Sep 2001 The LTC1871 Achieves the Industry's Highest Efficiency for Single-Ended Boost, Flyback and SEPIC Topologies

DESIGN FEATURES
The LTC1871 Achieves the Industry’s
Highest Efficiency for Single-Ended
Boost, Flyback and SEPIC Topologies
by Tick Houk
Introduction
The LTC1871 is a general-purpose
DC/DC controller IC optimized for
use in boost, SEPIC and flyback converter applications. It provides flexible,
high performance operation in a small
MS10 package in order to extend the
range of applications from single-cell,
lithium-ion battery portable electronics up to high voltage, high power
telecommunications equipment.
The LTC1871 is distinguished from
conventional current mode controllers because the current control loop
can be closed by sensing the voltage
drop across the power MOSFET switch
instead of across a discrete sense
resistor, as shown in Figure 1a. This
sensing technique provides the maximum efficiency possible for a
single-ended, current mode converter.
For applications where the drain of
the power MOSFET exceeds 36V, or
when more accurate control of the
maximum current is important, the
LTC1871 can also be used with a
conventional sense resistor, as shown
in Figure 1b.
Capable of operating with an input
voltage from 2.5V to 32V (36V Abs
Max), the LTC1871 also exhibits low
quiescent current. When operating in
continuous conduction mode (CCM),
the quiescent current drawn by the
VIN
VOUT
L
9
COUT
10
VIN
VOUT
L
7
M1
SENSE
GND
6
COUT
7
M1
10
6
GND
Figure 1a. Sensing on the drain
of the power MOSFET
+
LTC1871
GATE
GND
26
RUN pin below 1.248V, the total quiescent current drops to a very low
10µA.
The operating frequency of the converter can be programmed over a
50kHz-to-1MHz range by means of a
VIN
+
LTC1871
GATE
The LTC1871 is
distinguished from
conventional current mode
controllers because the
current control loop can be
closed by sensing the
voltage drop across the
power MOSFET switch
instead of across a discrete
sense resistor. This sensing
technique provides the
maximum efficiency
possible for a single-ended,
current mode converter.
9
VIN
SENSE
IC is typically only 550µA plus the
current required to switch the gate of
the external power MOSFET (IQTOT =
550µA + QG • fOSC). In Burst Mode™
operation, at light loads, this total
quiescent current (IQTOT) can drop to
as low as 250µA. Finally, when the
chip is in shutdown mode, with the
GND
single resistor from the FREQ pin to
ground. In addition, for systems where
the switching frequency of the converter needs to be controlled by an
external clock, the LTC1871 can be
synchronized using the MODE/SYNC
pin.
A Two-for-One Converter:
High Efficiency 5V Output
Boost Converter Operates
from a 2.5V or 3.3V Input
Figure 2 illustrates a 5V output boost
regulator that can operate from either
a 2.5V or a 3.3V input supply. Having
a single converter that can operate
from different supply voltages greatly
simplifies component purchasing and
production control for volume manufacturers. This particular design takes
advantage of the No RSENSE™ technology in order to maximize efficiency
and reduce board space and total
cost. Operating at a switching frequency of 300kHz enables the use of
a small, inexpensive 1.8µH inductor
from Toko. A Siliconix/Vishay SO-8
power MOSFET (the Si9426, which
has a maximum RDS(ON) of 16mΩ at
VGS = 2.5V and 13.5mΩ at VGS = 4.5V)
and an International Rectifier surface mount diode (30BQ015) were
chosen for the 2.0A output current
level. A combination of Sanyo
POSCAPs and low ESR Taiyo Yuden
ceramic capacitors was used to reduce
the output ripple to below 60mVP-P. It
should be noted that the output current of a converter such as this can
easily be scaled by the choice of the
components around the chip without
modifying the basic design.
Figure 1b. Sensing on the source
of the power MOSFET
Linear Technology Magazine • September 2001
DESIGN FEATURES
VIN
2.5V to 3.3V
L1
1.8µH
D1
1
2
RC
22k
CC2
47pF
CC1
6.8nF
3
4
R1
12.1k
R2
1%
37.4k
1%
5
RT
80.6k
1%
CIN: SANYO POSCAP 6TPA47M
COUT1: SANYO POSCAP 6TPB150M
COUT2: TAIYO YUDEN JMK316BJ106ML
CVCC: TAIYO YUDEN LMK316BJ475ML
SENSE
RUN
VIN
ITH
INTVCC
FREQ
GATE
MODE/SYNC
VOUT
5.0V/2A
9
+
LTC1871
FB
10
GND
(619) 661-6835
(408) 573-4150
8
7
6
M1
CVCC
4.7µF
X5R
+
COUT1
150µF
6.3V
(×2)
CIN
47µF
6.3V
D1: INTERNATIONAL RECTIFIER 30BQ015
L1: TOKO DS104C2 B952AS-1R8N
M1: SILICONIX/VISHAYSi9426
COUT2
10µF
6.3V
X5R
(×2)
GND
(310) 322-3331
(847) 699-3430
(800) 554-5565
Figure 2. 2.5V to 3.3V input, 5.0V/2A output boost converter
Figure 3 illustrates the efficiency
curves for this converter at input voltages of 2.5V and 3.3V; Figure 4
illustrates the load step response.
For applications where the maximizing the efficiency at very light loads
(for example, < 100µA) is a high priority, the current in the output divider
can be decreased to a few microamps
and Burst Mode operation applied
(the MODE/SYNC pin connected to
ground). In applications where fixed
frequency operation is more critical
than low current efficiency, or where
the lowest output ripple is desired,
pulse-skipping mode operation should
be used and the MODE/SYNC pin
should be connected to the INTVCC
pin. This allows discontinuous conduction mode (DCM) operation down
to near the limit defined by the chip’s
minimum on-time (about 175ns).
Below this output current level, the
converter will begin to skip cycles in
order to maintain output regulation.
Figure 5 shows the light load switching waveforms for Burst Mode and
pulse-skipping mode operation for the
converter in Figure 2 and Figure 6
illustrates the difference in measured
efficiency.
If synchronized operation is
required, a logic-level clock signal
can be applied to the MODE/SYNC
pin. In this case, the turn-on of the
power MOSFET will correspond to the
rising edge of the clock signal. Because
the internal slope compensation is
increased by 30% during synchronized operation, setting the nominal
operating frequency of the converter
to 75% of the external clock frequency
will result in the same net slope
compensation.
The Fargo Converter: A 4.5V
to 15V Input, 12V/1.5A
Output SEPIC Converter
The primary advantage of the SEPIC
converter is that the input voltage can
be greater than or less than the output voltage. Such a converter is
important in applications where the
input voltage is either poorly regulated or experiences significant
transients and the output voltage falls
within this input range. A perfect
example of such an environment exists
in automotive, battery-supplied systems. Under normal conditions, the
battery voltage in a car is approximately 13.7V. At very low ambient
temperatures, such as are experienced in Fargo, North Dakota in the
winter (–45°F without wind chill is
not uncommon), the current required
by the starter motor to crank a large
100
95
VOUT
50mV/DIV
90
EFFICIENCY (%)
85
VIN = 3.3V
80
75
2.0A
VIN = 2.5V
70
IOUT
0.5A/DIV
65
60
0.5A
55
50
0.001
0.01
0.1
1.0
OUTPUT CURRENT (A)
10.0
Figure 3. Efficiency vs input voltage
for Figure 2’s converter
Linear Technology Magazine • September 2001
100µs/DIV
Figure 4. Load-step response for Figure 2’s converter
27
DESIGN FEATURES
VOUT
20mV/DIV
VOUT
20mV/DIV
IL
0.5A/DIV
IL
0.5A/DIV
5µs/DIV
5µs/DIV
Figure 5a. Pulse-skip switching waveforms
at light load for Figure 2’s circuit
Figure 5b. Burst Mode switching waveforms
at light load for Figure 2’s circuit
The SEPIC converter illustrated in
Figure 7 can operate with a DC supply as low as 4.5V and also takes
advantage of the No RSENSE technology in order to maximize efficiency.
For this design, a coupled inductor
from BH Electronics was chosen,
along with International Rectifier’s
IRF7811W power MOSFET (which has
a maximum RDS(ON) of 12.5mΩ at VGS
= 4.5V). A Taiyo Yuden ceramic
capacitor was chosen for the DC coupling capacitor because of its low ESR
and high RMS current capability.
Figure 8 shows the efficiency for this
converter at three input voltages and
Figure 9 shows the typical load step
response.
This design greatly benefits from
the internal 5.2V low dropout regulator, or LDO, inside the LTC1871.
Having such a wide input voltage
range (4.5V–15V) would normally
V-8 engine can drag the battery voltage down below 5V. Certain systems
in the car must be fully operational
during this so-called cold cranking
(for example, the engine control unit),
hence a converter is needed that can
maintain regulation under these low
input conditions.
100
95
90
PULSE-SKIPPING
MODE
EFFICIENCY (%)
85
BURST MODE
OPERATION
80
75
70
65
60
55
50
0.001
0.01
0.1
1.0
OUTPUT CURRENT (A)
10.0
Figure 6. Efficiency vs operating mode
for Figure 2’s converter
present problems for the gate drive to
the power MOSFET. At low DC input
voltages, any drop between the chip
supply and the gate driver output
would reduce the VGS of the MOSFET
to as low as 3.3V (4.5V minus a typical drop of 1.2V through an NPN
output stage), thereby requiring the
use of a sublogic-level power MOSFET rated at a VGS of 2.5V. With the
PMOS-output LDO in the LTC1871
and a strong CMOS gate driver, however, the full supply voltage is applied
to the gate of the MOSFET, providing
maximum efficiency and more flexibility in the choice of power MOSFETs.
At the other end of the input range,
when the supply is 15V, the 5.2V LDO
output in the LTC1871 limits the
drive to the power MOSFET, thereby
minimizing gate oxide stress and
maximizing reliability.
VIN
4.5V to 15V
•
R3
1M
L1*
1
2
SENSE
RUN
VIN
ITH
LTC1871
RC
33k
CC2
47pF
CC1
6.8nF
3
R1
R2 12.1k
1%
105k
1%
4
RT
80.6k
1%
5
CDC
10µF
10
FB
FREQ
MODE/SYNC
D1
9
+
8
INTVCC
GATE
GND
7
6
CIN, COUT1: KEMET T495X476K020AS
CDC, COUT2: TAIYO YUDEN TMK432BJ106MM
D1: INTERNATIONAL RECTIFIER 30BQ040
M1: INTERNATIONAL RECTIFIER IRF7811W
L1, L2: BH ELECTRONICS BH510-1007 (*COUPLED INDUCTORS)
L2*
M1
CVCC
4.7µF
X5R
+
CIN
47µF
•
COUT1
47µF
20V
(x2)
VOUT
12V
1.5A
(2A PEAK)
COUT2
10µF
25V
X5R
(×2)
GND
(408) 986-0424
(408) 573-4150
(310) 322-3331
(612) 894-9590
Figure 7. 4.5V to 15V input, 12.0V/2A output SEPIC converter
28
Linear Technology Magazine • September 2001
DESIGN FEATURES
100
90
VIN = 5V
VOUT = 12V
VIN = 12V
95
VIN = 4.5V
VOUT (AC)
200mV/DIV
EFFICIENCY (%)
85
80
VIN = 15V
75
1.5A
70
IOUT
0.5A/DIV
65
60
O.5A
55
50
0.001
0.01
0.1
1.0
OUTPUT CURRENT (A)
10.0
50µs/DIV
Figure 8. 12V output SEPIC converter
efficiency vs input voltage and load current
VIN = 15V
VOUT = 12V
A “SLIC” Flyback
Power Supply
VOUT (AC)
200mV/DIV
Figure 10 illustrates a multiple-output telecom power supply designed
for use in subscriber line interface
circuits (SLICs). The input to the SLIC
power supply is some form of battery
(lead acid or lithium-ion, for example)
so that talk battery power can be
provided to POTS (plain old telephone
system) phones during an AC line
failure (or rolling blackout). The output voltage is typically proportional
to the distance the subscriber line
runs from the local hub to a house or
office, in order to compensate for the
impedance of the loop. Multiple output supplies are used to supply groups
of users at different distances from
the hub.
1.5A
IOUT
0.5A/DIV
O.5A
50µs/DIV
Figure 9. 12V output SEPIC converter load-step response; top, VIN = 5V; bottom, VIN = 15V
GND
•
+
4
VIN
7V TO 12V
R1
51k
C3
10µF
25V
X5R
R2
150k
CR
1nF
+
CC2
100pF
VOUT1
–24V
200mA
CIN
220µF
16V
TPS
•
T1
1, 2, 3
C4
10µF
25V
X5R
D4
+
C5
10µF
25V
X5R
•
•
LTC1871
FB
INTVCC
FREQ
RT
120k
+
+
COUT
3.3µF
100V
VIN
ITH
RC
82k
D3
5
SENSE
RUN
CC1
1nF
D2
MODE/SYNC
GATE
GND
f = 200kHz
M1
+
C1
4.7µF
X5R
C2
4.7µF
50V
X5R
6
RF1
10k
1%
RS
0.012
6
(310) 322-3331
C8
0.1µF
(561) 752-5000
4
+
M1: INT'L RECTIFIER IRL2910
D2–D4: INT'L RECTIFIER 10BQ060
T1: COILTRONICS VP5-0155
(PRIMARY = 3 WINDINGS IN PARALLEL)
1
–
10k
RF2
196k
1%
3
VOUT2
–72V
200mA
LT1783
2
Figure 10. High power, dual-output SLIC supply
Linear Technology Magazine • September 2001
29
DESIGN FEATURES
The –24V output of this supply
uses one secondary winding in a
capacitively coupled flyback configuration, whereas the –72V output
uses the other two windings in a
conventional flyback mode. The –24V
output uses an LT1783 op amp to
level-shift the feedback voltage, and
the –72V output is obtained by stacking additional windings on the –24V
output. A size 5, 6-winding Versapak
transformer (VP5-0155) was used for
convenience, with three windings connected in parallel on the primary to
satisfy the primary current demand.
Unlike the previous, low voltage
boost and SEPIC designs, which take
advantage of the No RSENSE technology, this flyback converter places
significant stress on the drain of the
power MOSFET. As a result, a 100V
BVDSS device is used (International
Rectifier’s IRL2910), along with a conventional 12mΩ sense resistor in the
MOSFET source (the absolute maximum voltage rating for the SENSE
pin on the LTC1871 is 36V). The
increase in losses due to this sense
resistor are relatively small in this
system (approximately 1%), due to
the high input voltage.
For systems where control of the
maximum output current is more
important than overall efficiency, the
use of a sense resistor can improve
performance. The initial tolerance of
a discrete sense resistor is commonly
better than ±5%, whereas the initial
tolerance of the RDS(ON) of a power
MOSFET is typically ±20%–30%. In
addition, the temperature coefficient
of the discrete resistor can easily be
an order of magnitude lower than for
a power MOSFET (whose R DS(ON)
increases approximately 50% from
25°C to 125°C).
The resistor divider formed by R1
and R2 is used to detect an undervoltage condition on the input supply
in order to shut down the converter
when the battery pack has discharged
below 5.0V. For a falling input voltage
(a discharging battery), the RUN pin
on the LTC1871 is compared to an
internal micropower 1.248V reference. If the RUN pin falls below this
LT1683/LT1783, continued from page 20
components, eliminating the need for
the grounded caps on the input filter.
electrical isolation of the output. An
overwinding coil on the transformer
provides the supply voltage to the
part.
An input filter is used to attenuate
the low frequency harmonics so the
supply can pass FCC Class B regulations. The LT1738 reduces the high
frequency harmonics and EMI noise
Authors can be contacted
at (408) 432-1900
30
Conclusion
Two new controllers have been created
that allow designers to confidently
tackle ultralow noise applications.
The LT1683 and LT1738 not only
provide normal controller functions
but also regulate the main compo-
threshold, the chip shuts down and
the quiescent current drops to 10µA
in order to reduce the load on the
battery. The hysteresis on the RUN
pin comparator was chosen to be
100mV in order to compensate for the
rise in the unloaded battery voltage
(or other input supply) and to provide
good noise immunity in general. In
this particular design, the rising input
start-up threshold is approximately
5.4V. The optional capacitor CR could
be used to give the converter some
ride-through capability for brief input transients.
Conclusion
The LTC1871 is a versatile control IC
optimized for a wide variety of singleended DC/DC converter topologies.
Flexible, high performance operation
is provided in a small, convenient
MS10 package in order to improve
efficiency, reduce the size and weight
of the power supply, and reduce the
total component and manufacturing
cost.
nents of EMI in a switcher, thus easily
reducing output noise. DC/DC converters can now be used in application
areas such as precision instrumentation and noise-sensitive wideband
communication equipment without
the uncertain noise implications associated with normal converter design
solutions.
for
the latest information
on LTC products,
visit
www.linear-tech.com
Linear Technology Magazine • September 2001