LT1737IGN datasheet(22/28 Pages) LINER

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LT1737

1737f

TYPICAL APPLICATIO S

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 VCC 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.

ILOAD (A)

1737 F09

0.01

0.1

VIN = 8V

VIN = 15V

VIN = 25V

ILOAD (mA)

5.00

VIN = 8V

1000

1737 F10

4.75

250

500

750

1250

5.25

VIN = 15V

VIN = 25V

Figure 9. Efficiency vs ILOAD

Figure 10. Output Regulation

50mV/DIV

AC COUPLED

VIN = 15V

µs/DIV

1737 F11a

ILOAD = 900mA

20MHz BANDWIDTH LIMITED

Without L/C Filter

50mV/DIV

AC COUPLED

VIN = 15V

µs/DIV

1737 F11b

ILOAD = 900mA

20MHz BANDWIDTH LIMITED

With L/C Filter

Figure 11

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