CIRCUITS AND METHOD FOR SIGNAL SUMMATION WITHIN A PULSE WIDTH MODULATION SYSTEM
Publication Date: 2004-Mar-16
The IP.com Prior Art Database
Keywords Pulse width Modulation (PWM), comparator, signal summation, voltage follower
Pulse width Modulation (PWM), comparator, signal summation, voltage follower
Pulse width modulation is a common technique used in voltage regulation. As customers demands for increased PWM performance has evolved they have required larger frequency compensation capacitors. Unfortunately these large compensation capacitors occupy a large portion of the semiconductor die resulting in increased die cost and reduced functionality per unit area. Therefore, it would be a competitive advantage to devise a circuit employing PWM having increased electrical performance, small die size and low capacitance.
Detailed Description Of The Drawings
A conventional PWM signal generation scheme is shown in Fig. 1. The constant triangular signal Vtrg generated on the capacitor C and variable modulating signals Vmod are summed together in the summer block. The amplitude sum Vsum of these signals is compared with Vref and a PWM output signal is generated as shown in the graphs in Fig. 2.
Fig. 3 shows the summation of two voltages referred to the ground by summation of the current signals Itrg and Imod converted from Vtrg and Vmod voltages. Resultant current Isum is converted on the resistor R back to the voltage Vsum, which is compared with Vref.
In another method shown in Fig. 4, a modulating signal is directly converted to current Imod. The triangular voltage Vtrg generated on the capacitor C is converted through operational amplifier OP_AMP, transistor Q1 and resistor Rtrg to the current Itrg. OP_AMP keeps voltage on the resistor Rtrg equal to the Vtrg. Thus, Itrg = Vtrg/Rtrg. This current is mirrored through M4 and M5 and summed with Imod. Resultant current Isum creates voltage Vsum on the resistor R. This voltage is compared by PWM comparator with reference voltage Vref and PWM signal is generated. The disadvantage of this arrangement is that structure of operational amplifier requires frequency compensation by capacitor and occupies a significant portion of the die.
Fig.5 shows in one embodiment of the instant invention, the triangular voltage Vtr...