ) can be achieved, using a MOS linear regulator.  A bipolar linear regulator will consume more Iq but this can be typically traded-off against a lower cost and intrinsic reverse polarity protection (PNP), as compared to the MOS linear regulator. 

While lower Iq levels are possible to achieve, a minimum feature set has to be considered such as a reset or watchdog circuit.  The primary power supply powering the key blocks, such as the micro-controller or CAN needs, should be able to reset the micro-controller (MCU) if some power supply glitch occurs that causes the MCU to ‘lock-up’.  

The input voltage compliance of the linear regulator is determined by the voltage drop-out of its high side switch.  Typically this can be as low as 200 mV.  An important facet of the linear regulator is that as the input to output voltage differential falls below the voltage drop-out of the high-side switch, the output voltage will follow the input voltage as it falls minus the drop-out voltage.  If the load, typically an MCU, accepts a low tolerance input supply then the linear regulator output will still be active, allowing the MCU more time to check its inputs, retain memory settings, and shut-down in an orderly manner.   

So, to be or not to be?  If the system can be switched off then, not to be – low Iq regulators are not a necessary feature of such a system.  If the system can’t be switched off then, to be – low Iq regulators are a necessary feature.   Which approach, whether linear, hybrid linear-SMPS or full SMPS depends upon the system level requirements and the trade-offs that can or cannot be made.

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