The challenge

Over 5% of the world’s population – 360 million people – has disabling hearing loss (328 million adults and 32 million children) and use a hearing aid. People with more severe or profound hearing losses may benefit from cochlear implants. Cochlear, based in Australia, is a pioneer and industry leader in cochlear implants for people with hearing loss.

A cochlear implant consists of an internal and external component. The internal component is surgically inserted under the skin behind the ear, and a narrow wire is threaded into the inner ear. The external component, which looks somewhat like a behind-the-ear hearing aid, is connected to the internal one through the skin via an external magnetic disk.

Incoming sounds are converted to electrical currents and directed to a number of contact points on the internal electrode-wire. This operation creates an electrical field which directly stimulates the auditory nerve, thus bypassing the defective inner ear. Unlike hearing aids, cochlear implants convert sound waves to electrical impulses and transmit them to the inner ear, providing people with the ability to hear sounds and potentially better understand speech without reading lips.

Integrated Circuits have helped the medical world to diagnose, cure or improve a patient’s life. Especially in the field of medical implants, new applications like treatments for epilepsy or depression are rapidly emerging. This is made possible by a combination of medical knowledge of the human body and improvements in IC design that allow more intelligence at low power consumption in a small volume.

Since the available power in a medical implant is extremely limited, high efficiency is required. This can be achieved by using different power domains for different functions. For nerve stimulation, voltages above 10V are needed as well. All these elements lead to the use of a dedicated power management system in a high voltage technology.

The solution

AnSem developed for Cochlear a highly efficient 24V power management system that generates all internal low and high voltage supplies required in the cochlear implant to perform the high voltage nerve stimulation. The implementation combines a compact design, requiring a minimum of external components, with advanced switch mode power supplies, LDOs and safety functions.

The heart of this system is a single inductor multiple output (SIMO) switched inductor regulator. The SIMO derives three different output supplies at 20V, 1.1 and 1.5V from a single input supply that has a range between 3.2V and 10V. The main advantage of a SIMO regulator is that all supplies share the same inductor. This is important since the inductor must be an external component and the available volume in a medical implant is limited.

Since the output load of the system is low, the quiescent power consumption of the system had to be reduced significantly. This is realized at system level and at circuit level by using digital logic, specifically adapted for ultra low-power applications. The power management system further consists of bandgap circuits, bias current generation and linear regulators. Smart logic controls the power on and power off sequence, generates the power on reset and checks that all supplies are in their valid operating area.

Safety is a primary focus throughout the design: Robust design techniques to overcome process, voltage and temperature variations have been applied. Furthermore the design can also cope with one time events like cosmic rays due to specific design techniques at system and block level.

Key technical statistics

• High efficiency embedded power management in a HV process
• Contains high voltage current sources for nerve stimulation
• Current calibrated to < 1% accuracy using OTP
• Single inductor multiple output switched mode power supply with wide loading range
• Low drop out 15 V regulator
• Advanced start-up control and power level monitoring circuitry