DN1019 - Ultralow Power Converter’s Control Scheme Eliminates Audible Switching Noise

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Ultralow Power Converter’s Control Scheme Eliminates Audible
Switching Noise – Design Note 1019
James Yasuhara
Introduction
An essential component of a noise-free audio device is
a clean power supply, but few switching regulators can
operate at high efficiency while keeping the switching
frequency out of the audio band. The LTC ®3620 fills this
void. It is a high efficiency 15mA buck regulator with a
programmable minimum switching frequency, making it
possible to virtually eliminate audible switching noise. The
internal synchronous switches and low quiescent current
of this buck regulator provide the ability to maintain high
efficiency, while its small footprint makes it ideal for tiny,
low power audio applications.
Operation
To maximize efficiency, the LTC3620 employs a variable
frequency architecture that adjusts its switching frequency
to match the load current. Of course, a variable frequency
scheme is a potential noise problem for an audio device
if the switching frequency is allowed to enter the audio
spectrum. The LTC3620 avoids this problem by locking
its frequency to a user-set minimum value for low load
currents. While operating within this lock range, the
charge per switching cycle is adjusted to supply the
appropriate amount of current. Outside of this range, at
higher loads, the switching frequency increases and the
charge per pulse is at its maximum value. Similarly, at
extremely light loads, the charge per switching cycle is
at its minimum and the frequency decreases to maintain
regulation. The load current range for frequency locking
is determined by the inductor size and the programmed
minimum switching frequency and therefore, may be
tailored to the user’s specific application.
Most regulators employ burst or pulse-skipping modes
to maintain regulation at extremely light loads. These
modes, while good for efficiency, suffer from relatively
large output voltage ripple and indiscriminately switch
in the audio range. Because the LTC3620 reduces the
charge per pulse rather than the frequency, it produces
low output voltage ripple while maintaining good efficiency
and minimizing audio switching noise. Figure 1 shows a
typical application with output ripple and efficiency.
Selecting the Minimum Switching Frequency
An internal set 50kHz clock can be used by setting FMIN/
MODE to 0V. Alternatively, the user can apply the minimum
frequency clock to the FMIN/MODE pin. For applications
that are not sensitive to audio noise, the frequency clamp
can be defeated by setting the FMIN/MODE pin high. In
this mode the charge per switching cycle is constant and
there is no lock range, as shown in Figure 2.
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owners.
25
20
1µF
CER
LOBATB
SW
LTC3620
FMIN/MODE VFB
GND
22µH
22pF
432k
523k
VOUT
1.1V
1µF
CER
∆VOUT (mVP-P)
RUN
VIN = 5.5V
15
VIN = 3.6V
10
90
5
70
0
10
5
OUTPUT CURRENT (mA)
15
DN1019 F01b
2.0
60
50
40
30
10
0
2.5
80
20
DN1019 F01a
EFFICIENCY
0
0.1
1.5
VIN = 3V
FMIN/MODE = 0V
VOUT = 1.1V
VOUT = 1.8V
VOUT = 2.5V
1
LOAD CURRENT (mA)
1.0
LOSS
10
DN1019 F01c
Figure 1. Typical LTC3620 Application Circuit and Associated VOUT Ripple, Efficiency and Power Loss
11/10/1019
0.5
0
POWER LOSS (mW)
VIN
3.0
100
VOUT = 1.1V
FMIN/MODE = 0V
L = 22µH
EFFICIENCY (%)
VIN
2.9V TO 5.5V
During load step transients within the lock range, the frequency momentarily deviates from its locked value. At the
same time, the charge per switching cycle adjusts to bring
SWITCHING FREQUENCY (kHz)
1000
200kHz, EXTERNAL
FMIN/MODE = 0V
FMIN/MODE = VIN
100
TA = 25°C
VIN = 3.6V
VOUT = 1.1V
10
0.01
0.1
1
LOAD CURRENT (mA)
10 20
DN1019 F02
Figure 2. Switching Frequency Drops with Decreasing Load
But Stays Constant at a Preset Frequency. Only at Very Light
Loads Does the Switching Frequency Continue to Drop
reference, making low output voltages possible. A fixed
output version, the LTC3620-1, uses internal feedback
resistors to set a 1.1V output, reducing the number of
external components as well as the solution footprint.
Small Solution Size Using 0603 SMT Package
Inductor
For minimum size, Figure 4 shows the LTC3620-1 (1.1V
fixed output version) operating with only an inductor, an
input capacitor and an output capacitor. The feedback
resistors and compensation are internal to the IC, and the
use of the low battery detect pull-up resistor is optional. If
greater efficiency is required, other slightly larger inductors with lower ESRs may be used. One such inductor is
shown in Figure 5 along with an efficiency comparison
between the 0603 and the larger inductor.
VIN
2.9V TO 5.5V
the switching frequency back to the desired value. This
allows for better load regulation during load steps. During
transients outside of the lock range, only the frequency
adjusts. Figure 3 shows typical switching characteristics
and the spectral content of VOUT while in the lock range.
VIN
RUN
1µF
CER
VOUT
1.1V
15mA
1µF
CER
DN1019 F04
Figure 4. Schematic and Layout for LTC3620-1
Minimum Solution Size
100
52.5kHz
–81.4dBm
VIN = 3.6V
FMIN/MODE = 0V
95
90
–100
EFFICIENCY (%)
POWER RATIO (dBm)
LTC3620-1
GND
–60
–80
22µH
FMIN/MODE VFB
Additional Features
The LTC3620 is equipped with a soft-start circuit to ensure smooth start-up transients. A low battery detector
–120
–140
–160
LOBATB
SW
12.5kHz
8kHz/DIV
VOUT = 1.1V
VIN = 3.6V
FMIN/MODE = 0V
TA = 25°C
92.5kHz
DN1019 F03
Figure 3. Switching Waveform with 1mA Load and Its
Corresponding Spectral Content
provides a warning when the input voltage falls below
3.0V. An undervoltage lockout is also included to prevent
battery damage by turning off the part when the input
voltage falls below 2.8V.
Li-Ion to 1.1V/15mA
Figure 4 shows the LTC3620 converting a Li-Ion battery
or USB input (2.9V–5.5V) to a 1.1V output at 15mA. The
synchronous switches are internal to the part, which
increases efficiency and eliminates the need for external
Schottky diodes. The VFB voltage is servoed to a 0.6V
Data Sheet Download
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Linear Technology Corporation
85
80
75
VOUT = 2.5V LPS3010
VOUT = 1.8V LPS3010
VOUT = 1.2V LPS3010
VOUT = 2.5V CBMF 1608T
VOUT = 1.8V CBMF 1608T
VOUT = 1.2V CBMF 1608T
70
65
60
1
10
IOUT (mA)
100
DN1019 F05
Figure 5. Larger Inductor Layout and Inductor-Dependent
Efficiency Comparison
Conclusion
The LTC3620 combines two usually mutually exclusive
features—high efficiency at light loads and the ability to minimize switching noise in the audio frequency
range—making it ideal for low power audio applications.
A complete power solution consumes minimal space
because of the LTC3620’s 2mm × 2mm package and need
for very few external parts, all of which are available in
the extremely compact 0603 component size.
For applications help,
call (408) 432-1900, Ext. 3725
dn1019f LT/TP 1110 • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
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●
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 LINEAR TECHNOLOGY CORPORATION 2010
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