DN130 - Power Supplies for Subscriber Line Interface Circuits

Power Supplies for Subscriber Line Interface Circuits
Design Note 130
Eddie Beville
As the demand for world wide networking grows, so
will the need for advanced data transmission products.
In particular, ISDN services have become popular because of the recent development of the Internet. ISDN
provides higher speed data transmission than standard
modems used in PCs. Also, ISDN supports the standard
telephone interface (voice and fax), which includes the
Subscriber Line Interface Circuit. A Subscriber Line
Interface Circuit requires a negative power supply for
the interface and the ringer voltages. The power supplies
described herein are designed for these applications.
Specifically, these designs address the AMD79R79
SLIC device with on-chip ringing.
CIRCUIT DESCRIPTIONS
LT®1171 Supplies –23.8V at 50mA and
–71.5V at 60mA
Figure 1 shows a current mode flyback power supply
using the LT1171CQ device. This current mode device
has a wide input voltage range of 3V to 60V, current
limit protection and an on-chip 65V, 0.30Ω bipolar
D3
T1 MURS120
33μH
9V TO 18V
7
VIN
C1
330μF
35V
+
VR1
LT1171
VSW
VC GND FB
1
3
+
:1
5
D1
P6KE-43A
D5
1N4687 OR
4 MMSZ4V3T1
1
D2
1N5817 1:
2
D4
MURS120
6
–23.8V
50mA
5
D6
MBRS1100T3
+
:1
2
4
R1
1k
C2
0.33μF
4
3
R2
71.5k
1%
R4
1k
R3
1.24k
1%
6
C6
0.01μF
2
3
V+
U1
LT1006
V–
4
Figure 1
06/96/130_conv
7
+
C5
0.1μF
–
PRIMARY
INDUCTANCE: 33μH
PRIMARY
PEAK CURRENT: 2A
PRIMARY DCR: 0.1Ω
SECONDARY DCR: 0.5Ω
C4
68μF
63V
–71.5V
60mA
:1
9V TO 18V
PE-68488
PULSE ENGINEERING
C3
120μF
35V
switch. The input voltage range for the circuit is 9V to
18V. This circuit is intended for small wall adapters that
power ISDN boxes. The output voltages are –23.8V at
50mA and –71.5V at 60mA.
The circuit shown in Figure 1 uses the LT1171 in standard flyback topology. The transformer’s turns ratio is
1:1:1:1, where 23.8V appears across each secondary
winding and the primary during the switch off time. The
remaining secondary windings are stacked in series to
develop –47V. The –47V section is then stacked onto the
–23.8V section to get –71.5V. This technique provides
very good cross regulation, lowers the voltage rating
required on the output capacitors and lowers the RMS
currents, allowing the use of cheaper output capacitors.
Either the –23.8V output or the –71.5V output can be
at full load without effecting the other corresponding
output. The circuit’s step response is very good; no significant overshoot occurs after either output is shorted
and released. Also, the transformer windings are all
quadrafilar to lower the leakage inductance and cost.
LT1269 Supplies –23.5V at 60mA and –71.5V at
120mA from 5V Input
Figure 2 shows a current mode flyback power supply
using the LT1269CQ device. This current mode device
has a wide input voltage range, current limit protection
and an onboard 60V, 0.20Ω bipolar switch. The input
voltage range for the circuit is 5V to 18V. This design
provides a wider input voltage range and greater output
power than that of Figure 1. The output voltages are
–23.5V at 60mA and –71.5V at 120mA (8.6W). This
circuit is designed to power two SLIC devices. The
circuit operation is identical to Figure 1, except for a
larger switching regulator device (VR1) and a different
transformer (T1). These changes allow for 5V operation
and higher output power. This circuit is designed for
full load on the –71V or –23.5V output. This accommodates the ringing on two SLICs or off hook on two
DN130 F01
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
Figure 1 Bill of Materials
REFERENCE
DESIGNATOR QUANITITY PART NUMBER
C1*
1
ECA-1VFQ331
C2, C5
1
0805
C3
1
UPL1V121MPH
C4
1
UPL1J680MPH
C6
2
0805
D1
1
P6KE-43A (MOT), TGL41-43A (GI)
D2
1
1N5817
D3, D4
2
MURS120
D5
1
1N4687, MMSZ4V3T1
D6
1
MBRS1100T3
R1, R4
2
0805
R2
1
0805
R3
1
0805
T1*
1
PE-68488
U1
1
LT1006S8
VR1*
1
LT1171CQ
*Changes and Additions for Figure 2’s Circuit
C1
1
205A100M
D5, D7
2
1N4001
R5, R6
2
T1
VR1
1
1
HMOO-96553
LT1269CQ
DISCRIPTION
Capacitor, 330μF, 35V HFQ
Capacitor, 0.33μF Ceramic
Capacitor, 120μF, 35V Plastic
Capacitor, 68μF, 63V Plastic
Capacitor, 0.01μF
Diode, 0.5W Zener
Diode, 1A Schottky
Diode, Ultrafast
Diode, Zener
Diode
Resistor, 1k, 5% SMT
Resistor, 71.5k, 1% SMT
Resistor, 1.24k, 1% SMT
Transformer
IC
IC
T1 MURS120
33μH
2
+
C1
100μF
20V
D1
P6KE-43A
VIN
D5
D7
1N4001 1N4001
VR1
LT1269
VSW
VC GND FB
1
3
10
D2
1N5817 1:
1
+
9
D4
MURS120
3
D6
MBRS1100T3
–23.5V
60mA
+
:0.85
2
C3
120μF
35V
:1.25
C4
–71.5V
68μF
120mA
63V
8
R1
1k
C2
0.33μF
4
R2
71.5k
1%
:0.85
7
5
R5
50k
1/4W
:0.85
HM00-96553
BI TECHNOLOGIES
(714) 447-2656
6
5V TO 18V
R4
1k
R3
1.24k
1%
7
6
C6
0.01μF
2
+
C5
0.1μF
–
PRIMARY
INDUCTANCE:
33μH TO 35μH
PRIMARY
PEAK CURRENT: 3.5A
PRIMARY DCR: 0.065Ω
SECONDARY
WINDINGS
≈30AWG WIRE.
TURN RATIOS ARE
SO NOTED ON THE
TRANSFORMER
3
V+
U1
LT1006
V–
4
R6
50k
1/4W
DN130 F02
Figure 2
Data Sheet Download
www.linear.com
Linear Technology Corporation
TELEPHONE
Nichicon
Nichicon
(708) 843-7500
Motorola or Equiv
Motorola or Equiv
Motorola or Equiv
Motorola or Equiv
Motorola or Equiv
Pulse Eng
LTC
LTC
Capacitor, 100μF, 20V OS-CON Sanyo
Diode
Resistor, 50k 0.25W SMT or
Through Hole
Transformer
BI Technology
IC
LTC
SLICs. R5 and R6 are preload resistors for maintaining
an accurate –23.5V output at full load with the –71V
output at minimum load.
D3
5V TO 18V
VENDOR
Panasonic
(408) 432-1900
(408) 432-1900
(619) 661-6835
(714) 447-2656
(408) 432-1900
LAYOUT AND THERMAL CONSIDERATIONS
Printed circuit board layout is an important consideration in the design of switching regulator circuits. A
good ground plane is required for all ground connections. The path from the input capacitor to the primary
winding of the transformer is a high current path, and
requires a short, wide copper trace (0.080" to 0.1"). The
VSW pin connection also needs a short, wide copper
trace. R1 and C2 need to be placed close to VR1. The
secondary windings can be connected to their associated components with 0.025" to 0.030" traces. The
feedback circuitry needs to be placed close to the FB
pin of VR1. Place C5 close to U1 to decouple the op
amp power supply. The LT1171CQ and LT1269CQ are
surface mount devices that require about a 1" copper
pad for heat sink mounting. Heat sinking is most critical for the LT1269CQ because of its high output power.
Also, vias from the copper pad to the internal ground
layers are highly recommended.
BILL OF MATERIALS
A bill of materials has been provided with each schematic.
For applications help,
call (408) 432-1900
dn130f_conv LT/GP 0696 185K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1996
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