ADVANCED INFORMATION
TK654xx
APPLICATION INFORMATION (CONT.)
The industry is presently in a frame of mind that unless the switcher is PWM with a fixed frequency (to fix the harmonics
away from the RF), the wireless RF section will not be clean. It is the intention of this application to open the mind of system
designers to consider this new concept which addresses a real issue. Several major IC manufacturers of PWM step down
converters are taunting the “Burst Mode” capability. This can be misleading since under the burst mode, the fixed
frequency of the PWM is no longer present. (Therefore harmonics are all over the place). Furthermore, the purpose of
theBurstModeistoallowthesystemtogointoastandbymodeinordertosavebatterylife. YetthesestateoftheartPWMs
with Burst mode ability still lack in response time due to the architecture which requires the coil current to slowly build up.
This build up in current is a barrier to the present day requirements.
This Buck and Linear Wireless application proposes a system as shown in Fig. 12, which allows for all 3 important aspects
of the design:
1)
2)
3)
Clean RF
High efficiency (At full, mid, and light loads)
Low standby currents with fast “wake up” response.
The approach shown in Fig. 12 requires a new architecture for the Buck (Step down) switching converter with very fast
transient characteristics, specifically during the transition from a Buck mode to Linear mode while a RF transmission/
reception is eminent or during a wake up out of standby mode.
WIRELESS PORTABLE SYSTEM USING BUCK AND LINEAR REGULATORS
Power
Amp
Buck
Enable
Buck
*
LDO
Linear
Enable
Linear
Transmit
Receive
µ Controller
* Not necessary if LDO is active low
FIGURE 12: Buck and Linear embedded to improve wireless system efficiency, standy time, and clean RF
Page 22
September 1999 TOKO, Inc.
TOKO [ TOKO, INC ]
