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Chapter 4
for control stability. The only effect ESR will have is on ripple voltage. A good solution
is to parallel a low-ESR ceramic capacitor (about 1 F) with the higher-capacitance
electrolytic or tantalum (about 10 F). The ceramic capacitor reduces 600 kHz ripple from
the bursts of charging and the electrolytic reduces ripple at the control frequency.
4.8 Multiple Phase PWM Controllers
The demands of power supplies for Pentium
®
class CPUs are signifi cantly different from
low power and traditional CPU needs. A Pentium, Athalon, or Opteron CPU requires very
low voltage at tens of amps. A typical Pentium 4 power supply must supply 1.4 V at 65A.
All of the controllers mentioned so far are single-phase controllers. Current mode PWM
controller-based regulators can be operated in parallel to increase current capacity. Many
IC manufacturers produce control ICs that operate multiple power supplies in parallel
with nonoverlapping phases. The LT3730 is a representative buck controller IC designed
for Intel portable computer applications. It can operate up to 600 kHz per phase. Since
the phases do not overlap, this gives a ripple frequency of 1.8 MHz.The inductor for each
phase can be one-third as large as would be required in a single-phase design. The output
capacitor can also be one-third the size of a comparable single-phase design. The input
and output capacitor ripple current decreases as you add additional phases. Increasing the
number of phases increases the effi ciency of the power supply by reducing the losses due to
ripple current in the capacitors. Polyphase operation is also possible for boost converters.
Polyphase operation of charge pump controllers can also provide similar improvements in
effi ciency by effectively increasing the frequency of operation.
4.9 Resonant Mode Controllers
One way to increase effi ciency and reduce the stress on the switching components is to
design the control and fi lter circuit so that the switch turns on and off at zero current or
zero voltage.
Resonant mode switching circuits use constant on-time with variable off-time (as in the
TL497) to frequency modulate the current provided to the output circuit. In this case,
the inductance and capacitance of the output fi lter are selected so that the response is
resonant at the switching frequency. The frequency is adjusted so that the switch is turned
on and off at either zero voltage points or zero current points of the output waveform. The
UC1860 is a representative resonant mode switching regulator IC.
Resonant controllers have found very limited use because of the diffi culty of design
compared with ordinary square wave control. The advantages of resonant mode have been
minimized because of advances in MOSFET technology.

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