DN261 - Chip Set Offers Low Cost Alternative to 48V Telecom Modules

Chip Set Offers Low Cost Alternative to 48V Telecom Modules
Design Note 261
Kurk Mathews
The demand for high performance 48V input telecom
modules has never been higher. The latest generation
modules offer high current, high efficiency alternatives
to traditional low voltage potted supplies. Although these
drop-in solutions are attractive, their higher cost can
quickly add up as product volumes increase. Replacing a module with an on-board solution based on the
new LT®1681/LTC ®1698 chip set significantly reduces
system cost and takes full advantage of board area,
minimizing the need to derate output power at elevated
ambient temperatures.
Isolated 48V to 3.3V Supply
The schematic in Figure 3 presents a complete 36VIN to
72VIN to 3.3VOUT/20A power supply. Total component
cost is typically below $30 (50k pcs). The 2-transistor
forward converter utilizes low voltage primary switches
and provides for recovery of the transformer’s mutual
and leakage energy. The LTC1698 secondary controller
synchronizes with the LT1681 via a small pulse transformer and drives secondary synchronous MOSFETs.
Efficiency is shown in Figure 2. The LTC1698 includes an
error amplifier and optocoupler drive buffer, eliminating
the output feed-forward path and loop compensation
issues associated with ‘431 type references. A margin
pin allows the output voltage to be adjusted ±5%.
Other output voltages can be realized by substituting
components into the same basic circuit. Contact the
07/01/261_conv
Figure 1. 3.3V/20A Supply in Half-Brick Footprint
95
36VIN
90
EFFICIENCY (%)
The new LT1681 controller contains all the necessary
functions for a synchronous forward converter and
works together with the LTC1698 secondary controller
for isolated applications. The LT1681 features include
current-mode operation up to 250kHz, 75V high side
driver, leading edge blanking, input under/over voltage
protection, synchronous gate output and a thermal
shutdown pin. The LTC1698 interfaces directly with
the LT1681 providing an error amplifier, current limit
and output over voltage protection in addition to synchronous gate drivers.
72VIN
85
48VIN
80
75
70
2
4
6
8
10 12 14
CURRENT (A)
16
18
20
dn261f02
Figure 2. 3.3V Efficiency (See Figure 3)
factory for more information including half-brick layout
information (see Figure 1).
Conclusion
The LT1681/LTC1698 combine to provide a low cost,
discrete alternative to telecom modules. The LT1681/
LTC1698 features reduce circuit complexity. High efficiency synchronous operation combined with the
thermal advantages of an on board supply make the
LT1681/LTC1698 the ideal choice for high current, low
cost isolated converters.
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.
Data Sheet Download
www.linear.com
Linear Technology Corporation
●
C2
1.5μF
100V
C3
1.5μF
100V
FAX: (408) 434-0507 ● www.linear.com
0.1μF
C26
68μF
20V
+
MMBZ5248LT1
18V
10k
MMBZ5245LT1
15V
267k
0.25W
1nF
24k
1.24k
1%
56k
20k
73.2k
1%
BAS21
20
17
0.1μF 100V
19
18
16
11
12
15
BAS21
BAS21
BAT54
330pF
10k
BAT54
MMBT3906LT1
Q12
ZVN3310F
10Ω
1μF
MMBT3906LT1
5VREF
6
82pF
3
4
4700pF
7
8
3k
3300pF
10Ω
10
3
1t
4
t6
1k
5
8
S
2
1 T1
4
2
14
5
10k
15
3
4700pF
1k
220pF
S
Q1
12
11
16 2
100Ω
2k
0.25W
6
1k
1000pF
S
0.1μF
VAUX
OPTODRV
MARGIN
OVPIN
3
4
10
PGND GND PWRGD ICOMP
LTC1698
1.24k
1%
13
7
9
L2
2.35μH
2.43k
1%
dn261f03
1k
0.33μF
1.78k
1%
3.01k
Q6, Q15, Q17
FDS6680A
w3
MMBZ5240BLT1
10V
0.22μF
50V
MBR0540
4.7Ω
1000pF
100V
0.022μF
1000pF
100V 10Ω
0.25W
10Ω
0.25W
VDD ISNS ISNSGND FG CG VCOMP
8
SYNC
VFB
1
4.7μF
16V
Q13
FZT690
Q5, Q14
FDS6680A
w2
C1
2200pF
250V
3.3Ω
5
7
VSEC
Figrue 3. 36V-72V DC to 3.3V/20A Isolated Synchronous Forward Converter
52.3k
1%
5
0.1μF
T2
ISO1
5VREF MOC207
4 3 1
7
6
3.3Ω
BAT54
0.025Ω
1/2W
Q3
MURS120T3
MMBT3906LT1
MURS120T3
10Ω
VCC VBST BLKSENS TG BSTREF BG SENSE IMAX PGND 13
SG
2
LT1681
OVLO
9
1
V
SS
V
F
THERM
SYNC
SGND
5V
REF
SET
C FB
SHDN
14
C4
1.5μF
100V
L3 1mH
C1: MURATA ERIE GHM3045X7R222K-GC
C2, C3, C4: VITRAMON VJ1825Y155MXB
C5 TO C8: 330μF 10V KEMET T510X337K010AS
C26: AVX TPSE686M020R0150
ISO1: QT OPTOELECTRONICS MOC207
L1: COILCRAFT DO1608C-332
L2: PULSE P1977 PLANAR INDUCTOR
L3: COILCRAFT DO1608C-105
Q1, Q3: SILICONIX Si4486EY
Q5, Q6, Q14,Q15, Q17: FAIRCHILD FDS6680A
Q12: ZETEX ZVN3310F
Q13: ZETEX FZT690
T1: PULSE P1976 PLANAR TRANSFORMER
T2: MIDCOM 31264R
VIN
–
VIN+
t
L1
3.3μH
t
t
(408) 432-1900
For applications help,
call (408) 432-1900
dn261f_conv LT/TP 0701 305K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
© LINEAR TECHNOLOGY CORPORATION 2001
+
TRIM
S
VOUT–
C5 TO C8
330μF
10V
w4
VOUT+
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