DN279 - Microprocessor Core Supply Voltage Set by I2C Bus Without VID Lines

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Microprocessor Core Supply Voltage Set by I2C Bus
without VID Lines – Design Note 279
Mark Gurries
Introduction
Many modern CPUs run at two different clock speeds,
where each speed requires a different core operating
voltage to assure optimum performance. These voltages
are documented in the manufacturer’s VID (voltage
identification) section of the CPU specification. Some
new DC/DC converters (the LTC®1909, for example) have
built-in VID control that supports dual programmable
output voltages, but many existing converters do not.
The LTC1699 is a precision 2-state resistive divider that
uses a simple SMBus interface to allow VID control on
non-VID-enabled DC/DC converters. No dedicated VID
lines are required.
How it Works
DC/DC converters maintain consistent output voltage
by comparing the output voltage, through an accurate
voltage divider, against an internal reference and adjusting the output to compensate for differences. The
LTC1699 is a 2-state resistive divider that replaces the
fixed voltage divider in the feedback circuit, thus allowing the circuit to support two different voltage outputs.
It is specifically designed to work with DC/DC PWM
converters that use an internal 0.8V reference, such as
the LTC1702A, LTC1628, LTC1735 and the LTC1778.
The LTC1699, on command, can choose between two
programmable, precision output voltages. The two
voltages are programmed via 5-bit VID words sent over
the commonly used SMBus (System Management Bus)
serial interface, precluding the need for dedicated VID
control lines. The host system then has two methods
to switch between the two voltages: digitally through
the SMBus interface or via a logic signal at the select
(SEL) pin. When CPU voltage is in regulation, the
LTC1699 provides a power good signal that can be used
to inform the CPU and satellite systems that power is
up to specification. An enhanced version of the IC, the
LTC1699EGN, expands the power sequencing control
and status lines to coordinate multiple DC/DC converters
that manage other CPU system voltages, such as those
for the I/O and clock supplies (see Figure 2).
Since accurate CPU voltages are critical for reliable
CPU operation, the voltage dividers in the LTC1699 are
accurate to within ±0.35%. There are three versions of
the LTC1699 to support different Intel CPUs and the
unique voltage tables based on their 5-bit VID codes. The
LTC1699-80 covers the Intel mobile specification while
the desktop standards are covered by the LTC1699-81
for the VRM8.4 specification, and the LTC1699-82 for
VRM9.0 specification (see Table 1).
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
VCC
U1
LTC1699-81
VRON
SDA
SCL
SEL
5
3
4
1
2
7
11
14
15
VRON
SDA
SCL
SEL
NC
NC
NC
GND
GND
5V
R9
100k
PGOOD
CPU_ON
FB
IO_ON
CLK_ON
PGOOD
R1
39k
VIN
C6
0.01μF
8
13
9
10
6
12
SENSE 16
VCC
C8, 0.01μF
R2, 11k
C15
220pF
R4
10k
M1
U2
LTC1778
1
2
3
4
5
6
7
8
C18
2200pF
16
RUN/SS BOOST
15
PGOOD
TG
14
VRNG
SW
13
FCB
PGND
12
ITH
BG
11
SGND
INTVCC
10
VIN
ION
9
VFB
EXTVCC
R5
715k, 1%
5VDC
C14, 0.1μF
C1
10μF
25V
D1
C19, 0.01μF
C7
0.22μF
R3
1Ω
C13
4.7μF
6.3V
C9
22μF
6.3V
D2
+
+
C10
180μF
4V
C1, C2, C3, C4, C9: TAIYO YUDAN
C5: SANYO
C10, C11: PANASONIC, SPCL. POLY.
D1: CMDSH-3
D2: DIODES INC., B340A
L1: TOKO, 919AS-1RON
M1: Si4884DY
M2: Si4874DY
Figure 1. SMBus Controlled High Efficiency DC/DC Converter
03/02/279_conv
C4
10μF
25V
L1, 1μH
M2
C12
0.1μF
C2
10μF
25V
+
C3
10μF
25V
VIN
5V
C5
TO 20V
10μF
35V
ALUM
GND
VOUT
1.3V TO
C11 3.5V
180μF AT 10A
4V
GND
DN279 F01
VIN
VIN
OUTPUT
ADJUST
SMBus
INTERFACE
VOUT
VCORE
ON/OFF
DC/DC
CONVERTER*
LTC1699EGN
CPU/DSP/
FPGA/ASIC
VIN
VOUT
ON/OFF
DC/DC
CONVERTER
MEMORY/
CPU I/O
ON/OFF
DC/DC
CONVERTER
GRAPHIC I/C
CPU CLOCK/
ETC
*WITH 0.8V REFERENCE
DN279 F02
Figure 2. SMBus Power Sequencing and Multiple DC/DC
Converter Control with the 16-Pin SSOP Package of the
LTC1699EGN
Table 1. Selection of VOUT Ranges and VRMs with the
Combination of the LTC1699 and an Appropriate DC/DC
Converter
PART NUMBER
VOUT RANGE
VRM COMPATIBILITY
LTC1699-80
0.9V to 2.0V
Mobile CPU
LTC1699-81
1.3V to 3.5V
VRM8.4
LTC1699-82
1.075V to 1.85V
VRM9.0
Why Use an SMBus?
An SMBus is easy to implement and is growing as a
system control standard. The SMBus was developed
as a low power 2-wire serial interface to standardize
the control and monitoring of the system support
functions other than the CPU, originally defined for
portable computers with intelligent rechargeable batteries. Most portable computers today use the SMBus
for more than just battery control. It has evolved as
the standard method of power flow control, system
temperature monitoring and cooling control. It is now
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supported by popular operating systems and is integral
to current PC design standards. Controlling the CPU
voltage via SMBus is the next logical step, eliminating
the need for proprietary control interfaces.
The SMBus does have some limitations. The SMBus
version 1.0 standard has no error checking protocol,
a potentially significant problem for modern CPUs that
do not fare well when provided the wrong voltage.
Although the newer SMBus v1.1 standard includes an
optional error checking protocol, it is not widely used.
Because most systems that traditionally use SMBus
are error tolerant, upgrading current designs to the
SMBus v1.1 protocol means a significant increase in
communications software and hardware complexity. To
address these issues and still take advantage of SMBus
benefits, Linear Technology has developed several
special protocol procedures and recommendations
that provide ways to eliminate errors without use of
v1.1 error checking.
The first is to allow the host to write and read the
preprogrammed voltage values as often as needed to
verify the value. The second is that a programmed value
is not activated until the host sends two SMBus “ON”
or “OFF” commands, one after the other. If any bit is
out of place in the “ON” or “OFF” values, the preceding
voltage programming command is rejected.
The LTC1699 also features two special lockout functions.
The first is to ignore “ON” commands until the voltage
registers are set up. In addition, when two valid “ON”
command sequences are received, the VID registers are
locked out to prevent changes while the power supplies
are operating. Finally, the LTC1699 implements the new
SMBus v1.1 logic levels for improved signaling integrity.
Together these techniques offer robust and safe control
of the CPU voltage using the popular SMBus.
Desktop/Portable VID DC/DC Converter
Figure 1 shows a typical implementation of a core
voltage regulator using the LTC1778 controller and
the LTC1699. The equivalent circuit is available as a
monolithic IC in the LTC1909.
For applications help,
call (408) 432-1900
dn279f_conv LT/TP 0302 371.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2002
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