power wireless implantable electronics is presented herein. The system is .... and N3 transistors, the main source of error is due to IB gen- erated via the P4-P5 ...
A Low Power Battery Management System for Rechargeable Wireless Implantable Electronics Pengfei Li, Student Member, IEEE, Rizwan Bashirullah, Member, IEEE, Jose C. Principe, Fellow, IEEE
I. INTRODUCTION Low power wireless interfaces for implantable multichannel recording microelectrodes can facilitate the study of neural signal activity and long-term bio-compatibility in behaving animals while minimizing the risks of skin irritation and possible infections due to tethered percutaneous connectors [1]-[3]. These integrated wireless interfaces often share a common framework that consist of an external component placed in very close proximity (100Wh/kg) and are available in miniature pintype form factors specially designed for medical implant applications [4]. Batteries can be charged or discharged at various rates and is usually specified in terms of the “C rate,” where C is the battery nominal capacity expressed in terms of mAh (milliamps hours) or Ah (amps-hours). Fig.2a shows a typical charge/discharge profile for a lithium ion battery. The battery is charged at a constant current (CC) rate to its upper threshold of 4.2V, and then switches to constant voltage mode (CV) until its current drops to about 4% of initial current. In order to maximize the number of charge cycles and battery lifetime, CC rates of 1C are typically used, although higher rates are often employed to achieve shorter charge cycles. Based on the charging profile, a standard Li-ion battery charger needs to realize three fundamental functions: constant current (CC) control, constant voltage (CV) control and end-of-charge detection (about 4% of the constant current). Fig.2b shows a basic topology used by most integrated battery chargers [5]-[9]. The basic cell consists of a PMOS transistor biased in saturation to provide constant current fast charging until the cell voltage (VBAT) reaches a maximum voltage, beyond which a constant voltage control loop (not shown) is activated. Charging is typically terminated when the voltage across the external sensing resistor (RSENSE) indicates a current reduction of 4% of its initial constant current. B. Proposed Battery Control Loop Accurate current measurement for end-of-charge detection requires the insertion of a high precision external sensing resistor (RSENSE) in the battery charging path. A low value is typically chosen to maintain the maximum voltage drop across RSENSE to within 100mV so as not to affect the charging profile and minimize the power dissipation – this is particularly important when the dynamic range of the battery current is large. This implies a high resolution (VRES) comparator with low input referred offset (
