ADVANCED INFORMATION
TK654xx
APPLICATION INFORMATION (CONT.)
New microprocessors are earnestly addressing this power issue by reducing the voltage. Battery manufacturers are
wakinguprightbeforetheendofthecenturywiththeintroductionofthe“polymerelectrolytebattery”whichallowsforhigher
volumetric and gravimetric densities. Furthermore, these types of batteries come with much better form factors.
Unfortunately the wireless transmit and receive section of the portable system is very sensitive and requires very clean
analog voltages to keep a clean reception and transmission. This RF requirement reigns over the power management
section of the system and creates a dilemma to the system engineer. How do you extend the battery life with “efficient
switchers” when they are so “dirty” in performance compared to “linears”?
The industry has made attempts to get away from a sole linear system shown in Fig. 10 by considering a “PWM” switcher
in front of the linear as shown if Fig. 11.
The system of Fig. 10 has the advantage of very clean voltages for the RF section, but lacks high efficiency. Specifically
as the processor voltage drops and the battery makers refine the Li-Polimer battery (which ranges from 2.7V min to 4.2V
max).
The system of Fig. 11 has the advantage of high efficiency followed by clean RF due to the linears, but lacks the ability
to turn off (turn on) fast when the system is in standby operation. This is specifically due to the PWM architecture which
uses Continuous Conduction Mode (CCM) to build up the current in the coil.
WIRELESS PORTABLE SYSTEM USING PWM SWITCHER TO IMPROVE EFFICIENCY
Power
Amp
Enable A
Enable B
Enable A
µ Controller
Enable B
Enable C
Enable C
FIGURE 11: PWM Switcher embedded to improve wireless system efficiency
September 1999 TOKO, Inc.
Page 21
TOKO [ TOKO, INC ]
