2.7V to 38V VIN Range, Low Noise, 250mA Buck-Boost Charge Pump Converter

design features
2.7V to 38V VIN Range, Low Noise, 250mA Buck-Boost
Charge Pump Converter
George H. Barbehenn
The LTC3245 is a buck-boost regulator that dispenses with the traditional inductor,
and instead uses a capacitor charge pump. The LTC3245’s input voltage range is
2.7V to 38V, and it can be used without a feedback divider to produce one of two
fixed output voltages, 3.3V and 5V, or programmed via a feedback divider to any
output voltage from 2.5V to 5.5V. Maximum output current is 250mA. As a result of
its buck-boost topology, the LTC3245 is capable of regulating a voltage above or
below the input voltage, allowing it to satisfy automobile cold crank requirements.
The LTC3245 achieves efficiency of
80% when delivering 5V, 100m A from
a 12V source, significantly higher efficiency than an LDO, making it possible
to avoid the space and cost requirements of an LDO with a heat sink. The
LTC3245 is available in an exposed
pad MSOP12 or 3mm × 4mm DFN12.
Because N and M are integers, a straight
charge pump cannot be used to produce an arbitrary output. Instead the
controller modifies VIN to produce VIN′,
which is then fed to the charge pump.
The charge pump can operate in one of
three modes, buck, LDO or boost, resulting in ½VIN′, VIN′ or 2VIN′, respectively.
CHARGE PUMP OPERATION
By properly controlling both VIN′ and
the operating mode of the charge pump
any arbitrary voltage can be achieved.
When operating in buck mode, the
input current is approximately half
that of an equivalent LDO, offering a
significant efficiency improvement.
Figure 3 shows a simplified block diagram
of the LTC3245 converter. Charge pumps
can operate as N/M × VIN converter, where
N and M are integers. ½, 1, and 2 are the
simplest forms and only require one flying
capacitor. Higher order N and M require
more flying capacitors and switches.
1µF
SEL2
BURST
SEL1
80
C–
PGOOD
GND
350
EFFICIENCY
70
VOUT
OUTS/ADJ
400
VIN = 12V
500k
VOUT = 5V
IOUT UP TO 250mA
10µF
300
60
250
50
200
40
PLOSS
30
20
10
0.1
150
PLOSS (mW)
VIN
90
Figure 2. Efficiency of the
converter in Figure 1
LTC3245
VIN = 2.7V TO 38V
To decouple the LTC3245, place a
3.3µ F ~ 10µ F MLCC capacitor as close to
the VIN pin as possible. One way to move it
closer is to limit the voltage rating on the
capacitor, which helps minimize the size
of the cap, and the smaller it is, the nearer
the VIN pin it can be placed. For instance,
although LTC3245 is rated to operate up to
38V input, for an automotive supply, an
MLCC with 16V rating should be sufficient.
EFFICIENCY (%)
C+
The LTC3245 charges the flying capacitor each switching cycle, so VIN must be
sufficiently decoupled to minimize EMI.
A decoupling capacitor with a short,
low inductance supply connection, but
a high inductance ground connection,
1µF
Figure 1. A 5V output
buck-boost converter
INPUT RIPPLE AND EMI
100
50
1
10
IOUT (mA)
100
0
1000
January 2014 : LT Journal of Analog Innovation | 9
The LTC3245 achieves efficiency of 80% when delivering
5V, 100mA from a 12V source, significantly higher efficiency
than an LDO, making it possible to avoid the space
and cost requirements of an LDO with a heat sink.
C+
VIN′
C–
VOUT
CHARGE PUMP
BUCK MODE: VOUT = ½VIN′
LDO MODE: VOUT = VIN′
BOOST MODE: VOUT = 2VIN′
is not very effective. The ideal situation
is when the supply connection is short
and wide, and the ground connection
is an area fill with a very wide connection to the exposed pad on the LTC3245.
when properly decoupled, the LTC3245
does not present any issue when striving to meet government regulations
for radiated or conducted emissions.
The assumption is made that VIN does not
have a very long connection back to a
low impedance supply. If the input supply
is high impedance, or the connection to
the input supply is longer than 5cm, it is
recommended that the supply be decoupled with additional bulk capacitance, as
needed. In many cases, 33µ F is adequate.
The detail of the charge pump block
(Figure 3) suggests that the flying capacitor is only involved in the charge pump
(a)
10 | January 2014 : LT Journal of Analog Innovation
(b)
60
110
50
100
90
40
30
CISPR 22 CLASS B LIMIT
20
10
0
–10
LTC3245
VIN = 14V
ILOAD =0.25A
–20
–30
80
CISPR 25 CLASS 3
BROADBAND LIMIT
70
60
50
40
30
20
LTC3245
VIN = 13V
VOUT = 5V
10
0 100 200 300 400 500 600 700 800 900 1000
FREQUENCY (MHz)
Figure 4 shows measured radiated and
conducted signatures of the LTC3245,
tested in a microchamber in accordance with CISPR25. As can be seen here,
The minimum capacitance of the flying
capacitor must be 0.4µ F. Since polarized capacitors are not allowed, the
most appropriate capacitor is MLCC.
MLCC capacitors with enough capacitance to meet the 0.4µ F are likely Class
II dielectric capacitors, with strong voltage
Figure 4. Radiated (a) and conducted (b) emissions
AMPLITUDE (dBµV/m)
The LTC3245 can be optimized for light
load efficiency or low output ripple by
choosing high efficiency Burst Mode®
operation or low noise mode. Burst
Mode operation features low quiescent
current and hence higher efficiency at
low load currents. Low noise mode
trades off light load efficiency for
lower output ripple at light loads.
CHOOSING THE FLYING CAPACITOR
The flying capacitor cannot be polarized, such as an electrolytic or tantalum
capacitor. The voltage rating of the flying capacitor should be about 1V more
than the output voltage, such as using a
6.3V flying capacitor for a 5V output.
AMPLITUDE (dBµV/m)
LTC3245
VIN
itself. However, the flying capacitor is
also involved in the variable attenuator
that generates VIN′. Consequently, the
capacitor should not be chosen based
on straightforward calculation, but
instead by observing a few constraints.
CFLY
Figure 3. Detail of the charge pump block
DETECTOR = PEAK HOLD
RBW = 120kHz
VBW = 300kHz
SWEEP TIME = 680ms, ≥10 SWEEPS
# OF POINTS = 501
0
0.1
1
10
FREQUENCY (MHz)
LOAD = 240Ω WITH33µF ELECTROLYTIC
& CERAMIC INPUT CAP
DETECTOR = PEAK
RBW = 9kHz
VBW = 30kHz
SWEEP TIME = 3.7ms, ≥10 SWEEPS
# OF POINTS = 501
100
design features
The OUTS/ADJ pin is used either for sensing VOUT for fixed 3.3V and 5V outputs
or as the feedback pin for an adjustable output voltage. It is connected directly to
the output when using the fixed values. An adjustable output can be set anywhere
between 2.5V and 5V through the choice of suitable feedback resistors.
coefficients on their capacitance. The
voltage coefficient of the capacitance
is a function of the maximum voltage, so a capacitor of maximum voltage of 16V operating at 5V will have
much more in-circuit capacitance than
a 6.3V capacitor of the same nominal
capacitance and size, operating at 5V.
So, a 0.47µ F, 6.3V, Class II dielectric
capacitor operating at 5V will likely not
meet the minimum capacitance, while
a 0.47µ, 50V, Class II dielectric capacitor likely will. A capacitor such as the
TDK C1005X5R1C105K 1µ F, 16V, 0402
is suitable for most applications.
OUTPUT CAPACITOR
The choice of output capacitor value
is a trade-off between ripple and step
response. As the output capacitance is
increased, the ripple decreases but the step
response is also increasingly overdamped.
The required voltage rating of the output capacitor is the output voltage of the
regulator, so a 6.3V capacitor would suffice
for a 5V output. Nevertheless, as discussed
above, Class II dielectric capacitors lose
more than half their nominal capacitance
at their rated voltage. Consequently, it
may be necessary to choose a larger capacitor when operating close to the rated voltage of the capacitor, to minimize ripple.
A good compromise between ripple and
response is a capacitor with a capacitance, at bias, of 10× ~ 20× the flying
capacitor. This means 10µ F to 20µ F for
SHUTDOWN
the recommended flying capacitor value
of 1µ F. Since Class II capacitors lose
a little more than half their capacitance at rated voltage, this indicates a
47µ F nominal capacitance capacitor.
The LTC3245 can also be placed in
shutdown to reduce the quiescent current to just 4µ A. Pull both SEL1 and
SEL2 low to shutdown the LTC3245.
ADJUSTABLE OUTPUT
PGOOD
Besides the two fixed output voltage values
of 3.3V and 5V, it is possible to program
the output voltage of the LTC3245 using
feedback resistor as shown in Figure 5.
PGOOD is an active high, open drain
signal that indicates the output of the
LTC3245 is in regulation. The threshold
for the PGOOD indication is 90% of the
desired feedback or sense voltage.
Adjustable output mode is achieved
by setting SEL2 low and SEL1 high. The
OUTS/ADJ pin is used either for sensing
the output for fixed output voltages or as
the feedback pin for an adjustable output
voltage. It is connected directly to the
output when using the fixed values. For
adjustable output, the feedback reference
voltage is 1.200V ±2%. The output can be
set anywhere between 2.5 and 5V, through
the choice of suitable feedback resistors.
CONCLUSION
The LTC3245 is a switched capacitor buckboost DC/DC converter that produces a
regulated output (3.3V, 5V or adjustable)
from a 2.7V to 38V input. No inductors are required. Low operating current
(20µ A with no load, 4µ A in shutdown)
and low external parts count (three small
ceramic capacitors) make the LTC3245
ideal for low power, space-constrained
automotive and industrial applications. n
1µF
C+
PGOOD
VIN
2.7V TO 38V
VIN
3.3µF
50V
LTC3245
BURST
PGOOD
470k
VOUT
SEL1
SEL2
Figure 5. A 3.6V output
buck-boost converter
C–
100k
OUTS/ADJ
GND
200k
VOUT
3.6V
250mA
47µF
6.3V
January 2014 : LT Journal of Analog Innovation | 11
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