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LT1737IGN#TRPBF

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22
LT1737
1737fa
TYPICAL APPLICATIO S
U
Overall power supply efficiency and output regulation
versus input voltage and load current may be seen in
Figures 9 and 10. Available output current is a function of
input voltage, varying from 650mA with 8V input to
1100mA with 25V input.
In cases when the output switching noise is objectionable,
the optional output L/C filter shown may be added. The
oscilloscope photos in Figure 11 show the dramatic reduc-
tion in output voltage ripple with the optional filter.
Note: It is theoretically possible to extend the input voltage
range of this topology higher by raising the breakdown
voltage ratings on Q1, U2 and M1, while adjusting the
transformer windings as necessary. However this ap-
proach is generally undesirable as the relatively fixed
supply current required by the LT1737 generates more
and more wasted heat in linear regulator U2 as input
voltage is increased. The LT1725, a close “cousin” of the
LT1737 is recommended in such instances.
The LT1725 is very similar to the LT1737, but it contains
an integral wide hysteresis undervoltage lockout (UVLO)
circuit that monitors the V
CC
voltage. When used in
conjunction with a 3-winding transformer to provide both
device power and output voltage feedback information,
this allows for a “trickle charge” start-up from an input
voltage of up to hundreds of volts. The LT1725 is thus well
suited to operate from “telecom” input voltages of 48V to
72V, or even offline inputs up to several hundred volts! See
the LT1725 data sheet for further information.
I
LOAD
(A)
20
50
40
30
90
80
70
60
1737 F09
EFFICIENCY (%)
0.01
1
0.1
V
IN
= 8V
V
IN
= 15V
V
IN
= 25V
I
LOAD
(mA)
0
OUTPUT VOLTAGE (V)
5.00
V
IN
= 8V
1000
1737 F10
4.75
250
500
750
1250
5.25
V
IN
= 15V
V
IN
= 25V
Figure 9. Efficiency vs I
LOAD
Figure 10. Output Regulation
50mV/DIV
AC COUPLED
V
IN
= 15V 1µs/DIV 1737 F11a
I
LOAD
= 900mA
20MHz BANDWIDTH LIMITED
Without L/C Filter
50mV/DIV
AC COUPLED
V
IN
= 15V 1µs/DIV 1737 F11b
I
LOAD
= 900mA
20MHz BANDWIDTH LIMITED
With L/C Filter
Figure 11
23
LT1737
1737fa
TYPICAL APPLICATIO S
U
APPLICATION WITH 2-WINDING TRANSFORMER
The previous application example utilized a 3-winding
transformer, the third winding providing only feedback
information. Additional circuitry may be employed to
provide feedback information, thus allowing the trans-
former to be reduced to a 2-winding topology. (The cost
and size savings associated with the transformer often
make this a preferable alternative. Furthermore, a variety
of manufacturers offer off-the-shelf dual wound magnet-
ics which often can be applied as 1:1 transformers.)
Figure 12 shows an LT1737 configured for operation with
a dual wound toroid, the Coiltronics #CTX150-4
OCTA-PAC
TM
. A ground referred version of the flyback
voltage waveform is now provided by components Q1, R2,
R1
0.27Ω
0.5W
IRC TYPE LR 2010
M1
IRFL014
R10
5.1Ω
5%
13
4
T1
COILTRONICS
CTX150-4
V
IN
2
1737 F12
R6
100k
5%
R5
150k
5%
C5
47pF
50V
NPO
6
8
7
3
910 15
R9
33k
5%
R8
240k
5%
16
2
14 13 12 4 5 11 1
C3
1nF
25V
X7R
R7
4.7k
5%
R11
100Ω
5%
D4
1N5252
D3
MBRS1100
R4
75k
5%
C1: AVX TPS TANTALUM (TPSE226M035R0300)
C2: AVX TPS TANTALUM (TPSE336M025R0200)
D1, D3: MOTOROLA 100V, 1A SCHOTTKY DIODE
D2: SIGNAL DIODE
D4: 24V, 500mW ZENER DIODE
R3
3.01k
1%
C8
0.1µF
25V
Z5U
C4
0.1µF
25V
Z5U
C6
470pF
50V
X7R
C7
470pF
50V
X7R
•
•
D1
MBRS1100
C2
33µF
25V
+
R13
7.5k
5%
V
OUT
15V
250mA
R12
100Ω
5%
M1: INT’L RECTIFIER 60V, 0.2Ω N-CH MOSFET
D2
BAS16
Q1
2N3906
R2
35.7k
1%
C1
22µF
35V
+
PGND
I
SENSE
GATE
V
C
FB
V
CC
UVLO3V
OUT
SGNDR
CMPC
R
OCMP
MINENAB
LT1737
ENDLYSFSTOSCAP t
ON
Figure 12. 12V-18V to Isolated 15V Converter
I
LOAD
(mA)
1
60
EFFICIENCY (%)
70
80
90
10 100 1000
1737 F13
50
40
30
20
V
IN
= 12V
V
IN
= 15V
V
IN
= 18V
Figure 13. Efficiency vs I
LOAD
I
LOAD
(mA)
0
OUTPUT VOLTAGE (V)
15.0
V
IN
= 12V
1737 F14
14.5
100
200
300
15.5
V
IN
= 15V
V
IN
= 18V
Figure 14. Load Regulation
OCTA-PAC is a trademark of Coiltronics, Inc.
24
LT1737
1737fa
TYPICAL APPLICATIO S
U
R3 and D2. (Diode D2 prevents reverse emitter/base
breakdown in Q1 when MOSFET M1 is in the “ON” state.)
The raw flyback voltage at the drain of MOSFET M1 minus
the V
BE
of Q1 is converted to a current by R2 and then back
to a voltage at R3. Or, stated mathematically:
VV V
R
R
FB FLBK BE
=
()
⎛
⎝
⎜
⎞
⎠
⎟
–
3
2
Resistor R13 provides an initial pre-load to the supply
output to improve light load regulation. Resistor divider
R8/R9 sets the undervoltage lockout threshold at nomi-
nally 10.4V for turn-on, with turn-off about 600mV lower.
Overall power supply efficiency and output regulation
versus input voltage and load current may be seen in
Figures 13 and 14.
5V
IN
APPLICATION
The LT1737 is a bipolar technology IC specified to operate
down to a minimum input supply voltage of 4.5V. Although
its GATE pin drives “low” nearly to ground, its “high”
capability is limited by a headroom requirement of roughly
2V
BE
s. Thus when operating at a worst case 4.5V supply,
the GATE output will only drive up to a nominal 3V or so.
Fortunately, MOSFETs are now available with specified
performance at this level of gate voltage.
The circuit shown in Figure 15 provides an isolated 5V
output from an input between 4.5V and 5.5V. Two Si9804
low gate voltage MOSFETs are paralleled to handle the
primary-side current—up to 12A peak. This circuit pro-
vides more output power than the previous examples. It
PGND
I
SENSE
GATE
V
C
FB
R1
0.02Ω
M1
SI9804
10 4
9
T1
COILTRONICS
VP5-0083
V
IN
3
V
CC
UVLO3V
OUT
SGND
1737 F15
R
CMPC
R
OCMP
MINENAB
LT1737
R6
51k
5%
R5
51k
5%
C5
47pF
50V
NPO
6
8
7
3
910 15
16
2
14 13 12 4 5 11 1
ENDLYSFSTOSCAP t
ON
C3
1nF
25V
X7R
R7
2.2k
5%
R8
10Ω
5%
D4
1N5240
D3
MBR0520
R4
75k
5%
C1A-B, C2A-B: SANYO POSCAP (10TPB220M)
D1: MOTOROLA 35V, 8A SCHOTTKY DIODE
D2: SIGNAL DIODE
D3: MOTOROLA 20V, 0.5A SCHOTTKY DIODE
D4: 10V, 500mW ZENER DIODE
M1, M2: SILICONIX/VISHAY 25V, 0.023Ω N-CH MOSFET
R1: 5 × 0.10Ω, 1W (IRC LR2512)
T1: COILTRONICS TRANSFORMER
R3
3.01k
1%
C8
1µF
25V
Z5U
C4
0.1µF
25V
Z5U
C9
1nF
25V
X7R
R11
51Ω
5%
11
2
12
1
M2
SI9804
C6
4.7nF
50V
X7R
C7
2.2nF
50V
X7R
•••
•
5
8
•
6
7
•
D1
MBRD835L
C2A
220µF
10V
+
C2B
220µF
10V
+
R10
270Ω
5%
V
OUT
5V
3A
R9
15Ω
5%
D2
BAS16
Q1
2N3906
R2
11.0k
1%
C1A
220µF
10V
+
C1B
220µF
10V
+
Figure 15. 4.5V-5.5V to Isolated 5V Converter
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LT1737IGN#TRPBF

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Buy LT1737IGN#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Hi Pwr Iso Fly Cntr
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