Aug 1998 VID Voltage Programmer for Intel Mobile Processors

DESIGN FEATURES
VID Voltage Programmer
for Intel Mobile Processors
Microprocessor manufacturers’
relentless push for higher speed and
lower power dissipation, especially in
areas of mobile laptop computer processors, is forcing supply voltages to
these processors to a level previously
thought impossible or impractical. In
fact, the supply voltage has become
so critical that different microprocessors demand different yet precise
supply voltage levels in order to function optimally.
To accommodate this new generation of microprocessors, LTC
introduces the LTC1706-19 VID
(voltage identification) voltage programmer. This device is a precision,
digitally programmable resistive
divider designed for use with an entire family of LTC’s DC/DC converters
with onboard 1.19V references. These
converters include the LTC1433,
LTC1434, LTC1435, LTC1435A,
LTC1436, L TC1438, LTC1439,
LTC1538-AUX, LTC1539 and
LTC1624. (Consult the factory for
future compatible DC/DC converter
VIN
4.5V–22V
1
CSS
0.1µF
2
3
CC2
220pF
40k
VID0
VCC
VCC
40k
VID1
SGND
VCC
40k
VID3
Figure 2. LTC1706-19 block diagram
TG
SW
BOOST
6
SGND
VOSENSE
FB
VID2
INTVCC
5
RFB2
SWITCH-CONTROL
LOGIC
RFB2
RUN/SS
RC
10k
RFB1
13k
40k
VIN
ITH
SENSE
VCC
COSC
CC
1000pF
51pF
VCC
LTC1435A
COSC
43pF
by Peter Guan
BG
PGND
RF
4.7Ω
13
CF
0.1µF
16
+
M1
Si4410DY
CIN
10µF, 30V
×2 R
SENSE
VOUT
1.30V–
2.00V/7A
0.015Ω
14
L1 3.3µH
DB*
12
0.22µF
+
+
11
10
SENSE– SENSE+
7
8
1000pF
SENSE
VCC
15
4.7µF
M2
Si4410DY
D1
MBRS
-140T3
FB
LTC1706-19
VID VID VID VID
0 1 2 3
COUT
820µF
4V
×2
GND
*DB = CMDSH-3
FROM µP
Figure 1. Intel Mobile Pentium II processor VID power converter
Linear Technology Magazine • August 1998
13
DESIGN FEATURES
VIN
4.8V–20V
VCC
2.7V–5.5V
LTC1624 1000pF
LTC1706-19 3
10µF
VCC
7
6
VID0
SENSE
8
VID1
100pF
1
VID2
2
5
VID3
FB
470pF
GND
4
6.8k
1
2
3
4
SENSE–
VIN
ITH/RUN BOOST
VFB
GND
TG
SW
8
7 0.1µF
6
RSENSE
0.05Ω
+
Si4412DY
VOUT
1.3V–2.0V
5
10µH
MBRS340T3
CIN
22µF
35V
×2
+
COUT
100µF
10V
×2
Figure 3. High efficiency SO-8, N-channel switching regulator with programmable output
products.) The LTC1706-19 is fully
compliant with the Intel mobile VID
specifications and comes in a tiny
SO-8 package. Four digital pins are
provided to program output voltages
from 1.3V to 2.0V in 50mV steps with
an accuracy of ±0.25%.
Figure 1 shows a VID-programmed
DC/DC converter for an Intel mobile
processor that uses the LTC1435A
and LTC1706-19 to deliver 7A of output current with a programmable VOUT
of 1.3V to 2.0V from a VIN of 4.5V to
22V. Simply connecting the LTC170619’s FB and SENSE pins to the
LTC1435A’s VOSENSE and SENSE–
pins, respectively, closes the loop
between the output voltage sense and
the feedback inputs of the LTC1435A
regulator with the appropriate resistive divider network, which is
controlled by the LTC1706-19’s four
VID input pins.
Figure 2 shows a simplified block
diagram of the LTC1706-19. A 40k
resistor in series with a diode from
VCC pulls up each VID input pin.
Therefore, the VID pin must be
grounded or driven low to produce a
digital low input, whereas a digital
high input can be generated by either
floating the VID pin or connecting it to
V CC. Series diodes from V CC are
included to prevent the inputs from
being damaged or clamped by a
potential higher than VCC. This allows
the LTC1706-19 to be fully TTL compatible and operational over a VID
input voltage range that is much
higher than VCC. When all the inputs
are high, the LTC1706-19 has a typi-
14
cal quiescent current of 0.1µ A from
VCC, because all active devices are
turned off. However, due to the pullup resistors on each of the VID
programming inputs, each grounded
input contributes approximately
68µ A, (VCC – 0.6)/40k of supply current in a 3.3V system.
The top feedback resistor in the
block diagram, R FB1 , connected
between SENSE and FB, is a 15k
resistor whose value is not modified
by the state of the VID program inputs.
The bottom feedback resistor, RFB2,
however, is modified by the four VID
inputs. The precision of the ratio between RFB2 and RFB1 results in a
±0.25% output accuracy.
Table 1 shows the VID inputs and
their corresponding output voltages.
VID3 is the most significant bit (MSB)
and VID0 is the least significant bit
(LSB). When all four inputs are low,
the LTC1706-19 sets the regulator
output voltage to 2.00V. Each
increasing binary count is equivalent
to decreasing the output voltage by
50mV. Therefore, to obtain a 1.30V
output, the three MSBs are left floating while only VID0 is grounded. In
cases where all four VID inputs are
tied high or left floating, such as when
no processor is present in the system,
a regulated 1.25V output is generated at VSENSE.
Figure 3 shows a combination of
the LTC1624 and the LTC1706-19
configured as a high efficiency stepdown switching regulator with a
programmable output of 1.3V to 2.0V
from an input of 4.8V to 20V. Using
only one N-channel power MOSFET,
the two SO-8 packaged LTC parts
offer an extremely versatile, efficient,
compact regulated power supply.
Table 1. VID inputs and corresponding output voltages
Code
VID3
VID2
VID1
VID0
Output
0000
GND
GND
GND
GND
2.00V
0001
GND
GND
GND
Float
1.95V
0010
GND
GND
Float
GND
1.90V
0011
GND
GND
Float
Float
1.85V
0100
GND
Float
GND
GND
1.80V
0101
GND
Float
GND
Float
1.75V
0110
GND
Float
Float
GND
1.70V
0111
GND
Float
Float
Float
1.65V
1000
Float
GND
GND
GND
1.60V
1001
Float
GND
GND
Float
1.55V
1010
Float
GND
Float
GND
1.50V
1011
Float
GND
Float
Float
1.45V
1100
Float
Float
GND
GND
1.40V
1101
Float
Float
GND
Float
1.35V
1110
Float
Float
Float
GND
1.30V
Linear Technology Magazine • August 1998
DESIGN FEATURES
Figure 4 shows the LTC1436A-PLL
and the LTC1706-19, a combination
that yields a high efficiency low noise
synchronous step-down switching
regulator with programmable 1.3V to
2V outputs and external frequency
synchronization capability.
Besides the LTC family of 1.19Vreferenced DC/DC converters, the
LTC1706-19 can also be used to pro-
10k
VIN
4.5V–22V
1
24
PLL LPF PLLIN
2
CSS
0.1µF
3
4
COSC
VIN
RUN/SS
TGL
TGS
LTC1436A-PLL
ITH
SW
CC
510pF
INTVCC
RC
10k
BOOST
6
8
put will range from 1.27V to 2.03V in
steps of 50.8mV.
The LTC1706-19 is the ideal companion chip to provide precise,
programmable low-voltage outputs for
an entire family of LTC DC/DC converters. Its compact size, compatibility
and high accuracy are just the right
features for today’s portable electronic
equipment.
EXTERNAL
FREQUENCY
SYNCHRONIZATION
0.1µF
COSC
39pF
100pF
gram the output voltages of regulators with different onboard references.
Figure 5 shows the LTC1706-19
programming the output of the
LT1575, an UltraFast™ transient
response, low dropout regulator that
is ideal for today’s power-hungry desktop microprocessors. However, since
the LT1575 has a 1.21V reference
instead of a 1.19V reference, the out-
SGND
BGL
VOSENSE
PGND
18
+
CIN
22µF, 35V
×2
M1
Si4412DY
21
19
M3
IRLML2803
20
L1
3.3µH
RSENSE
0.02Ω
VOUT
1.30V–
2.00V/5A
D B*
17
0.22µF
+
+
4.7µF
16
M2
Si4412DY
15
SENSE– SENSE+
9
10
SENSE
VCC
22
D1
MBRS
-140T3
FB
LTC1706-19
VID VID VID VID
0 1 2 3
COUT
100µF
10V
×2
GND
*DB = CMDSH-3
FROM µP
1000pF
Figure 4. High efficiency, low noise, synchronous step-down switching regulator with programmable output and external synchronization
VIN
12V
VCC 3.3V
LT1575
LTC1706-19 3
7
8
1
2
VCC
6
SENSE
VID0
1
2
VID1
1µF
VID2
FBK
VID3
5
3
4
SHDN
IPOS
VIN
INEG
GND
GATE
FB
COMP
8
3.3V
7
6
5.1Ω
IRFZ24
+
5
220µF
GND
4
VOUT
1.27V–2.03
IN 50.8mV STEPS
7.5k
24µF
10pF
1000pF
Figure 5. UltraFast transient response, low dropout regulator with adjustable output voltage
Linear Technology Magazine • August 1998
15
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