DN296 - LTC1871 No RSENSE Controller Is Small and Efficient in Boost, Flyback and SEPIC Applications

No RSENSE Controller Is Small and Efficient in Boost, Flyback
and SEPIC Applications
Design Note 296
Tick Houk
Introduction
The increase in demand for high bandwidth signal processing in telecom systems has driven a corresponding
increase in demand for space-efficient boost, flyback
and SEPIC power supplies. The LTC ®1871 addresses
these specific needs by providing a high performance,
single-ended current mode DC/DC controller IC in a
small, 10-lead MSOP package. The LTC1871 is efficient
and easy to use, mainly due to features such as No
RSENSE ™ current mode control, a user-programmable
operating frequency (50kHz to 1MHz), a strong gate
driver, programmable undervoltage lockout, synchronization capability and a wide input range (2.5V to 36V).
is sensed during the on-time, thereby providing the
control loop with a “lossless” method of measuring the
switch current. This technique provides the maximum
efficiency possible for a single-ended current mode
converter. It also saves board space and reduces the
cost of the power supply in applications where the drain
of the power MOSFET is less than 36V (the absolute
maximum rating of the SENSE pin). It should be noted
that the output voltage and maximum output current
of this supply can easily be scaled by the choice of
the components around the chip without modifying
the basic design.
An operating frequency of 300kHz is programmed
using a single resistor to ground, allowing the use
of a small, inexpensive 1.8μH inductor from Toko. A
Siliconix/Vishay SO-8 power MOSFET (Si9426) and an
International Rectifier surface mount diode (30BQ015)
A High Efficiency 5V, 2A Networking Logic Supply
Figure 1 illustrates a small, high efficiency networking
5V logic supply that can operate from either a 2.5V or
a 3.3V input supply. This design takes advantage of
LTC’s proprietary No RSENSE technology to provide true
current mode control without an external current sense
resistor. The voltage drop across the power MOSFET
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered
trademarks and No RSENSE is a trademark of Linear Technology Corporation. All
other trademarks are the property of their respective owners.
f = 300kHz
1
2
RC
22k
3
CC2
47pF
4
RT
CC1
6.8nF 80.6k
1%
5
SENSE
RUN
VIN
ITH
LTC1871
FB
INTVCC
FREQ
GATE
MODE/SYNC
R1
12.4k
1%
VIN
2.5V to 3.3V
CIN
47μF
X5R
GND
L1
1.8μH
10
D1
9
8
VOUT
5V
2A
COUT
100μF
X5R
(X2)
M1
7
6
CVCC
4.7μF
X5R
GND
R2
38.3k
1%
DN296 F01
D1: INTERNATIONAL RECTIFIER 30BQ015
L1: TOKO DS104C2 B952AS-1R8N
M1: SILICONIX/VISHAY Si9426
Figure 1. A High Efficiency 5V, 2A Networking Logic Supply
10/02/296_conv
were chosen for the 2A output current level, and a low
ESR ceramic output capacitor keeps the output ripple
below 60mVP-P.
A 2 Square Inch, 12V Non-Isolated Flyback
Housekeeping Supply for Telecom Applications
The LTC1871 is also ideally suited for telecom input
applications (36V to 72V input), where a single-switch
flyback topology results in low component count and
small size. Figure 2 illustrates a 12V, 0.4A housekeeping power supply which occupies only two square
inches of board space. The converter operates in
discontinuous mode (DCM) at 200kHz and provides
power to other analog and digital ICs on the PC board.
A standard, 6-winding VERSA-PAC transformer (VP10076) is connected with three windings in series on
the primary and three in parallel on the secondary. A
small SuperSOT-6 MOSFET from Fairchild provides
the primary switch, along with a surface mount diode
from International Rectifier.
VIN
36V TO 72V
CIN
1.5μF
100V
X7R
R1
604k
1%
R2
26.7k
1%
Q1
t
VOUT
12V
0.4A
4, 5, 6
t
COUT
47μF
X5R
t
SENSE
3
VIN
D2
LTC1871
FB
INTVCC
FREQ
MODE/SYNC
R3
12.4k
1%
M1
GATE
RT
120k
T1:
M1:
Q1:
D1:
D2:
D3:
2
R6
10Ω
ITH
CC1
6.8nF
D3
T1
D1
9.1V
RUN
RC
20k
t
1
f = 200kHz
CC2
100pF
Programmable Undervoltage Lockout Provides
Clean Start-Up and Power-Down
In order to provide well controlled start-up and powerdown, the RUN pin on the LTC1871 monitors the input
voltage—at resistive divider R1-R2—and turns off the
converter when the input drops below 29.5V. An 8%
hysteresis level provides increased noise immunity
(UV+ is 31.8V). The optional capacitor C1 can be used
to provide “ride-through” capability for short-duration
input dropouts.
R5
51k
UV + = 31.8V
UV – = 29.5V
C1
1nF
OPTIONAL
Due to the high input voltage, a pre-regulator—consisting of transistor Q1, diode D1 and resistor R5—is
needed to provide start-up power to the LTC1871. Once
the 12V output is in regulation, D2 provides DC power
to the IC and Q1 turns off. This bootstrapping technique
ensures that the gate drive power is provided by the
switcher itself, thus maintaining the highest efficiency
possible. A 5.2V LDO within the LTC1871 provides a
regulated supply for the gate driver, which is capable
of driving MOSFETs with gate charges up to 100nC.
GND
C2
4.7μF
X5R
RS
0.12Ω
R4
110k
1%
DN296 F02
COILTRONICS VP1-0076 (3 WINDINGS IN SERIES ON PRIMARY, 3 IN PARALLEL ON SECONDARY)
FAIRCHILD FDC2512 (150V, 0.5 OHM)
ZETEX FZT605 (120V)
ON SEMICONDUCTOR MMBZ5239BLT1 (9.1V)
ON SEMICONDUCTOR MMSD4148T11
INTERNATIONAL RECTIFIER 10BQ060
Figure 2. A Small, Non-Isolated 12V Flyback Telecom Housekeeping Supply
Data Sheet Download
www.linear.com
Linear Technology Corporation
For applications help,
call (408) 432-1900
dn296f_conv LT/TP 1002 351.5K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2002
Similar pages