May 2002 Small 1.25A Step-Down Regulator Switches at 4MHz for Space-Sensitive Applications

DESIGN FEATURES
Small 1.25A Step-Down Regulator
Switches at 4MHz for Space-Sensitive
Applications
by Damon Lee
Introduction
Cell phones, pagers, PDAs and other
portable devices are shrinking, and
as they shrink, the demand for smaller
components grows. The ubiquitous
switching regulator, which solves the
problem of creating a constant voltage from inconstant batteries, is not
exempt from the demand to become
smaller. One way to shrink regulator
circuitry is to increase the switching
frequency of the regulator, allowing
the use of smaller and less costly
capacitors and inductors to complete
the circuit. Another way is to shrink
the switcher itself by putting the
switcher and MOSFETs in a small
monolithic package. The LTC3411
DC/DC converter does both.
The LTC3411 is a 10-lead MSOP,
synchronous, step-down, current
mode, DC/DC converter, intended for
medium power applications. It operates within a 2.5V to 5.5V input voltage
range and switches at up to 4MHz,
making it possible to use tiny capacitors and inductors that are under
2mm in height. By using the LTC3411
in a small MS10 package, a complete
DC/DC converter can consume less
than 0.3 square inches of board real
estate, as shown in Figure 1.
The output of the LTC3411 is adjustable from 0.8V to 5V. For
Figure 1. A complete DC/DC converter can
take less than 0.3in2 of board space.
battery-powered applications that
have input voltages above and below
the output, the LTC3411 can also be
used in a single inductor, positive
Buck-Boost converter configuration.
A built-in 0.11Ω switch allows up to
1.25A of output current at high efficiency. OPTI-LOOP® compensation
allows the transient response to be
optimized over a wide range of loads
and output capacitors.
Efficiency takes on grave importance in battery-powered applications,
and the LTC3411 keeps efficiency
high. Automatic, power saving Burst
Mode® operation reduces gate charge
losses at low load currents. With no
load, the converter draws only 62µA,
and in shutdown, it draws less than
1µA, making it ideal for low current
applications.
The LTC3411 uses a current mode,
constant frequency architecture that
benefits noise sensitive applications.
Burst Mode operation is an efficient
solution for low load current applications, but sometimes noise
suppression takes on more importance than efficiency, especially in
telecommunication devices. To reduce
noise problems, the LTC3411 provides a pulse-skipping mode and a
forced-continuous mode. These
modes decrease the ripple noise and
improve noise filterability. Although
not as efficient as Burst Mode
operation at low load currents, pulseskipping mode and forced continuous
mode can still provide high efficiency
for moderate loads (see Figure 3). In
dropout, the internal P-channel MOSFET switch is turned on continuously,
thereby maximizing the usable battery life.
A High Efficiency 2.5V StepDown DC/DC Converter with
all Ceramic Capacitors
The low cost and low ESR of ceramic
capacitors make them a very attrac100
BURST MODE
OPERATION
95
100k
VIN
2.5V TO 5.5V
90
22µF
PGOOD
BURST MODE
PULSE SKIPPING MODE
SVIN
SYNC/MODE
SGND
ITH
LTC3411
13k
2.2µH
VOUT
2.5V/1.25A
SW
22µF
887k
SHDN/RT
1000pF
PVIN
VFB
EFFICIENCY (%)
PGOOD
85
PULSE SKIP
75
70
VIN = 3.3V
VOUT = 2.5V
CIRCUIT OF FIGURE 6
65
PGND
324k
C1, C2: TAIYO YUDEN JMK325BJ226MM
L1: TOKO A914BYW-2R2M (D52LC SERIES)
412k
60
1
100
1000
10
LOAD CURRENT (mA)
10000
3411 G04
3411 F01
Figure 2. Step-down 2.5V/1.25A regulator
Linear Technology Magazine • May 2002
FORCE CONTINUOUS
80
Figure 3. Efficiencies for the circuit
shown in Figure 2, under different
operating modes
9
DESIGN FEATURES
VOUT
10mV/
DIV
VOUT
10mV/
DIV
VOUT
10mV/
DIV
IL1
100mA/
DIV
IL1
100mA/
DIV
IL1
100mA/
DIV
2µs/DIV
VIN = 3.3V
VOUT = 2.5V
ILOAD = 50mA
CIRCUIT OF FIGURE 2
2µs/DIV
VIN = 3.3V
VOUT = 2.5V
ILOAD = 50mA
CIRCUIT OF FIGURE 2
2µs/DIV
VIN = 3.3V
VOUT = 2.5V
ILOAD = 50mA
CIRCUIT OF FIGURE 2
Figure 4. Burst Mode operation
Figure 5. Pulse skipping mode
Figure 6. Forced continuous mode
tive choice for use in switching regulators. Unfortunately, the ESR is so
low that it can cause loop stability
problems. Solid tantalum capacitor
ESR generates a loop zero at 5KHz to
50KHz that is instrumental in giving
acceptable loop phase margin. Ceramic capacitors remain capacitive to
beyond 300KHz and usually resonate
with their ESL before ESR becomes
effective. Also, ceramic caps are prone
to temperature effects, requiring the
designer to check loop stability over
the operating temperature range. For
these reasons, great care must be
taken when using only ceramic input
and output capacitors. The LTC3411
helps solve loop stability problems
with its OPTI-LOOP phase compensation adjustment, allowing the use
of ceramic capacitors. For details,
and a process for optimizing compen-
C1
22µF
VIN
2.5V
TO 5V
100k
R2
887k
C7
10pF
PVIN
PGND
SVIN
SW
LTC3411
PGOOD
R1
280k
sation components, see Linear Technology Application Note 76.
A typical application for the
LTC3411 is a 2.5V step-down converter using only ceramic capacitors,
as shown in Figure 2. This circuit
provides a regulated 2.5V output, at
up to 1.25A, from a 2.5V to 5.5V
input. Efficiency for the circuit is as
high as 95% for a 3.3V input as
shown in Figure 3.
Although the LTC3411 is capable
of operating at 4MHz, the frequency
in this application is set for 1MHz by
R4 to improve the efficiency. Also, the
availability of capacitors and inductors capable of 4MHz operation is
limited.
Figures 3 through 6 show the tradeoff between noise and efficiency for
the different modes for the circuit.
Figure 3 shows the efficiencies, while
Figures 4, 5 and 6 show the output
L1
3.3µH
SGND
VIN
PGOOD
VFB
SYNC/MODE
ITH
SHDN/RT
R3
13k
C3
1000pF
D1
M1
C2
22µF
×2
+
VOUT
3.3V/
400mA
C4
47µF
BM
Single Cell Li-Ion to 3.3V
DC/DC Converter
R4
324k
3411 TA02
C1, C2: TAIYO YUDEN JMK325BJ226MM
C4: SANYO POSCAP 6TPA47M
D1: ON SEMICONDUCTOR MBRM120L
L1: TOKO A915AY-3R3M (D53LC SERIES)
M1: SILICONIX Si2302DS
voltage and inductor current for different operating modes.
Burst Mode operation is the most
efficient for low current loads, but it is
also generates the most complicated
noise patterns. Figure 4 shows how
Burst Mode operation produces a
single pulse or a group of pulses that
are repeated periodically. By running
cycles in periodic bursts, the switching losses—dominated by the gate
charge losses of the power MOSFET—
are minimized. Figure 5 shows how in
pulse skipping mode, the LTC3411
continues to switch at a constant
frequency down to very low currents,
minimizing the ripple voltage and
ripple current. Finally, Figure 6 shows
how in forced continuous mode, the
inductor current is continuously
cycled, creating a constant ripple at
all output currents. Forced continuous mode is particularly useful in
noise-sensitive telecom applications
since the constant frequency noise is
easy to filter. Another advantage of
this mode is that the regulator is
capable of both sourcing and sinking
current into a load. This mode is
enabled by forcing the mode pin to
half of VIN.
(408) 573-4150
(619) 661-6835
(602) 244-6600
(847) 699-3430
(800) 554-5565
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 2.5V. When a device requires
a voltage output that falls somewhere
Figure 7a. Single inductor, positive, buck-boost converter
10
Linear Technology Magazine • May 2002
DESIGN FEATURES
85
VIN = 4V
VIN = 2.5V
VIN = 3V
VIN = 3.5V
75
70
65
60
fO = 1MHz
55
10
100k
LOAD CURRENT (mA)
1000
It’s Only 2mm High: 2MHz,
Li-Ion to 1.8V Converter
The LTC3411 is a monolithic, stepdown regulator that switches at high
frequencies, lowering component
costs and board real estate requirements of DC/DC converters. Although
the LTC3411 is designed for basic
buck applications, its architecture is
versatile enough to produce an
efficient single inductor, positive buckboost converter, due in part to its
power saving Burst Mode operation
and the OPTI-LOOP compensation
feature.
3411 TA03
Figure 7b. Efficiency for the
circuit in Figure 7a
the middle of the Li-Ion operating
range, say 3.3V, a simple buck or
boost converter does not work. One
solution is a single inductor, positive
buck-boost converter, which allows
the input voltage to vary above and
below the output voltage.
In Figure 7, the LTC3411 is used in
a single Inductor, positive buck-boost
configuration to supply a constant
VIN
2.5V
TO 4.2V
square inches. In the spirit of keeping
things as small as possible, this circuit uses tantalum capacitors for their
relatively small size when compared
to equivalent ceramic capacitors.
The downside to running at a higher
frequency is that efficiency suffers a
little due to higher switching losses.
The efficiency for this particular circuit peaks at 93% with VIN = 2.5V.
In some applications, minimizing the
height of the circuit takes prime importance. One method of lowering the
DC/DC converter height is to run the
LTC3411 at the 2MHz switching frequency, which allows one to use
low-profile inductors and capacitors.
Figure 8 shows a circuit built with low
profile components to produce a 2mm
tall (nominal), 1.8V step-down converter that occupies less than 0.3
Conclusion
100
95
C6
1µF
+
C1
33µF
R5
100k
PVIN
PGOOD
SVIN
SW
LTC3411
L1
1µH
C4 22pF
SYNC/MODE
ITH
C7
47pF
R3
15k
C3
470pF
SGND PGND
+
C2
33µF
C5
1µF
VOUT
1.8V
AT 1.25A
VFB
SHDN/RT
R4
154k
2.5V
90
PGOOD
R1
698k
R2
887k
85
EFFICIENCY (%)
EFFICIENCY (%)
80
3.3V with 400–600mA of load current, depending on the battery voltage.
This circuit is well suited to portable
applications because none of the components exceed 3mm in height.
The efficiency varies with the input
supply, due to resistive losses at high
currents and to switching losses at
low currents. The typical efficiency
across both battery voltage and load
current is about 78%.
3.6V
80
75
70
4.2V
65
60
3411 TA04
VOUT = 1.8V
fO = 2MHz
55
50
C1, C2: AVX TPSB336K006R0600
(207) 282-5111
C4, C5: TAIYO YUDEN LMK212BJ105MG (408) 573-4150
L1: COILCRAFT DO1606T-102
(847) 639-6400
Figure 8a. Tiny 1.8V/1.25A step-down converter uses low profile components
1
100
1000
10
LOAD CURRENT (mA)
10000
3411 TA05
Figure 8b. Efficiency for the
circuit in Figure 8a
LT3420, continued from page 8
the target output voltage is reached,
DONE goes high while CHARGE is
also high. The output of A1 goes high,
which forces CHARGE high regardless of the PWM signal. The part is
now in the Refresh mode. Once the
refresh period is over, the DONE pin
goes low, allowing the PWM signal to
drive the CHARGE pin once again.
This function can be easily implemented in a microcontroller. Figure 9
shows the input current for the cirLinear Technology Magazine • May 2002
cuit of Figure 1 as the duty cycle of the
PWM signal is varied.
Conclusion
The LT3420 provides a highly efficient and integrated solution for
charging photoflash capacitors. Many
important features are incorporated
into the device, including automatic
refresh, tightly controlled currents
and an integrated power switch, thus
reducing external parts count. The
LT3420 comes in a small, low profile,
MSOP-10 package, making for a complete solution that takes significantly
less PC board space than more traditional methods. Perhaps most
importantly, the LT3420 provides a
simple solution to a complicated high
voltage problem, freeing camera designers to spend time on other
important matters, like increasing the
pixel count or adding new camera
features.
11