ENERGY SAVING, SELF-REGULATING BACKUP CELL RECHARGER

Publishing Venue

Motorola

Related People

Abstract

This circuit meets the need of a backup cell to be quickly recharged without excessively loading the charg- ing source, and without wasting main cell charge once the backup cell is fully recharged. Referring to Figure 1, a backup cell (BT2) supplies retention voltage to an IC when the main source (VW) is off. During this time, the Vm source drops to just below one cell voltage, minus a diode drop, as in a paging receiver. When Vdd comes back on, we desire to recharge BT2 through Vda. The backup cell is prevented from discharging during reten- tion mode through any path other than the IC, hence extending retention time for a given cell capacity.

MOTOROLA Technical Developments Volume a October 1999

ENERGY SAVING, SELF+IEGULATING BACKUP CELL RECHARGER

by James E. Berry, Patrick Kung and Mark T. Stair

   This circuit meets the need of a backup cell to be quickly recharged without excessively loading the charg- ing source, and without wasting main cell charge once the backup cell is fully recharged. Referring to Figure 1, a backup cell (BT2) supplies retention voltage to an IC when the main source (VW) is off. During this time, the Vm source drops to just below one cell voltage, minus a diode drop, as in a paging receiver. When Vdd comes back on, we desire to recharge BT2 through Vda. The backup cell is prevented from discharging during reten- tion mode through any path other than the IC, hence extending retention time for a given cell capacity.

   With Vu on, Ql is supplied with sufficient base voltage to be saturated, allowing R2 to set the base cur- rent of Q2. Q2 is always saturated while Vdd is on, so that the IC will have sufficient supply voltage for standby or active mode. R3 sets the maximum charging current for ET2 by:

Ich,max=(Vdd - VQ2 sat - VBT2 min)/R3,

RI, R2 and R4 are selected so as to insure that both transistors are saturated, yet without drawing excessive bias current. The R4/(Fi4 + Rl) ratio is selected so that for worst case VddOrr, Ql still conducts no collector cur- rent. Collector(charging) current through Q2 is self regulating, as shown by the above equation, and approaches zero as the c...