As energy efficiency is becoming a must in many industries, mastery of the quiescent current starts being a designer’s obligation.
The quiescent current is the minimum value necessary for the DC-DC converter to remain enabled when not in use. It is the rated current used in on condition with minimum functionality: i.e., the integrated circuit is switched on and ready to operate.
The quiescent current is absorbed by the IC and flows entirely to the ground. The total current absorbed also depends on the inductor input current, which in turn depends on the load (unlike the quiescent current) and the conversion efficiency.
A low quiescent current directly results in high conversion efficiency, especially with light loads. Lower efficiency means higher power dissipation and shorter battery life. In the approximate case, the quiescent current is simply the difference between the input power and output power, divided by input voltage. Similar to switching regulators, the quiescent current is the polarization current and the gate drive current.
Renesas presented a pair of new 42 V dual synchronous controllers with a quiescent current of 6 µA and integrated MOSFET drivers that provide primary power stage solutions for Renesas R-Car H3 and R-Car M3 SoCs.
Speaking with Edward Kohler, strategy marketing manager, Automotive EV and Power Business Unit at Renesas, he said, “We approach low quiescent current operation from two levels. We first optimize the device to operate with minimum current. We then look at the application level and tailor the device further to enable system-level operating current savings.”
He added, “In addition to offering low quiescent current, these controllers are designed to mitigate EMI. The ISL78263 and ISL78264 (figure 1) controllers are able to operate across a very wide frequency range with a constant switching frequency. The devices have a resistor -programmable adjustable oscillator that can be set from 200kHz to 2.2MHz and also offer an optional synchronization input that allows the system to more precisely control the frequency with a clock. The wide adjustability range gives maximum flexibility to the system to choose which frequency band to operate, while the ability to operate with constant switching frequency forces the energy generated to stay within the selected frequency band. Typically, in automotive environments, it is desirable to operate just above or just below the Medium Wave AM broadcast band. Operating below the band typically offers better DC/DC conversion efficiency while operating above the band provides for maximum EMI mitigation and reduces the size and weight of the electromagnetic components.”
Quiescent Current
In order to optimize the device itself, Renesas developed a modulator architecture based on a strategy that allows for scaling of the operating current with load current. “This way, the ISL78263 and ISL78264 can lower their quiescent current when it really matters, at light load. To realize this capability, the IC employs several design features,” said Kohler.
The modulator architecture makes some functional blocks necessary only for high-load operation, allowing them to be completely powered off in the light load condition. This requires special design and management to make sure these functional elements are only powered off when appropriate and that they can be re-enabled almost instantaneously to respond if the system has a sudden increase in operating current. For those blocks that must remain active continuously, the circuit topology is optimized to operate from a low supply voltage with ultra-low bias current.
“Minimizing both the supply voltage and the operating current minimizes the required operating power which we ultimately translate into system input current,” said Kohler. “Finally, we add advanced noise immunity measures that ensure that the circuits operating with ultra-low bias current cannot be disturbed.”
In addition to optimizing the circuits and architecture of the device, Renesas examined the application-level use-case and looked for opportunities to minimize the system’s operating current. “First, we built in a supply switchover function that allows the device to operate the internal circuits from a secondary, low voltage bias supply when present,” said Kohler. “This allows the device to switch from using the system input, which is typically higher, over to a lower voltage. In most cases, the application will utilize this function to power the ICs internal circuits from its own output, taking advantage of the switching regulator’s step-down efficiency gain to further minimize current draw from the higher system supply. In addition, we also integrated a low-current voltage feedback network within the IC. This allows the system to eliminate an external voltage feedback network, which would typically be composed of a pair of resistors between the output and ground and would consume a relatively high level of static current. Combining all of these design techniques, we have been able to achieve typical quiescent current of 6uA, which is the lowest figure for any automotive 40V rated step-down controller.”
