Design Solutions 9 - 2-Phase DC/DC Controller Reduces Capacitanceand Increases Battery Life in Portable Applications

Design Solutions 9
June 1999
2-Phase DC/DC Controller Reduces Capacitance
and Increases Battery Life in Portable Applications
The LTC®1628 dual high efficiency DC/DC controller brings
the considerable benefits of 2-phase operation to portable
applications for the first time. Notebook computers, PDAs,
handheld terminals and automotive electronics will all
benefit from the lower input filtering requirement, reduced
electromagnetic interference (EMI) and increased efficiency associated with 2-phase operation.
coming from the switches, greatly reducing the overlap
time where they add together (see Figure 1). The result is
a significant reduction in total RMS input current, which,
in turn, allows less expensive input capacitors to be used,
reduces shielding requirements for EMI and improves real
world operating efficiency.
Figure 2 compares the input waveforms for a representative 1-phase dual switching regulator to the new LTC1628
2-phase dual switching regulator. An actual measurement
of the RMS input current under these conditions shows
that 2-phase operation lowers the input current from
2.53ARMS to 1.55ARMS.
Why the need for 2-phase operation? Before the LTC1628,
constant-frequency dual switching regulators operated
both channels in phase (i.e., 1-phase operation). This
means that both switches turned on at the same time,
causing current pulses of up to twice the amplitude of
those for one regulator to be drawn from the input capacitor and battery. These large amplitude current pulses
increased the total RMS current flowing from the input
capacitor, requiring the use of more expensive input
capacitors and increasing both EMI and losses in the input
capacitor and battery.
Although this is an impressive reduction in itself, remember that the power losses are proportional to IRMS2,
meaning that the actual power wasted is reduced by a
factor of 2.66. The reduced input ripple voltage also means
less power lost in the input power path, which could
include batteries, switches, trace/connector resistances
and protection circuitry (see Figure 4). Improvements in
both conducted and radiated EMI also directly accrue as a
result of the reduced RMS input current and voltage.
With 2-phase operation, the two channels of the dual
switching regulator are operated 180 degrees out of
phase. This effectively interleaves the current pulses
, LTC and LT are registered trademarks of Linear Technology Corporation.
VIN
+
CIN
1-PHASE OPERATION
I5
I3.3
IIN
IIN = I5 + I3.3
I5
5V
OUTPUT
STAGE
2-PHASE OPERATION
I5
I3.3
DUAL
OUTPUT
SWITCHING
REGULATOR
3.3V
OUTPUT
STAGE
I3.3
IIN
Figure 1. With 2-Phase Operation, the Two Channels of the Dual Switching Regulator are Operated 180 Degrees Out of Phase. This
Effectively Interleaves the Current Pulses Coming from the Switches, Greatly Reducing the Overlap Time Where They Add Together
1
Design Solutions 9
Typical Single-Phase
LTC1628 2-Phase
5V SWITCH: 20V/DIV
3.3V SWITCH: 20V/DIV
INPUT CURRENT: 5A/DIV
INPUT VOLTAGE: 500mV/DIV
IIN(MEAS) = 2.53ARMS
IIN(MEAS) = 1.55ARMS
(a)
(b)
Figure 2. Actual Input Waveforms Comparing Single-Phase and 2-Phase Operation for Dual
Switching Regulators Converting 12V to 5V and 3.3V at 3A Each. The Reduced Input Ripple
with the LTC1628 2-Phase Regulator Allows Less Expensive Input Capacitors, Reduces
Shielding Requirements for EMI and Improves Efficiency
3.0
SINGLE-PHASE
DUAL CONTROLLER
INPUT RMS CURRENT (A)
2.5
2.0
1.5
2-PHASE
DUAL CONTROLLER
1.0
0.5
0
VO1 = 5V/3A
VO2 = 3.3V/3A
0
10
20
30
INPUT VOLTAGE (V)
40
Figure 3. The 2-Phase Solution Results in Significantly Reduced
RMS Ripple Current, Which in Turn, Allows Less Expensive Input
Capacitors and Increases Real World Operating Efficiency
Of course, the improvement afforded by 2-phase operation is a function of the dual switching regulator’s relative
duty cycles which, in turn, are dependent upon the input
voltage VIN (Duty Cycle = VOUT/VIN). Figure 3 shows how
the RMS input current varies for 1-phase and 2-phase
operation for 3.3V and 5V regulators over a wide input
voltage range.
It can be readily seen that the advantages of 2-phase
operation are not limited to a narrow operating range, but
in fact extend over a wide region. A good rule of thumb for
most applications is that 2-phase operation will reduce the
input capacitor requirement to that for just one channel
operating at maximum current and 50% duty cycle.
2
A final question: If 2-phase operation offers such an
advantage over 1-phase operation for dual switching
regulators, why hasn’t it been done before? The answer is
that, while simple in concept, it is hard to implement.
Constant-frequency, current mode switching regulators
require an oscillator-derived “slope compensation” signal
to allow stable operation of each regulator at over 50%
duty cycle. This signal is relatively easy to derive in
1-phase dual switching regulators, but required the development of a new and proprietary technique to allow
2-phase operation. In addition, isolation between the two
channels becomes more critical with 2-phase operation
because switch transitions in one channel could potentially disrupt the operation of the other channel.
The LTC1628 is proof that these hurdles have been surmounted. The new device offers unique advantages for the
ever expanding number of high efficiency power supplies
required in portable electronics.
LTC1628 Operation
The LTC1628 switching regulator performs high efficiency DC/DC voltage conversion while maintaining constant frequency over a wide range of load current, using a
2-phase current mode architecture. The 2-phase approach
results in 75% less power loss (and heat generated) in the
input source resistance because dissipated power is
proportional to the square of the RMS current as illustrated in Figure 4.
Design Solutions 9
What is completely ignored in bench testing of a supply is
the power loss internal to the battery due to switcher input
ripple current being forced to flow through the battery. By
reducing the amplitude of this ripple term, the internal
battery power losses are also reduced. Figure 5 shows the
measured impedance of a typical Li-Ion battery pack as a
function of frequency. Clearly, at the switching frequencies of today’s regulators the battery is a lot more lossy
than would be expected at DC. This picture starts to look
even worse when one considers that a switcher’s input
current waveform is trapezoidal, and therefore harmonic
rich. Much of the energy contained in this input ripple
current is at frequencies above 1MHz even though the
fundamental switching frequency is only 200kHz to 300kHz.
Because of these additional losses, efficiency measured
with a lab supply as the input source will produce results
that are somewhat optimistic. The only way to accurately
characterize the system efficiency is to measure actual run
time with various converter designs.
The input ripple frequency is also double the individual
controller’s switching frequency, further reducing the
10
ACTUAL
MEAS. 1-PHASE 2-PHASE
2.53A
1.55A
IRMS
IMPEDANCE (Ω)
VIN(RMS)
DC/DC
CIN
PCB/
CONN
RBATT
ESR
ESRBATT
PROTECTION
• CIN LOSS = (IRMS)2 • ESR
FOR ESR = 0.05Ω:
1-PHASE LOSS = 0.32W
2-PHASE LOSS = 0.12W
POWER SAVED = 0.2W
0.1
• HIDDEN LOSS = [VIN(RMS)]2/RBATT
0.01
0.01
CAN BE ≥ 0.5W WITH 1 PHASE
ESL
1
0.1
+
D3
M1
INTVCC
TG1
CB1, 0.1µF
BOOST1
BOOST2
M2
BG1
+
COUT1
47µF
6.3V
27pF
COUT1: EEFCDOJ470R
COUT2: EEFCDOG560R
R1
20k
1%
CC1
220pF
RC1
15k
D1, D2: MBRM140T3
D3, D4: CMDSH-3
D2
SENSE2 +
CS2
1000pF
–
VOSENSE1
R2
105k
1%
M4
BG2
EXTVCC
SENSE1
VOUT1
5V
5A
L2
6.3µH
PGND
SENSE1+
CS1
1000pF
CB2, 0.1µF
SW2
LTC1628
SGND
RSENSE1
0.01Ω
VIN
5.2V TO 28V
M3
TG2
SW1
D1
CIN
22µF
50V
CER
1µF
CER
D4
VIN
L1
6.3µH
1000 10000
Figure 5. Typical Li-Ion Battery Impedance
Figure 4. The Lower RMS Ripple Current of the
2-Phase Solution Increases System Efficiency
by Reducing Power Losses Inside the Battery
4.7µF
1
10
100
FREQUENCY (kHz)
ITH1
RUN/SS1
SENSE2
RSENSE2
0.01Ω
–
VOUT2
3.3V
5A
VOSENSE2
ITH2
RUN/SS2
CSS1
0.1µF
CSS2
0.1µF
CC2
220pF
RC2
15k
R3
20k
1%
R4
63.4k
1%
COUT2
56µF
4V
+
27pF
M1, M2, M3, M4: FDS6680A
Figure 6. LTC1628 High Efficiency Dual 5V/3V Step-Down Converter
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 9
input capacitance requirement. Reducing peak currents
and doubling the radiated frequency significantly reduces
EMI related problems. Figure 6 illustrates a typical 5V/3.3V
dual output step-down converter circuit using the LTC1628.
The internal oscillator frequency is set by the voltage
applied to the FREQSET pin. The frequency can be continuously varied over a 140kHz to 300kHz range by applying an
external voltage of 0V to 2.4V to the FREQSET pin.
High efficiency is made possible by selecting either of two
low current modes: 1) Burst ModeTM operation for maximum efficiency and 2) low noise, constant frequency,
burst disable mode for only slightly less efficiency. Constant frequency is desirable in applications requiring minimal electrical noise. Figure 7 shows the LTC1628 efficiency with Burst Mode operation enabled.
Burst Mode operation allows the output MOSFETs to
“sleep” between several PWM switching cycle periods of
normal MOSFET activity. Bursting starts at approximately
20% of maximum designed load current.
The burst disable mode allows heavily discontinuous, low
audio noise, constant-frequency operation down to approximately 1% of maximum designed load current. This
mode results in the elimination of switching frequency
subharmonics over 99% of the output load range. Switching cycles start to be dropped at approximately 1% of
maximum designed load current in order to maintain
proper output voltage.
The FCB input pin allows the selection of the low current
operating mode of both switching regulator controllers.
Burst disable mode is selected when the FCB pin is tied to
INTVCC. Tying the FCB pin to ground potential forces
controller 1 into PWM or forced continuous mode. Both
controllers can be forced into PWM mode by tying the FCB
pin to ground and tying the FLTCPL pin to INTVCC. In
forced continuous mode, the output MOSFETs are always
driven, regardless of output loading conditions.
As portable instruments continue to push for increased
functionality and longer run times, concerns regarding
board space and battery life become more dominant.
2-Phase operation allows the LTC1628 dual output DC/DC
controller to address both of these concerns by eliminating input filter capacitors and reducing the power lost in
the battery and the input power path. The LTC1628 can be
combined with the LTC1735/LTC1735-1 or LTC1736
DC/DC controllers to form a high efficiency multiple output
notebook PC power supply that uses a minimum of board
space. The LTC1628 can also be combined with the
LTC1702 or LTC1703 dual output DC/DC controllers to
form a similar notebook power supply using a 2-Step
system architecture with the CPU core and I/O supplies
derived from the 5V and 3.3V rails for faster transient
response and less heat generated near the CPU.
For a complete list of Linear Technology’s solutions for
Notebook PCs, visit www.linear-tech.com/ads/nbook.html.
Burst Mode is a trademark of Linear Technology Corporation.
100
5V OUTPUT
EFFICIENCY (%)
90
3.3V OUTPUT
80
70
60
VIN = 15V
50
0.001
0.1
0.01
1
OUTPUT CURRENT (A)
10
Figure 7. Efficiency for Both Outputs Exceeds 90%
Over a Wide Range of Load Currents
Figure 8. Additional Information Regarding Linear Technology’s
2-Phase DC/DC Controllers and Other Power Solutions for
Notebook PCs Can be Found at www.linear-tech.com/ads/
nbook.html
4
Linear Technology Corporation
dsol9a LT/TP 0699 2K REV A • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 ● FAX: (408) 434-0507 ● www.linear-tech.com
 LINEAR TECHNOLOGY CORPORATION 1999
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