Dec 2003 Save Board Space with a High Efficiency Dual Synchronous, 600mA, 1.5MHz Step-Down DC/DC Regulator

DESIGN FEATURES
Save Board Space with a High
Efficiency Dual Synchronous, 600mA,
1.5MHz Step-Down DC/DC Regulator
by Damon Lee
Introduction
100
The ever shrinking nature of cell
phones, pagers, PDAs and other portable devices drives a corresponding
demand for smaller components.
One way to shrink DC/DC regulator
circuitry is to increase the switching
frequency of the regulator, thus allowing the use of smaller and cheaper
capacitors and inductors to complete
the circuit. Another way is to combine the switcher and MOSFETs in
one small, monolithic package. The
LTC3407 DC/DC regulator does
both.
The LTC3407 is a 10-lead, dual,
synchronous, step-down, current
mode, DC/DC regulator, intended
for low power applications. It operates
within a 2.5V to 5.5V input voltage
range and has a fixed 1.5MHz switching frequency, making it possible to
use tiny capacitors and inductors
that are under 1.2mm in height. The
LTC3407 is available in small DFN and
MSOP packages, allowing two 600mA
DC/DC Regulators to occupy less than
0.2 square inches of board real estate,
as shown in Figure 1.
The outputs of the LTC3407 are
independently adjustable from 0.6V
to 5V. For battery-powered applications that have input voltages above
95
VOUT2 = 2.5V
AT 600mA
C3
10µF
C5, 22pF
R4
887k
EFFICIENCY (%)
and below the output voltage, the
LTC3407 can be used in a single inductor, positive buck-boost converter
configuration. A built in 0.35Ω switch
allows up to 600mA of output current
at high efficiency. Internal compensation minimizes external components
and board space.
Efficiency is extremely important in
battery-powered applications, and the
LTC3407 keeps efficiency high with an
automatic, power saving Burst Mode®
operation, which reduces gate charge
losses at low load currents. With no
load, both converters together draw
only 40µA, and in shutdown, the device
draws less than 1µA, making it ideal
for low current applications.
VIN
RUN1
SW1
VFB2
VFB1
GND
10
100
LOAD CURRENT (mA)
1000
The LTC3407 uses a current-mode,
constant frequency architecture that
benefits noise sensitive applications.
Burst Mode is an efficient solution for
low current applications, but sometimes noise suppression is a higher
priority. To reduce noise problems,
a pulse-skipping mode is available,
which decreases the ripple noise at
low currents. Although not as efficient
as Burst Mode at low currents, pulseskipping mode still provides high
efficiency for moderate loads, as seen
in Figure 2. In dropout, the internal
P-channel MOSFET switch is turned
on continuously, thereby maximizing
the usable battery life.
95
RESET
C4, 22pF
R2
R1 887k
442k
2.5V
90
VOUT1 = 1.8V
AT 600mA
1.8V
85
80
75
70
C2
10µF
L1, L2: MURATA LQH32CN2R2M33
Figure 3. Dual step-down regulator provides 1.8V and 2.5V at 600mA.
VIN = 3.3V
Burst Mode OPERATION
NO LOAD ON OTHER CHANNEL
65
60
C1, C2, C3: TAIYO YUDEN JMK316BJ106ML
1
100
L1
2.2µH
SW2
Linear Technology Magazine • December 2003
VIN = 3.6V
VOUT = 1.8V
NO LOAD ON OTHER CHANNEL
Figure 2. Comparison of Burst Mode operation
and pulse skip mode. This particular data is for
the circuit shown in Figure 3.
R5
100k
POR
LTC3407
R3
280k
75
60
Figure 1. Two DC/DC regulators occupy
less than 0.2in2 of board space.
MODE/SYNC
L2
2.2µH
PULSE SKIP
80
65
EFFICIENCY (%)
RUN2
85
70
VIN = 2.5V
TO 5.5V
C1
10µF
Burst Mode
90
1
10
100
LOAD CURRENT (mA)
1000
Figure 4. Efficiencies of the circuit in Figure 3
19
DESIGN FEATURES
VIN = 2.8V
TO 4.2V
C1
10µF
RUN2
VIN
RUN1
MODE/SYNC
VOUT2 = 3.3V
AT 200mA
RESET
POR
LTC3407
L2
10µH
D1
L1
2.2µH
SW1
SW2
C6
47µF
R4
887k R3
196k
C3
10µF
C1, C2, C3: TAIYO YUDEN JMK316BJ106ML
C6: SANYO 6TPB47M
D1: PHILIPS PMEG2010
VFB1
VFB2
VOUT1 = 1.8V
AT 600mA
C4, 22pF
M1
+
when using only ceramic input and
output capacitors. The LTC3407 was
designed with ceramic capacitors in
mind and is internally compensated
to handle these difficult design considerations. High quality X5R or X7R
ceramic capacitors should be used to
minimize the temperature and voltage
coefficients.
Figure 3 shows a typical application
for the LTC3407 using only ceramic
capacitors. This circuit provides a
regulated 2.5V output and a regulated
1.8V output, both at up to 600mA,
from a 2.5V to 5.5V input. Efficiency
for the circuit is as high as 95% as
shown in Figure 4.
R5
100k
R2
R1 887k
442k
GND
C2
10µF
L1: MURATA LQH32CN2R2M33
L2: TOKO A914BYW-100M (D52LC SERIES)
M1: SILICONIX Si2302
Figure 5. Single inductor, positive buck-boost regulator
and a buck regulator with maximum height < 2mm
90
100
95
2.8V
3.6V
60
50
2.8V
2mm Height Li-Ion, Single
Inductor, Buck-Boost
Regulator and Buck Regulator
3.6V
90
4.2V
70
EFFICIENCY (%)
EFFICIENCY (%)
80
4.2V
85
Lithium-Ion batteries are popular in
many portable applications because
of their light weight and high energy
density, but the battery voltage ranges
from a fully charged 4.2V down to a
drained 2.8V. When a device requires
an output voltage that falls somewhere
in the middle of the Li-Ion operating
range, such as the 3.3V I/O supply,
a simple buck or boost converter does
not work. One solution is a single inductor, positive buck-boost regulator,
which allows the input voltage to vary
above and below the output voltage.
In Figure 5, regulator 2 is configured
as a single inductor, positive buckboost regulator to supply a constant
3.3V with 200mA–400mA of load current, depending on the battery voltage.
The circuit is well suited to portable
80
75
70
40
30
65
VOUT = 3.3V
Burst Mode OPERATION
1
10
100
LOAD CURRENT (mA)
1000
60
VOUT = 1.8V
Burst Mode OPERATION
1
10
100
LOAD CURRENT (mA)
1000
Figure 6. Efficiencies for the circuit in Figure 4
A Power-On Reset (POR) output is
available for microprocessor systems
to insure proper startups. Internal
overvoltage and undervoltage comparators on both outputs will pull the POR
output low if the output voltages are
not within ±8.5%. The POR output is
delayed by 262,144 clock cycles (about
175ms) after achieving regulation, but
will be pulled low immediately when
either ouput is out of regulation.
phase margin. Ceramic capacitors, on
the other hand, remain capacitive to
beyond 300kHz and usually resonate
with their ESL before the ESR becomes
effective. Also, inexpensive ceramic capacitors are prone to temperature and
voltage effects, requiring the designer
to check loop stability over the operating temperature range. For these
reasons, great care is usually needed
High Efficiency 2.5V and 1.8V
Step-Down DC/DC Regulator
with all Ceramic Capacitors
The low cost and low ESR of ceramic
capacitors make them a very attractive
choice for use in switching regulators.
In addition, ceramic capacitors have
a benign failure mechanism unlike
tantalum capacitors. Unfortunately,
the ESR is so low that it can cause
loop stability issues. A solid tantalum capacitor’s ESR generates a
loop zero at 5kHz–50kHz that can be
instrumental in giving acceptable loop
20
VIN = 2.8V
TO 4.2V
C1
2×4.7µF
RUN2
VIN
MODE/SYNC
VOUT2 = 2.5V
AT 600mA
C3
2×4.7µF
R4
887k
SW2
VFB1
C1, C2, C3: TDK C1608X5ROJ475M
GND
RESET
L1
2.2µH
SW1
VFB2
R3
280k
R5
100k
POR
LTC3407
L2
2.2µH
C5, 22pF
RUN1
C4, 22pF
R2
R1 604k
402k
VOUT1 = 1.5V
AT 600mA
C2
2×4.7µF
L1, L2: SUMIDA CDRH2D11-2.2µH
Figure 6. Low profile (1.2mm) Lithium-Ion dual step-down regulator
Linear Technology Magazine • December 2003
DESIGN FEATURES
100
100
95
95
2.8V
90
4.2V
85
EFFICIENCY (%)
EFFICIENCY (%)
90
3.6V
80
75
70
4.2V
85
3.6V
80
75
70
VOUT = 1.5V
Burst Mode OPERATION
NO LOAD ON OTHER CHANNEL
65
60
2.8V
1
10
100
LOAD CURRENT (mA)
VOUT = 2.5V
Burst Mode OPERATION
NO LOAD ON OTHER CHANNEL
65
60
1000
1
10
100
LOAD CURRENT (mA)
1000
Figure 7. Efficiencies for the circuit in Figure 6
applications because none of the components exceed 2mm in height.
The efficiency varies with the input
supply, due to resistive losses at high
currents and to switching losses at low
currents. As shown in Figure 6, the
typical efficiency across both battery
voltage and load current is about 75%
for the 3.3V output, and about 90%
for the 1.8V output.
Power over Ethernet, continued from page 18
These changes do add diodes to
the PD, but it’s definitely for the
best. Thanks to Auto-MDI-X, today’s
Ethernet switches and routers work
equally well with or without crossover
cables. Because the PD now must also
accept power of either polarity, 802.3af
Power over Ethernet will work with
either type of cable, minimizing end
user confusion.
The 802.3af standard now defines
specific peak currents for class 1 and
2 PDs. While all PDs can draw 400mA
of inrush current when they are first
powered, current consumption thereafter depends on the class of the PD.
height requirement, and to occupy
less than 0.2in2. The circuit provides
2.5V and 1.5V outputs, each with up
to 600mA of load current. Two 4.7µF
ceramic capacitors are used on each
supply, due to the lack of availability of
low profile 10µF ceramic capacitors.
The efficiency is slightly lower due to
the higher series resistance of the low
profile inductors. A peak efficiency of
91% for the 1.5V output and 95% for
the 2.5V output is achieved with these
components, as seen in Figure 7.
Low Profile, 1.2mm Height,
Lithium-Ion Dual Supply
Conclusion
In some applications, minimizing the
height of the circuit takes prime importance. New low profile capacitors and
inductors can be combined with the
already low profile 1.1mm maximum
height of the LTC3407’s 10-lead MSOP
package. Figure 6 shows a circuit designed to meet a 1.2mm maximum
The LTC3407 is a dual monolithic,
step-down regulator that switches at
1.5MHz, minimizing component costs
and board real estate requirements
for DC/DC regulators. The small size,
efficiency, low external component
count, and design flexibility of the
LTC3407 make it ideal for portable
applications.
Class 0 and 3 PDs must still never draw
more than 400mA peak and 350mA
continuous. Class 1 PDs cannot exceed
120mA peak and consume more than
3.84W continuously and Class 2 PDs
must limit their peak current to 210mA
and continuously use no more than
6.49W. These peak current limits do
not require a class 1 or 2 PD to include
active current limiting (unless it has
more than 180µF of input bypass) as
long as its switching regulator stays
below the average power limit and
filtering keeps peak currents below
the specified maximum. While the
standard does not specifically ad-
dress this point, a PD does not have
to stay below these peak limits if VPORT
suddenly increases. Furthermore, the
standard does not allow a PSE to apply
a lower active current limit, ILIM, based
on a PD’s class. A PSE, however, may
monitor the PD’s current and decide
to disconnect it if PD is not staying
within the limits of its class, i.e. the
PSE may reduce the ICUT threshold for
a class 1 or 2 PD.
In addition to the changes listed
above there are some minor parametric changes. Refer to the IEEE
802.3af-2003 standard for further
details.
+ OR –
FROM
SIGNAL
PAIRS
+ OR –
+
+ OR –
FROM
SPARE
PAIRS
–
LTC4257
GND
SMAJ58A
RCLASS SIGDISA
C1
RPULLUP 4.7µF
51k
100V
PWRGD
RCLASS
VIN
VOUT
+
VIN
SWITCHING
POWER SUPPLY
SHDN
GND
+
3.3V
TO LOGIC
–
+ OR –
Figure 4. Typical PD application
Linear Technology Magazine • December 2003
21
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