Design Solutions 8 - 2-Step Power Conversion: Portable Power for the Future

Design Solutions 8
April 1999
2-Step Power Conversion:
Portable Power for the Future
As microprocessor operating voltages continue to
decrease, power conversion for CPU core power is
becoming a daunting challenge. A core power supply
must have fast transient response, good efficiency, and
low heat generation in the vicinity of the processor. These
factors will soon force a move away from 1-step power
conversion from battery or wall adapter to processor, to
2-step conversion where the CPU core power is obtained
from the 5V supply. While new to the portable arena,
distributed power systems using 5V as a bus voltage have
been used in large systems for many years. And although
it may not be absolutely necessary to adopt this architecture in portables today, the clock is ticking for the old
brute-force approach.
Let’s start with the biggest argument against 2-step
conversion: the perceived drop in efficiency and attendant heat generation in the 5V supply. Quick calculations
may give a false impression that efficiency significantly
decreases. It does not! Later in this paper we will show
accurate calculations of efficiency for 2-step power conversion based on actual demo board measurements that
show overall efficiency numbers equal to 1-step high
efficiency converters.
On the other hand, many benefits result from 2-step
conversion: more symmetrical transient response, lower
heat generation in the vicinity of the processor, and easy
modification for lower processor voltages in the future.
Peak currents taken from the battery are also reduced,
which leads to better battery efficiency that can often
improve upon the efficiency measured using laboratory
power supplies. Consequently, battery life in a real notebook computer may actually exceed that of 1-step
architectures.
, LTC and LT are registered trademarks of Linear Technology Corporation.
1-Step Approach for CPU Power
2-Step Approach for CPU Power
LDO
3 TO 4 SERIES
Li-Ion CELLS
(8.1V TO 16.8V)
12V
0.2A
5V
2.5A
(TOT 3A)
LTC1628
DUAL
OUTPUT
DC/DC
CONVERTER
12V
0.2A
LDO
3 TO 4 SERIES
Li-Ion CELLS
(8.1V TO 16.8V)
5V
2.5A
(TOT 6.2A)
LTC1628
DUAL
OUTPUT
DC/DC
CONVERTER
3.3V
3A
(TOT 3.5A)
LDO
LTC1735 OR
LTC1736 (VID)
DC/DC
CONVERTER
3.3V
3A
(TOT 5.3A)
2.5V
0.5A
LDO
1.XV
9A+
2.5V
0.5A
1.XV
9A
LTC1702 OR
LTC1703 (VID)
DC/DC
CONVERTER
LTC1624
DC/DC
CONVERTER
1.8V
3A
1.8V
3A
Figure 1. As CPU Core Voltages Continue to Drop, the Traditional 1-Step Approach Will Become Obsolete Due to Infinitesimal Duty
Cycles and Severely Skewed Transient Behavior. The 2-Step Approach Eliminates These Issues by Splitting the Conversion Into Two
Stages, Thus Yielding Faster Transient Response and Lower Heat Generation Near the CPU.
1
Design Solutions 8
The culprit lies in the duty cycle for a step-down switching
regulator, given by the ratio of VOUT to VIN. In 1-step power
conversion, the switch on-time must be very short because the step-down ratio is large. This gives a very fast
inductor current ramp-up and a much slower current
ramp-down. The inductance value must be large enough
to keep the current under control during the ramp-up. This
requires a larger inductance than for operation with a low
input voltage. Fast current rise and slow current decay
means that the transient response of the regulator is good
for load increases but poor for load decreases. The lower,
constant input voltage for a 2-step conversion process
not only yields a more symmetrical transient response,
but it completely eliminates the headaches associated
with optimizing loop dynamics over widely varying battery
and wall adapter voltages.
voltage converter can add up to five points of efficiency,
thereby minimizing heat generation near the processor.
Because the duty cycles are closer to 50% with 2-step
conversion, and there is less switching loss due to the
lower voltage swings, the switching frequency may also
be increased. This allows smaller, lower cost external
components to be used and further aids the transient
response.
The “Total Power Out” term must include not only the
power ultimately supplied to the CPU core, but also the
additional power supplied at each conversion from which
the CPU core voltage is derived. The “Total Power Lost”
term is the sum of the powers lost at every conversion and
is calculated from the respective operating efficiencies.
To minimize the high current PCB trace lengths, the core
supply must be located near the processor. With a 1-step
converter, the power lost is significantly higher than for
the second step of a 2-step conversion. Switching regulators for converting high input to low output voltages rarely
approach 90% efficiency. A properly designed 5V to core
Table 1 compares the power lost at each stage for 1-step
and 2-step CPU power conversions from a 12V input
voltage. Note that when all of the Figure 1 outputs are
considered, the overall efficiency is identical for 1-step
and 2-step conversion.
A common mistake when computing the efficiency of a
2-step power conversion system is to simply multiply the
efficiency of the first conversion by the efficiency of the
second conversion. While expedient, this method does
not reveal the overall system efficiency nor the distribution
of losses on the board. The correct approach to evaluate
2-step power conversion efficiency is to return to the
definition of efficiency:
Efficiency =
Total Power Out
• 100%
(Total Power Out + Total Power Lost)
Table 1. 1-Step vs 2-Step Operating Efficiency, VIN = 12V
1-Step for CPU Power
OUTPUT /ILOAD†
3.3V/1.5A
5V/1A
2-Step for CPU Power
DC/DC OUTPUT
POWER
EFFICIENCY*
POWER
LOST
OUTPUT/ILOAD†
DC/DC OUTPUT
POWER
EFFICIENCY*
POWER
LOST
5.61W
93%
0.42W
3.3V/1.5A
8.54W††
93%
0.64W
14.78W††
94%
0.94W
5W
95%
0.26W
5V/1A
1.6V/5.5A
8.8W
86%
1.43W
1.6V/5.5A
8.8W
90%
0.98W
1.8V/1.5A
2.7W
80%
0.68W
1.8V/1.5A
2.7W
92%
0.23W
2.5V/0.2A
0.5W
76%
0.16W
2.5V/0.2A
0.5W
76%
0.16W
CCFL
8W
90%
0.89W
CCFL
8W
90%
0.89W
TOTAL POWER
TO LOADS
30W
88.6%
3.84W
TOTAL POWER
TO LOADS
30W
88.6%
3.84W
Total Output Power
× 100%
Total Output Power + Total Power Lost
†Average output current; the 12V output is normally off.
††Including the additional power required for the 1.6V and 1.8V outputs.
*Efficiency =
2
Design Solutions 8
And what about the increased burden on the 5V regulator?
Table 1 reveals that while the power lost in the 5V supply
does increase with 2-step conversion, it is still less than
that lost in the 1-step CPU core supply. Furthermore,
power lost in the core and I/O supplies is in the worst
possible thermal environment for a notebook computer—
next to the processor. In this example, 2-step conversion
reduced the power dissipated in the vicinity of the CPU by
0.9W.
An additional concern sometimes voiced by power supply
designers is that there might be pitfalls from loading the
output of one switching regulator with the input of
another. In fact, the input current of a switching regulator
is directly proportional to its output voltage and current
and inversely proportional to its input voltage. This represents a benign load for an upstream switching regulator,
and cascaded switching regulators have been used in a
host of different power distribution applications over the
years. Today’s desktop computers, for example, use
precisely the same architecture as proposed here for
portables.
As time goes forward, microprocessor fabrication lithography will continue to shrink and force still lower CPU core
operating voltages and higher operating currents. 1.1V
supplies and 15A peak operating currents are already on
the horizon for portable systems. These demands will
render the traditional 1-step conversion approaches unworkable as a result of infinitesimal duty cycles and
severely skewed transient behavior.
Linear Technology has developed a third generation of
high efficiency DC/DC converters with unique features that
are ideal for implementing 2-step conversion strategies.
The LTC®1628 2-phase dual system power supply controller is an ideal solution for providing 3.3V and 5V
system power and the first conversion step in a 2-step
solution. For the highest first step conversion efficiency,
an LTC1625 No RSENSETM current mode controller can be
used to provide 5V at up to 10A. For the second step, the
LTC1702 and LTC1703 (VID option) 2-phase dual controllers convert 5V or 3.3V to CPU core and I/O supplies at
efficiencies of up to 95%. The LTC1702/LTC1703 require
no sense resistors and operate at 550kHz for fast transient
response and low external component cost.
No RSENSE is a trademark of Linear Technology Corporation.
LTC1703 Load Transient Response
VID
VOUT
50mV/DIV
1.5V
±7.5%
1.5V
VOUT
1.3V
100µs/DIV
2STEP F2
Figure 2. Operating at 550kHz from a 5V Supply Allows
the Use of a Much Smaller Inductor, Resulting in Excellent
Transient Response
100µs/DIV
2STEP F3
Figure 3. The LTC1703 Exhibits a Settling Time Less Than
100µs When VOUT is Changed Using the VID Control Inputs
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.
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Linear Technology Corporation
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com
E8
GND
E7
VOUT4
1.5V/12A
C53
1µF
RESET
SW1
5V ENABLE
FREQSET
STBYMD
FLTCPL
FCB
STDBY3 3V
STDBY 5V
3.3V ENABLE
+
C33
180µF
4V
R5
33k
C4
330pF
+
C5
56pF
+
R6
33k
C6
330pF
R18
1M
C34
180µF
4V
R2
1M
COREVENABLE
FAULT
FCB
1.8V ENABLE
D5
C35
180µF MBRD835L
4V
L3
0.8µH
ETQP6FOR8L
C7
56pF
R4
1M
C2
0.1µF
JP1
LATCH-OFF
DISABLE
8 7 6 5
Q8
IRF7811
3 2 1
TG1
RUN/SS2
TG2
SW2
BOOST2
BG2
PGND
INTVCC
EXTVCC
BG1
VIN
BOOST1
SW1
C37
0.22µF
15
16
17
18
19
20
21
22
23
24
25
26
27
28
C40
220pF
+
C13
4.7µF
C15
0.1µF
R11
10Ω
R19 18.7k 1%
C36
0.1µF
D6
MBR0520LT1
C10
0.1µF
C11
D2
0.1µF CMDSH-3
C12
1µF
D1 CMDSH-3
C14
0.1µF
50V
VID0
VID1
VID2
VID3
VID4
R21 1k
R20 C38 C39
100k 220pF 15pF
8 7 6 5
Q9
IRF7811
3 2 1
8 7 6 5
Q7
IRF7811
3 2 1
SENSE2+
SENSE2–
VOSNS2
ITH2
3.3VOUT
SGND
ITH1
FCB
STBYMD
FREQSET
VOSNS1
SENSE1–
SENSE1
+
FLTCPL
U1
LTC1628CG
RUNSS1
C8
1000pF
14
13
12
11
10
9
8
7
6
5
4
3
2
1
C9
1000pF
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Q1
IRF7805
IMAX2
B2
B3
B4
VCC
FB2
COMP2
RUN/SS2
FAULT
PGND
SW2
TG2
BG2
BOOST2
LOCATE LTC1703
NEAR PROCESSOR
B1
B0
SENSE
FB1
SGND
COMP1
RUN/SS1
FCB
IMAX1
SW1
TG1
BG1
BOOST1
PVCC
U2
LTC1703CG
C41
1µF
15
16
17
18
19
20
21
22
23
24
25
26
27
R22
24.9k
1%
28
Q5
IRF7807
5 6 7 8
1 2 3
C42
0.1µF
C43
0.22µF
D7
MBR0520LT1
L2
4.6µH
ETQP6F4R6H
C44
15pF
R23
100k
7 8
Q10A
NDS8926
1
C47
0.22µF
C48
1µF
+
+
2STEP F04
C46
180µF
4V
R24
8.06k
1%
C45
220pF
+
+
C29
0.1µF
50V
C21
180µF
4V
C19
150µF
6V
C16
0.1µF
50V
5 6
Q10B
NDS8926
3
L4
2.2µH
DO3316P-222
R13
0.010Ω
+
R12
0.005Ω
R9
0.007Ω
C17
150µF
6V
D4
MBRS130T3
D3
MBRD835L
L1
2.9µH
ETQP6F2R9L
Q2
IRF7805
Q4
IRF7807
5 6 7 8
1 2 3
5 6 7 8
1 2 3
5 6 7 8
Q3
IRF7805
1 2 3
1 2 3
5 6 7 8
C26
47pF
R16
20k
1%
R25
10.2k
1%
C49
150µF
6V
R26
1k
C50
2200pF
C51
150µF
6V
C28
100pF
C25
47pF
R15
20k
1%
R17
63.4k
1%
C23
100pF
R14
105k
1%
+
C22
22µF
50V
Figure 4. The LTC1628 and LTC1703 Are 2-Phase, Dual Output DC/DC Controllers That, When Combined,
Form a High Efficiency 4-Output Notebook PC Power Supply that Requires a Minimum of PC Board Area
+
C32
180µF
4V
C3
0.1µF
C1
0.1µF
R3
1M
R1
1M
R7
1M
R8
1M
+
J1
HEADER 10
R27
10Ω
C52
1µF
SGND GND
L5
0.33µH
DO3316P-331HC
C27
10µF
6.3V
C24
10µF
6.3V
E10
GND
E9
VOUT3
1.8V/3A
E6
GND
E5
VOUT2
3.3V/5A
E4
GND
E3
VOUT1
5V/4A
E2
GND
E1
VIN
7V TO 20V
Design Solutions 8
dsol8 LT/TP 0499 2K • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
 LINEAR TECHNOLOGY CORPORATION 1999
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