System and method for battery charging

Abstract

A charging circuit includes an N-channel metal-oxide-semiconductor field-effect transistor (NMOSFET) that controls a charging current to a battery, a charge pump that generates a driving signal based on a plurality of pulses, and a resistor coupled to a gate of the NMOSFE. The resistor and a capacitance of the gate of the NMOSFET form a low pass filter. The driving signal is filtered by the low pass filter to control a gate voltage of the NMOSFET. A variation of a gate-source voltage of the NMOSFET is proportional to a pulse density of the plurality of pulses. A variation of the charging current flowing through the NMOSFET to the battery is proportional to the pulse density.

1 . A charging circuit for charging a battery, said charging circuit comprising:
 an N-channel metal-oxide-semiconductor field-effect transistor (NMOSFET) that controls a charging current to a battery;   a charge pump that generates a driving signal based on a plurality of pulses; and   a resistor coupled to a gate of said NMOSFET, wherein said resistor and a capacitance of said gate of said NMOSFET form a low pass filter, wherein said driving signal is filtered by said low pass filter to control a gate voltage of said NMOSFET, wherein a variation of a gate-source voltage of said NMOSFET is proportional to a pulse density of said plurality of pulses, and wherein a variation of said charging current flowing through said NMOSFET to said battery is proportional to said pulse density.   
     
     
         2 . The charging circuit of  claim 1 , further comprising:
 a pulse generator that generates said plurality of pulses; and   a controller, coupled to said pulse generator, that controls said pulse density of said plurality of pulses according to a status of said battery.   
     
     
         3 . The charging circuit as claimed in  claim 2 , wherein said pulse density decreases when said charging current is greater than a first predetermined threshold. 
     
     
         4 . The charging circuit as claimed in  claim 3 , wherein said pulse density increases when said charging current is less than a second predetermined threshold that is less than said first predetermined threshold. 
     
     
         5 . The charging circuit as claimed in  claim 2 , wherein said controller controls said pulse density when a battery voltage of said battery charged by said charging current is less than a predetermined voltage threshold. 
     
     
         6 . The charging circuit as claimed in  claim 2 , wherein said controller controls said pulse density when a cell voltage for each cell of a plurality of cells charged by said charging current is less than a predetermined voltage threshold. 
     
     
         7 . The charging circuit as claimed in  claim 2 , further comprising:
 an oscillator that generates a plurality of clock pulses,   wherein said charge pump receives said plurality of clock pulses and generates said driving signal which turns on said NMOSFET.   
     
     
         8 . The charging circuit of  claim 7 , wherein said charging circuit powers off said oscillator and powers on said pulse generator when a voltage of said battery is less than a threshold, and wherein said charging circuit powers off said pulse generator and powers on said oscillator when said voltage is greater than said threshold. 
     
     
         9 . An electronic device comprising:
 a charger that charges a battery; and   a charging circuit, coupled to said battery and said charger, that controls a charging current from said charger to said battery, said charging circuit comprising:
 a pulse generator that generates a plurality of pulses to control an N-channel metal-oxide-semiconductor field-effect transistor (NMOSFET) coupled between said charger and said battery; 
 a charge pump, coupled to said pulse generator, that receives said plurality of pulses and generates a driving signal according to said pulses; and 
 a low pass filter coupled to said charge pump, wherein said driving signal is filtered by said low pass filter to control a gate voltage of said NMOSFET, wherein a variation of a gate-source voltage of said NMOSFET is proportional to a pulse density of said plurality of pulses, and wherein a variation of said charging current flowing through said NMOSFET to said battery is proportional to said pulse density. 
   
     
     
         10 . The electronic device of  claim 9 , further comprising:
 a controller, coupled to said pulse generator, that controls said pulse density of said plurality of pulses according to a status of said battery.   
     
     
         11 . The electronic device of  claim 9 , wherein said low pass filter is formed by a resistor coupled to a gate of said NMOSFET and a capacitance of said gate of said NMOSFET. 
     
     
         12 . The electronic device of  claim 9 , wherein said pulse density decreases when said charging current is greater than a first predetermined threshold. 
     
     
         13 . The electronic device of  claim 12 , wherein said pulse density increases when said charging current is less than a second predetermined threshold that is less than said first predetermined threshold. 
     
     
         14 . The electronic device of  claim 10 , wherein said controller controls said pulse density when a battery voltage of said battery charged by said charging current is less than a predetermined voltage threshold. 
     
     
         15 . The electronic device of  claim 10 , wherein said controller controls said pulse density when a cell voltage for each cell of a plurality of cells charged by said charging current is less than a predetermined voltage threshold. 
     
     
         16 . The electronic device of  claim 9 , further comprising:
 an oscillator that generates a plurality of clock pulses,   wherein said charge pump receives said plurality of clock pulses and generates said driving signal which turns on said NMOSFET.   
     
     
         17 . The electronic device of  claim 16 , wherein said charging circuit powers off said oscillator and powers on said pulse generator when a voltage of said battery is less than a threshold, and wherein said charging circuit powers off said pulse generator and powers on said oscillator when said voltage is greater than said threshold.