Datasheet

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