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The amount of output capacitance at the output of the

converter also determines the transient response of a

converter. The greater the output capacitance, the less

converter bandwidth is required to meet the transient

regulation. A 4-A transient response has been simulated

using a SPICE program for various output capacitance

characteristics (Table 1). A SPICE simulation with an 800-µF

output capacitor with 0.0125-

Ω ESR reveals that converter

with a 15-µs response time will meet the voltage regulation

limit of the Pentium VRE processor (Figure 4). This translates

into a converter with approximately 32-kHz unity gain

bandwidth, assuming a second-order response system with a

damping coefficient of 0.9.

In reality, approximately 25% of the requisite regulation limit

will be utilized by variations in the reference voltage and in the

resistor dividers. To increase power supply manufacturing

yields, it is also necessary to allow an additional tolerance in

voltage regulation of 10% or more, further increasing the

bandwidth requirement. If remote sensing is not available,

delayed sensing of the processor voltage will cause a further

drop.

Together, these constraints reduce the regulation limit of VRE

Pentium converter from ±75 mV to approximately ±45 mV. To

meet the ±45-mV regulation limit requires a converter unity

gain bandwidth of approximately 100 kHz. Obtaining a closed-

loop bandwidth of 100 kHz requires a converter switching

frequency of 375 kHz or greater. It also requires a ultra-fast

error amplifier. Typically, the error amplifier must provide a

bandwidth between 10 and 20 times the switching frequency

to correctly respond to the stimuli. In the case of the Pentium

converter, large bulk capacitors at the output of the converter

and at the microprocessor require even greater bandwidth.

FIGURE 3. Pentium Converter Requirements

TABLE 1. Transient Response

THE LINEAR REGULATION SOLUTION

In the past, linear regulators were the ideal solution for low-

power, 5-V/3-V conversion. Linear regulators provided an

inherently

quiet

power

system,

eliminating

EMI/EMC

problems. Stability and compensation issues were also

minimal, making the application and analysis as simple as

possible. The advantages of linear regulators remained

significant until power demands increased. Under these new

conditions, their disadvantages become obvious. With a 7-A

output current, a VRE converter will dissipate over 10 W of

power

into

an

already

blazing

hot

system,

requiring

cumbersome heat sinks. These increase manufacturing

difficulties and labor costs, which could easily offset the price

advantage of the linear regulator solution. Meanwhile, the

physical dimension requirements, 2.60

× 1.81 × 0.8 inches,

could be easily exceeded with a large heat sink. Additionally,

the cost savings of a linear regulator solution would be

transferred to the increased costs of a larger, noisier fan

required to circulate the additional heat over the circuit.

Output

Capacitance

(µF)

ESR

Response Time

for ±45-mV

Regulation

(µs)

BW for ±45-mV

Regulation

(kHz)

Response Time

for ±75-mV

Regulation

(µs)

BW for ±75-mV

Regulation

(kHz)

800

0.0125

5

95

15

32

700

0.0143

4

120

14

34

600

0.0167

3

160

13

37

500

0.020

2

240

12

40

400

0.025

1

480

11

43