PWM Converter. AN718 Datasheet

AN718 Converter. Datasheet pdf. Equivalent

Part AN718
Description Powering The Pentium Vre With The Si9145 Voltage Mode Controlled PWM Converter
Feature AN718 Vishay Siliconix AN718 Powering the Pentium™ VRE with the Si9145 Voltage Mode Controlled PWM .
Manufacture Vishay Intertechnology
Datasheet
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AN718 Vishay Siliconix AN718 Powering the Pentium™ VRE with AN718 Datasheet
AN718 Vishay Siliconix AN718 Powering the Pentium™ VRE with AN718 Datasheet
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AN718
AN718
Vishay Siliconix
AN718
Powering the Pentium™ VRE with the Si9145 Voltage Mode
Controlled PWM Converter
BENEFITS
• First and only Intel-approved switching converter solution to
provide static and dynamic voltage regulation for the
Pentium VRE microprocessor.
• Low cost
• Efficiency greater than 89% at 7 A
• Eliminates the need for heat sinks
• Supplies maximum currents required by P54C
• High operating frequency (recommended 375 kHz) allows
use of the smallest possible inductors and capacitors,
resulting in lowest voltage ripple and best transient
response
• Easily adjustable output voltage to meet the different specs
of P54C family.
• PCB layout available
• Available in SO-16 surface-mount package
A PROVEN SOLUTION FOR THE PENTIUM™
CONVERTER
Siliconix has developed the first and only Intel-approved
switching power supply solution for the Pentium VRE
microprocessor. Built on a leading-edge CBiC/D process, the
Si9145BY is the only voltage-mode controlled PWM IC
capable of switching up to 2 MHz and providing a 25-MHz
error amplifier. For the first time, a 100-kHz closed-loop
bandwidth switching converter can be designed to meet the
dynamic transient requirements of the Pentium
microprocessor, without adding numerous output capacitors.
The Si9145BY consistently satisfies Pentium VRE voltage
regulation limits within generous margins. Worst-case
assumptions for voltage regulation, as provided by Intel, are
displayed in Figure 2.
REGULATION REQUIREMENTS FOR THE VRE
PENTIUM CONVERTER
The Pentium VRE microprocessor has been designed to
operate faster and more efficiently than any of its
predecessors. To save power, a CPU clock has been
implemented with start/stop features, requiring an ultra-fast
response from the power supply. To obtain higher clock
speeds and to enhance manufacturing yields, the Pentium
requires specific output voltage levels with tight voltage
regulations (Figure 3). Within this stringent voltage regulation
limit, no overshoot or undershoot are permitted to exceed the
regulation limit, irrespective of their duration. Not surprisingly,
these new demands have overwhelmed the capabilities of
existing power supply designs based on conventional PWM
ICs.
Meeting the tight static and transient regulation demands of
the Pentium VRE microprocessor requires a certain amount of
decoupling capacitance both at the processor and at the
output of the power supply. Obviously, the greater the
decoupling capacitance, the easier it is to meet the transient
regulation. Since cost and space limit the amount of
capacitance one can use, Intel has recommended the use of
six 100-µF low-ESR tantalum capacitors and twenty-five 1-µF
ceramic capacitors.
FIGURE 1. Typical Application Circuit
Pentium™ is a trademark of Intel Corporation.
FaxBack 408-970-5600, request 70588
www.siliconix.com
FIGURE 2. Intel Measured Transient Response
1



AN718
AN718
Vishay Siliconix
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-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
Output
Capacitance
(µF)
800
700
600
500
400
ESR
0.0125
0.0143
0.0167
0.020
0.025
Response Time
for ±45-mV
Regulation
(µs)
5
4
3
2
1
BW for ±45-mV
Regulation
(kHz)
95
120
160
240
480
Response Time
for ±75-mV
Regulation
(µs)
15
14
13
12
11
BW for ±75-mV
Regulation
(kHz)
32
34
37
40
43
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.
FaxBack 408-970-5600, request 70588
2 www.siliconix.com





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