L DESIGN IDEAS
Save Board Space with a High
Efficiency Dual Synchronous,
400mA/800mA, 2.25MHz
Step-Down DC/DC Regulator by Damon Lee
Introduction
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
LTC3548 DC/DC regulator does
both.
The LTC3548 is a 10-lead MSOP/
DFN, 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 2.25MHz
switching frequency, making it possible to use low-profile capacitors and
inductors that are only 1mm high. The
LTC3548 is the latest in the LTC3407
and LTC3407-2 family of dual regulators and features an improved Burst
Mode ripple and two outputs of 400mA
and 800mA. It is available in small
MSOP and DFN packages, allowing
Figure 1. Two DC/DC regulators occupy
less than 0.2in2 of board space
two DC/DC Regulators to occupy less
than 0.2 square inches of board real
estate, as shown in Figure 1.
The outputs of the LTC3548 are
independently adjustable from 0.6V
to 5V. For battery-powered applications that have input voltages above
and below the output voltage, the
LTC3548 can be used in a single
inductor, positive buck-boost converter configuration (see data sheet
for details). Two built in 0.35Ω switch
provides high efficiency at maximum
output current. Internal compensation
minimizes external components and
board space.
120
VOUT1
100mV/
DIV
100
LTC3407-2
95
100
IL1
500mA/
DIV
IL1
500mA/
DIV
80
EFFICIENCY (%)
VOUT RIPPLE (mV)
90
VOUT1
100mV/
DIV
60
LTC3548
40
Burst Mode OPERATION
85
80
PULSE SKIP MODE
75
70
VIN = 3.3V
VOUT1 = 1.8V
LOAD = 100mA
4µs/DIV
Figure 2. Comparison of ripple for Burst Mode
operation of the LTC3548 and LTC3407-2.
44
Efficiency is extremely important in
battery-powered applications, and the
LTC3548 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. The
LTC3548 features an improved Burst
Mode ripple voltage, which is only
about one third of the ripple for the
LTC3407 and LTC3407-2, as shown
in Figure 2 and Figure 3.
The LTC3548 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 4. In dropout, the internal
20
VIN = 3.6V, VOUT = 1.8V
NO LOAD ON OTHER CHANNEL
CHANNEL 1; CIRCUIT OF FIGURE 3
65
VIN = 3.6V
0
0.1
1
10
100
LOAD CURRENT (mA)
1000
Figure 3. Burst Mode operation
output voltage ripple vs load current
for the LTC3548 and LTC3407-2.
60
1
10
100
LOAD CURRENT (mA)
1000
Figure 4. Efficiency of Burst Mode
operation and pulse skip mode
Linear Technology Magazine • March 2006
DESIGN IDEAS L
P-channel MOSFET switch is turned
on continuously, thereby maximizing
the usable battery life.
A Power-On Reset 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 output is out
of regulation.
A 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
phase margin. Ceramic capacitors, on
the other hand, remain capacitive to
beyond 300kHz and usually resonate
Digi-Tune Filters, continued from page 43
can be tuned to 110kHz (maximum f0
= 800kHz/[0.707 • 10]).
A Tunable Bandpass Filter
The –3dB bandwidth of a second order
filter is equal to the center frequency
(fCENTER) divided by the Q value (bandwidth = fCENTER/Q). The sensitivity of
the second order bandpass filter to
the tolerance of the integrator’s RC
values is proportional to the filter’s
Q. Typically with a Q ≤ 4, using a
±1% R and a ±5% C for the filter’s two
integrators is practical for a second
bandpass filter. The sensentivity of the
second order bandpass filter with Q >
4 increases rapidly for each unit of Q
increase and the filter’s two integrators should use ±1% RC components.
Linear Technology Magazine • March 2006
VIN = 2.5V*
TO 5.5V
C1
10µF
RUN2 VIN
MODE/SYNC
VOUT2 = 2.5V*
AT 400mA
C3
4.7µF
L2
4.7µH
C5, 68pF
R4
887k
RUN1
POR
LTC3548
SW2
SW1
VFB1
VFB2
R3
280k
C1, C2, C3: TAIYO YUDEN JMK212BJ106MG
C3: TAIYO YUDEN JMK212BJ475MG
GND
R5
100k
POWER-ON
RESET
L1
2.2µH
C4, 33pF
R2
R1 604k
301k
VOUT1 = 1.8V
AT 800mA
C2
10µF
L1: MURATA LQH32CN2R2M11
L2: MURATA LQH32CN4R7M23
*VOUT CONNECTED TO VIN FOR VIN ≤ 2.8V (DROPOUT)
Figure 5. Dual output step-down application yields 1.8V at 800mA and 2.5V at 400mA.
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 when using only ceramic input
and output capacitors. The LTC3548
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 5 shows a typical application
for the LTC3548 using only ceramic
capacitors. This circuit provides a
regulated 2.5V output and a regulated
1.8V output, at up to 400mA and
800mA, from a 2.5V to 5.5V input.
Figure 4 shows the bandpass filter of
Figure 1 tuned from 2kHz to 16kHz
using a 2kHz integrator frequency (R
= 205k, ±1% and C = 390pF, ±5%) and
an LTC6912-2 with gain settings 1, 2,
4, and 8. The tuned center frequencies responses of Figure 4 are 2.73%
lower than the design values of 2kHz,
4kHz, 8kHz and 16kHz and equal to
the error of the circuit’s RC values of
the two integrators (measured values
of aproximatelly 206k for each R and
403pF for each C). The gain error at
16kHz is due to the filter’s f0 frequency
approaching the maximum f0 for a Q
= 4 and a PGA gain equal to 8 (maximum f0 = 25kHz = 800kHz/{4 • 8]). The
maximum f0 frequency is a function
of the gain-bandwidth product of the
LTC6912-X op amps.
Conclusion
The LTC3548 is a dual monolithic,
step-down regulator that switches at
2.25MHz, 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
LTC3548 make it ideal for portable
applications. L
Other Filter Options
Figure 5 shows an example of a
second order notch filter. The notch
filter’s integrator frequency is 500Hz
(1/[2π • 316kΩ • 1000pF]) and with
PGA gains 1, 2, 4 and 8 the notch
frequency is tuned to 500Hz, 1kHz,
2kHz and 4kHz respectively. Any of
the filters discussed above can be
made into SPI-tunable fourth order
filters by cascading two second order
circuits. L
Notes
1 SPI is a synchronous communication protocol using
a 3-wire interface between a microprocessor and
a peripheral device
45