LM2742

HG

BOOT

LG

PGND

FB

SD

PWGD

FREQ

SS

SGND

EAO

PGND

+

+

Vo = 5V@1.8A

2 x 680 PF

16V

26 m:

47 PH, 2.7A

53 m:

Q1

10 PF

16V

10 PF

6.3V

+5V (low current source)

D1

D2

10k

1.37k

22n

56p

3.9n

61.9k

750

12n

267k

0.1P

2.2P

L1

Cox

Co1,2

680 PF

16V

1.54A

LM2742

HG

BOOT

LG

PGND

FB

SD

PWGD

FREQ

SS

SGND

EAO

PGND

+

+

+5V

Vo = 3.3V@3A

4.2 PH, 5.5A

15 m:

D1

Q1/Q2

1 PH, 6.4A

7.3 m:

2 x 680 PF

16V,

1.54A

LM78L05

To 2nd LM27x7

2.21k

0.1P

2k

2.2P

32.5k

12n

8.2p

1n

4.7n

2.37k

52.3k

Co1,2

680 PF

16V, 1.54A

10 PF

16V

10k

10 PF

25V

Cox

LM2742

www.ti.com

SNVS266C – MARCH 2004 – REVISED MARCH 2013

The circuits in Figure 35 are intended for ADSL applications, where the high switching frequency keeps noise out

of the data transmission range. In this design, the 1.8 and 3.3V outputs come up simultaneously by using the

same softstart capacitor. Because two current sources now charge the same capacitor, the capacitance must be

doubled to achieve the same softstart time. (Here, 40nF is used to achieve a 5ms softstart time.) A common

softstart capacitor means that, should one circuit enter current limit, the other circuit will also enter current limit.

The additional compensation components Rc2 and Cc3 are needed for the low ESR, all ceramic output

capacitors, and the wide (3x) range of Vin.

Figure 36. 12V Unregulated to 3.3V, 3A, 750kHz

This circuit shows the LM2742 paired with a cost effective solution to provide the 5V chip power supply, using no

extra components other than the LM78L05 regulator itself. The input voltage comes from a 'brick' power supply

which does not regulate the 12V line tightly. Additional, inexpensive 10uF ceramic capacitors (Cinx and Cox)

help isolate devices with sensitive databands, such as DSL and cable modems, from switching noise and

harmonics.

Figure 37. 12V to 5V, 1.8A, 100kHz

In situations where low cost is very important, the LM2742 can also be used as an asynchronous controller, as

shown in the above circuit. Although a a schottky diode in place of the bottom FET will not be as efficient, it will

cost much less than the FET. The 5V at low current needed to run the LM2742 could come from a zener diode or

inexpensive regulator, such as the one shown in Figure 36. Because the LM2742 senses current in the low side

MOSFET, the current limit feature will not function in an asynchronous design. The ISEN pin should be left open

in this case.

Copyright © 2004–2013, Texas Instruments Incorporated

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