Download Design Solutions 7 - A Low Cost Dynamic VID

Design Solutions 7
April 1999
A Low Cost Dynamic VID Power Supply
for Pentium® III Processors
INTRODUCTION
The latest Pentium® III mobile processors announced by
Intel require a 2-level power supply voltage that must be
dynamically switched. A power supply that satisfies these
requirements can be constructed using a DC/DC controller
containing an internal 5-bit DAC. However, substantial
external circuitry is still required to switch between the two
operating voltages. This paper presents a new solution
that satisfies Intel’s dynamic VID requirements, but with
lower cost, smaller PCB area and better performance than
competing solutions.
Many notebook computer manufacturers also prefer the
VID feature because it provides flexibility to change output
voltages to support future Intel processors, as well as
processors from AMD and Cyrix. When using a DC/DC
converter with a built-in 5-bit DAC, the output voltages
must be changed by modifying jumpers or zero-ohm
resistors on the motherboard. The alternative solution
presented in this paper also provides complete voltage
flexibility by simply changing the value of two resistors. In
fact, even greater flexibility is available, since voltages are
not limited to those specified in the 5-bit VID table.
signal must stay low until the new output voltage has
settled within specifications, but must not remain low
for more than 100µs.
The MAX1711 data sheet states: “The internal PGOOD
detector circuit monitors only output undervoltage;
PGOOD will probably go low during upward transitions,
but not downward.” For this reason, a transition detector and timer circuit must be added.
2. Rapid Output Voltage Change
The CPU core power supply output voltage must adjust
to the new programmed value in less than 100µs. In
order to meet this requirement, the MAX1711 must be
forced into PWM mode for a high-to-low voltage transition. The MAX1711 data sheet states: “If the minimum
load is very light, it may be necessary to assert forced
PWM mode (via SKIP) during the transition period to
guarantee some output sink current capability. Otherwise, the output voltage won’t ramp downwards until
pulled down by external load current.” External circuitry
must be added to drive the SKIP pin high during voltage
transitions.
3. Fast Transient Response
DYNAMIC VID USING THE MAX1711
The MAX1711 data sheet (Rev. 0; 11/98) shows an example of a mobile VID power supply (see Figure 1). Most
of the Figure has been taken directly from Maxim’s data
sheet (Figure 10, page 24); a 5-bit multiplexer and jumpers
have been added since they are required for dynamic
control of the output voltage.
This circuit is complex and expensive for the following
reasons (quotations are taken directly from the MAX1711
data sheet):
1. POWER_GOOD Generation
The POWER_GOOD signal must go low during each
change in output voltage. This is true whether changing
from low-to-high or high-to-low. The POWER_GOOD
Powering a Pentium III processor demands extremely
good transient response from the DC/DC converter.
Linear Technology’s OPTI-LOOPTM compensation technology allows use of fewer output capacitors, and
without ESR limitations. Maxim explains the difficulty
an engineer faces when trying to meet these requirements without OPTI-LOOP: “Selecting the output capacitors in dynamically adjusted VCORE applications
can be tricky due to trade-offs between capacitor capacity and ESR…It may be necessary to mix capacitor
types or use specialized capacitors such as those shown
in Figure 7 in order to achieve the required ESR while
staying within the min/max capacitance value window.”
, LTC and LT are registered trademarks of Linear Technology Corporation.
OPTI-LOOP is a trademark of Linear Technology Corporation.
Pentium is a registered trademark of Intel Corporation.
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Design Solutions 7
5V
VBATT
10V TO 22V
0.1µF
7
ON/OFF
0.22µF
9
VSEL
470pF
5
5V
15
1
REF
BST
DH
CC
LX
0.1µF
3.3V
10
13
*
*
*
*
BX
*
3
7
11
17
21
*
*
*
*
1A1
24
VCC
1B1
2A1
2B1
3A1
3B1
4A1
4B1
5A1
5B1
2
2
20
6
19
10
18
16
17
20
16
8
*
CMPSH3
V+
VDD
VCC
10µF
25V
×6
CERAMIC
1µF
20Ω
MAX1711
DL
GND
22 2Ω
24
IRF7805
0.1µF
23
1µH
20A
IRF7805
×2
PGND
D1
FB
D2
FBS
D3
GNDS
D4
PGOOD
TON
ILIM SKIP
14
*
*
1k
11
12
21
POWERGOOD
40k
1%
2N7002
*
*
*
4
14
18
22
*
20µF
CERAMIC
4
200k
1%
8
*
220µF
4V
×10
OS-CON
3
6
SN74CBT
3383
3.3V
+
SHDN
D0
VOUT
1.5V
15A
13
*
*
*
1A2
1B2
2A2
2B2
3A2
3B2
4A2
4B2
5B2
5A2
BE
1
GND
5
9
3.3V 0.1µF
15
TRANSITION
DETECTOR
19
23
12
1k
12
13
3M
A4
VCC
B4
Y4
14
1k
10
4
5
A3
Y3
1k
2
–
8
2N7002
1N4148
74HC86
3.3V
A2
B2
Y2
30k
1N4148
1000pF
30k
100k
A1
B1
820pF
5%
6
1000pF
1
100k
1%
MAX986
B3
GND
7
Y1
49.9k
1%
+
1N4148
1000pF
1k
100k
1%
11
1000pF
9
*INSTALL ZERO-OHM RESISTORS
AS NEEDED TO PROGRAM DESIRED
VOUT(L) AND VOUT(H)
2N7002
100k
2N7002
3
1N4148
2N7002
VID F01
Figure 1. MAX1711 Dynamic VID Implementation Requires Additional Circuitry for Operation
Figure 7 in the MAX1711 data sheet shows 6 × 47µF
ceramic output capacitors in parallel. However, because the ESR of these capacitors is too low for feedback loop stability, the user must also insert at least
5mΩ of PCB trace resistance in series with the high
current path. This PCB trace resistor consumes PCB
area and also reduces the DC/DC converter efficiency by
3.5% with a 10A load!
The dynamic VID circuit shown in the MAX1711 data
sheet (Figure 10, page 24) uses 10 × 220 µF
OS-CON capacitors plus a 20µF ceramic capacitor on
2
the output. This is expensive and consumes considerable PCB area.
4. Inrush Current Limiting
Using large amounts of output capacitance has another
side-effect: very large current pulses are required to
change the output voltage. The MAX1711 data sheet
states: “attempting to slew the output upward quickly
causes large current surges at the battery as the IC goes
into output current limiting during the transition.”
Design Solutions 7
This is particularly troublesome in the case of the
MAX1711 for two reasons: 1) large output capacitance
means more charge must be transferred to change the
output voltage, and 2) output current limiting is very
inaccurate. This second problem is a result of using the
lower MOSFET RDS(ON) for current limiting. The resultant wide current limit tolerance means that current
drawn from the battery during a low-to-high transition
may be much more than the maximum load current. In
order to prevent very large surge currents, Maxim’s
dynamic VID implementation must include extra circuitry to reduce the current limit threshold during
voltage changes.
below ground. The MAX1711 data sheet states: “If the
load can’t tolerate being forced to a negative voltage, it
may be desirable to place a power Schottky diode
across the output to act as a reverse-polarity clamp.”
DYNAMIC VID USING THE LTC1735-1
A simple dynamic VID power supply may also be constructed using the LTC1735-1 (see Figure 2). This solution
has several important advantages:
• Lower DC/DC converter cost
• Significantly less PCB area is required
• OPTI-LOOP compensation provides better transient
response
5. Multiplexing VID Values
The MAX1711 accepts a 5-bit VID digital programming
word to set the output voltage. However, because this
5-bit input must be switched between two different
values, a multiplexer and two sets of jumpers must be
connected to the MAX1711’s VID input (see Figure 1).
Two sets of resistors/jumpers and the 24-pin multiplexer require considerable additional PCB area.
• Complete output voltage programming flexibility down
to VOUT = 0.8V
• “Soft-Latch” output overvoltage protection
VID Resistor Selection
Instead of a 5-bit VID DAC, the LTC1735-1 relies on an
external 2-range voltage feedback divider. A voltage select
signal changes the feedback divider ratio, and, therefore,
the programmed output voltage. Any two output voltages
may be selected, with VOUT as low as 0.8V. Selecting different output voltages simply requires changing the values of two external resistors. Programming resistors can
be modified according to the following simple formulas:
6. Output Overvoltage Protection
The MAX1711 uses a fixed output overvoltage threshold of 2.25V nominal (2.29V maximum). This may be
high enough to cause damage to low voltage processors. In addition, if the overvoltage protection ever does
activate, a negative voltage can be delivered to the
processor as the output capacitor discharges and “rings”
4.7Ω
PGOOD
0.1µF
ON/OFF
47pF
330pF
LTC1735CGN-1
1
0.1µF
2
33k
3
47pF
4
5
47pF
VIN
4.5V TO 24V
1000pF 6
7
8
COSC
TG
BOOST
RUN/SS
ITH
SW
PGOOD
VIN
SENSE –
INTVCC
SENSE +
BG
VOSENSE
PGND
EXTVCC
SGND
10Ω
10Ω
16
FDS6680A
10µF
50V
CERAMIC
×2
0.22µF
15
14
1.2µH
13
12
11
+
4.7µF
10
9
VOUT
1.3V TO 1.5V
12A
0.004Ω
CMDSH-3
5V
INPUT
1µF
MBRS340T3
FDS6680A
×2
47pF
R1
10k
0.5%
47pF
R3
31.6k
1%
+
180µF
4V
SP
×4
2N7002
VSEL = 1: VOUT = 1.55V
VSEL = 0: VOUT = 1.3V
R2
15.8k
0.5%
GND
VID F02
Figure 2. LTC1735-1 Dynamic VID Implementation is Very Simple
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
3
Design Solutions 7
R2 =
R3 =
8k
VOUT (L) – 0.8V
8k
VOUT (H) – VOUT (L)
For a typical case of VOUT(L) = 1.3V and VOUT(H) = 1.55V,
R2 = 15.8k and R3 = 31.6k as shown in Figure 2. Resistors
R1 and R2 should be 0.5% tolerance, but resistor R3 may
be 1% tolerance.
VOUT
PGOOD
VSEL
VID F03
Figure 3. LTC1735-1 Voltage Programming
Response at IOUT = 100mA
POWER_GOOD Operation
Transient Response
Linear Technology’s OPTI-LOOP compensation allows
optimization of transient response, without limitations on
output capacitor ESR. Figure 5 compares the transient
response of the LTC1735-1 with OPTI-LOOP versus the
MAX1711. The MAX1711 transient response scope photograph was taken from the MAX1711 data sheet, and
uses 3 x 470µF Kemet T510 tantalum output capacitors.
VOUT
PGOOD
VSEL
VID F04
Figure 4. LTC1735-1 Voltage Programming
Response at IOUT = 9A
LTC1735-1
VOUT (50mV/DIV)
The PGOOD signal generated by the LTC1735-1 automatically goes low every time VOUT is changed by the voltage
select signal. Because the LTC1735-1’s internal power
good comparator operates on both low-to-high and highto-low transitions, no additional circuitry is required.
Figures 3 and 4 show the LTC1735-1 making both low-tohigh and high-to-low transitions. Figure 3 shows the
response with a 100mA load current, while Figure 4 uses
a 9A load current. Regardless of the load current, the
output voltage completes the voltage transition in less
than 30µs, easily meeting the 100µs specification requirement.
MAX1711
VIN = 15V
VOUT = 1.6V
IOUT = 30mA to 7A
COUT = 1500µF
Output Overvoltage Protection
The LTC1735-1 contains internal overvoltage protection
(OVP) circuitry that activates when the output rises to
7.5% above the programmed voltage. The OVP threshold
is not a fixed value, but moves as the output voltage is
changed. For example, if the output voltage is programmed
to 1.55V, the OVP will limit the output to 1.67V.
Demo Board
Linear Technology’s exclusive “soft-latch” circuitry provides robust OVP protection, but does not result in negative output voltages as with the MAX1711. Consequently,
no power Schottky diode is needed across the output.
The LTC1735-1 circuit shown in Figure 2 is available as a
demo board (part number DC267). Contact your LTC
representative to obtain a sample demo board for evaluation in your own laboratory.
4
Linear Technology Corporation
Time (10µs/DIV)
VID F05
Figure 5. LTC1735-1 vs MAX1711 Transient Response
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