1.5A Monolithic Buck-Boost DC/DC Converter with Up to 95% Efficiency Features 2.5V–15V Input and Output Voltage Ranges

1.5A Monolithic Buck-Boost DC/DC Converter with
Up to 95% Efficiency Features 2.5V–15V Input and Output
Voltage Ranges
Richard Cook
Multicell high capacity batteries are increasingly becoming
commonplace in handheld devices and industrial
instruments that receive their power from a variety of
sources. To maximize battery run time and support the
variety of power sources, voltage regulators in multipower
source systems must be able to maintain a constant output
voltage even as the input voltage source resides above,
below or equal to the output. This can be accomplished
using two separate power converters with two controller
ICs. A better solution is to use a single buck-boost DC/DC
converter, which offers a smaller, simpler and more efficient
design, attributes critically important in handheld devices.
The LTC3111 is a monolithic buckboost converter with input and output
voltage ranges of 2.5V to 15V and an
output current capability of 1.5A. It
allows conversion from a variety of
power sources such as single or multi
Li-ion cells, lead acid batteries, capacitor banks, USB cables or wall adapters.
architecture, effectively eliminating jitter
and EMI that can occur when crossing the
boundary between step-up and step-down
operation. This reduces or eliminates the
need for expensive filtering or shielding
for noise sensitive data conversion or
RF circuitry in the system. Selectable Burst
Mode® operation extends the operating time when battery-powered devices
are in idle by substantially reducing the
quiescent current of the power converter.
In addition to its wide operating range, the
LTC3111 features Linear Technology’s proprietary low noise buck-boost PWM control
Figure 2. One, two and three Li-ion cells can be used in this solution with the LTC3111’s accurate RUN
threshold feature
4.7µH
0.1µF
VIN
3V TO 12.6V
1 TO 3-CELL
Li-Ion
+
NUMBER
R
OF CELLS
274k
1
698k
2
3
1.13M
SW1
SW2
BST1
BST2
VIN
VOUT
10µF
LTC3111
0.1µF
680pF
COMP
BURST PWM
R
154k
24 | January 2015 : LT Journal of Analog Innovation
PWM/SYNC
RUN
SNSGND
SGND
26.1k
1M
33pF
27pF
FB
VCC
PGND
191k
1µF
20k
VOUT
5V
750mA
22µF VIN > 4V
Figure 1. LTC3111-based 18W solution
An accurate run threshold provides
the ability to precisely program the
turn‑on threshold voltage of the converter. The integrated fault protection
features include: current limit, thermal
shutdown and short-circuit protection, which provides robust operation
in harsh environments. For applications where component size is critical,
the 800kHz default switching frequency
can be synchronized up to 1.5MHz.
The LTC3111 based converter shown in
Figure 1 can generate 18W of power with
a 12V output. The solution footprint is
less than 180mm2 , more compact than a
controller-based buck-boost and much
more efficient than a complex dual-inductor SEPIC converter design at similar power
levels. The main external components are
limited to the input, output filter capacitors and the power inductor. The LTC3111
design features
In addition to its wide operating range, the LTC3111 features Linear Technology’s
proprietary low noise buck-boost PWM control architecture, effectively eliminating
jitter and EMI that can occur when crossing the boundary between step-up and
step-down operation. This reduces or eliminates the need for expensive filtering
or shielding for noise sensitive data conversion or RF circuitry in the system.
100
PWM
90 BURST MODE
OPERATION
80
TURN ON
THRESHOLD
3.3V
EFFICIENCY (%)
VOUT
1V/DIV
TURN OFF
THRESHOLD
3.0V
70
60
50
VIN
2V/DIV
VIN = 3.6V
VIN = 7.2V
VIN = 10.8V
40
30
0.1m
100ms/DIV
Figure 3. LTC3111 ramped input voltage response using the accurate run for
a single Li-ion solution
is offered in a thermally enhanced 16-lead
4mm × 3mm DFN or 16-lead MSOP package.
ACCURATE RUN THRESHOLD WITH
1-, 2- AND 3-CELL LI-ION
The LTC3111’s RUN pin can either be
used to enable/disable the converter via
digital select, or to set an accurate userprogrammable undervoltage lockout
(UVLO) threshold—by a resistive divider
from VIN to ground. The LTC3111’s
RUN threshold of 1.2V (±5% over temperature) allows customization of the
VIN
LTC4412
GATE
CTL
SENSE
GND
STAT
0.1
1 2
is greater than 3.3V and to turn off when
the input voltage drops below 3V.
This technique can be applied to two
or three series cell designs by changing
the value of R, as shown in the table for
Figure 2. The output voltage response to
a slowly ramped VIN for the single cell
case is shown in Figure 3. VOUT in the
single cell configuration turns on when
the input voltage reaches 3.3V and turns
off at 3V. Similarly, this plot can be scaled
for 2- and 3-cell cases, where turn-on/
Figure 2 shows an application circuit
where the accurate RUN pin threshold is
used to turn the LTC3111 converter on/
off when powered from a one, two or
three Li-ion cell battery. For the single cell
case, R is 267k, configuring the LTC3111
RUN pin to turn on when the input voltage
12V
ADAPTER
10m
ILOAD (A)
Figure 4. 5V output efficiency from one, two and three Li-ion cells
turn-on threshold voltage of the converter. Once enabled, 120mV of hysteresis
is introduced at the RUN pin, requiring
the source input voltage to drop 10%
before disabling power conversion.
Figure 5. LTC4412 PowerPath™
controller selects highest voltage input
to power the LTC3111 converter
1- OR 2-SERIES
Li-ION CELLS
1m
1N5819
4.7µH
0.1µF
SW1
SW2
BST1
BST2
VIN
VOUT
47µF
LTC3111
0.1µF
1200pF
COMP
BURST PWM
OFF ON
PWM/SYNC
1M
FB
VCC
PGND
VCC
20k
33pF
27pF
RUN
SGND
25.5k
22µF
VOUT
3.3V
1.2A
MBR0520L
316k
1µF
January 2015 : LT Journal of Analog Innovation | 25
The LTC3111 includes circuitry to minimize loop gain variation, resulting
in improved line transient response. Regulation for VOUT = 3.3V
remains within 50mV, or 1.5%, during a 20µs, 7.2V-to-12V, VIN rise
and fall transition with a 22µF output capacitor and 1A load.
100
EFFICIENCY (%)
80
VOUT
100mV/DIV
PWM
90
BURST MODE OPERATION
70
VIN
2V/DIV
60
50
40
30
0.1m
turn-off thresholds are 6.6V/6V and 9.9V/9V,
respectively. The accurate RUN feature
can also be applied to sources where
operation must be restricted to a minimum
input operating voltage such as a bank of
capacitors, lead acid or NiCd batteries.
Efficiency curves for the one, two and
three Li-ion cell designs operating at their
typical voltages are shown in Figure 4.
Peak efficiencies of greater than 90% are
achieved over all three battery voltages.
Note that the maximum load current
capability for a 5V output decreases
when the input voltage is less than 6V.
The LTC3111 data sheet provides performance curves showing maximum
output current capability versus input
voltage in PWM and Burst Mode operation for various output voltages to aid
in determining if the load can be supported over a specific input range.
VIN = 7.2V
VIN = 12V
1m
10m
ILOAD (A)
0.1
1 2
500µs/DIV
Figure 6. LTC3111 efficiency vs load current VOUT =
3.3V, VIN = 7.2V and 12V
Figure 7. Line response for VOUT = 3.3V, VIN stepped
from 7.2V to/from 12V
MULTIPLE INPUT SOURCES
Efficiency curves versus load current
for the 3.3V output, based on the two
input sources, are given in Figure 6.
Peak efficiencies of greater than 89%
are achieved. Selectable Burst Mode
operation with 49µ A of typical sleep
current extends high efficiency over
two decades of load current.
The wide operating range of the LTC3111
makes it easy to power devices from
multiple input sources. Figure 5 shows an
application where the LTC4412 PowerPath
controller (SOT-23 package) selects from
the higher of two input sources. The
LTC4412 maintains a 20mV forward voltage across the selected P-channel MOSFET,
keeping losses to a minimum. In this
circuit, the LTC4412 switches the input of
the LTC3111 to the greater of a 7.2V lithium
ion battery or 12V wall adapter.
4.7µH
0.1µF
VIN
5V
SW1
SW2
BST1
BST2
VIN
VOUT
10µF
LTC3111
0.1µF
2.5V < VOUT < 10V
680pF
COMP
VCC
PWM/SYNC
RUN
SNSGND
SGND
Figure 8. LTC3111 configured
as a variable output supply
26 | January 2015 : LT Journal of Analog Innovation
The LTC3111 includes circuitry to minimize loop gain variation, resulting in
improved line transient response. As
illustrated in Figure 7, VOUT regulation
is maintained within 50mV, or 1.5%,
26.1k
20k
R2
191k
0V < VCONTROL < 1.2V
VCONTROL
R3
162k
1µF
27pF
FB
VCC
PGND
1µF
VOUT
22µF
R1
1M
33pF
design features
The LTC3111 provides low noise buck-boost conversion for a variety of applications
requiring an extended input or output voltage range. The LTC3111’s ability to support heavy
loads makes it ideal for power hungry devices. Solution size and conversion efficiency benefit
from the 90mΩ internal N-channel MOSFET switches and thermally enhanced packages.
2.5
VOUT
2V/DIV
IOUT (A)
2
VCONTROL
500mV/DIV
1.5
1
0.5
5ms/DIV
0
VIN = 5V
INPUT CURRENT LIMIT = 2.3A
2
3
4
5
6
7
VOUT (V)
8
9
10
Figure 9. Variable output response using the
LTC3111
Figure 10. Maximum output current in PWM mode vs
output voltage for VIN = 5V
during the 20µs rise and fall transition with a 22µ F output capacitor and
1A load in stepdown operation.
Figure 9 shows the output voltage
response of a 0V to 1.2V ramped control
signal operating at 100Hz. The corresponding output voltage swings from
10V to 2.5V, providing an inverting gain of
6.2 from VCONTROL to VOUT. The low noise
PWM control provides low distortion and
high quality replication of the input signal.
VARIABLE OUTPUT VOLTAGE USING
THE LTC3111
For applications such as motor control,
lighting or power supply margin testing, the LTC3111 can be configured as
a variable voltage supply. This can be
accomplished in a number of ways.
Figure 8 shows one method: adding a
summing resistor between the FB pin
and a control voltage (VCONTROL).
The programmed output voltage can be
calculated using the following equation:
 R1  R1
VOUT = 0.8V 1+  + (0.8V − VCONTROL )
 R2  R3
where R1 is the resistor connected between
VOUT and FB, R2 is the resistor connected
from FB and ground and R3 is the resistor
connected from FB and VCONTROL .
When using the LTC3111 as a variable
output voltage regulator, the maximum
load current capability of the LTC3111
is reduced when VOUT > VIN (i.e., when
the part is in boost or step-up mode). As
Figure 10 shows, the maximum output
current capability is effectively reduced
by the step-up ratio of the converter.
For example, the output current capability when VOUT = 2VIN is roughly one
half the capability when VOUT = VIN . In
the example application above, a fixed
500m A load is applied to the output,
which the part is capable of supplying at
all output voltages. To ensure converter
stability, compensation values for this
application are determined at the highest boost ratio of VIN = 5V to VOUT = 10V.
SUMMARY
The LTC3111 provides low noise buckboost conversion for a variety of applications requiring an extended input or
output voltage range. The LTC3111’s
ability to efficiently support heavy load
currents makes it ideal for power hungry devices. Solution size and conversion
efficiency benefit from the 90mΩ internal
N-channel MOSFET switches and thermally
enhanced packages. Low quiescent current Burst Mode operation extends high
efficiency over several decades of load
current, enabling longer run times in
many battery powered applications. n
January 2015 : LT Journal of Analog Innovation | 27
Similar pages