Datasheet
LM2742
HG
BOOT
I
SEN
LG
PGND
FB
V
CC
SD
PWGD
FREQ
SS
SGND
EAO
PGND
+
+
V
IN
= 12V
Vo = 5V@1.8A
2 x 680 PF
16V
26 m:
47 PH, 2.7A
53 m:
R
FB2
R
FB1
C
C1
C
SS
R
FADJ
C
IN
C
BOOT
Q1
R
C2
C
C3
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
C
INX
Cox
Co1,2
680 PF
16V
1.54A
C
IN1
C
C2
R
C1
LM2742
HG
BOOT
LG
PGND
FB
SD
PWGD
FREQ
SS
SGND
EAO
PGND
+
+
+5V
V
IN
= 11 to 13V
Vo = 3.3V@3A
4.2 PH, 5.5A
15 m:
C
C1
R
FADJ
D1
Q1/Q2
1 PH, 6.4A
7.3 m:
2 x 680 PF
16V,
1.54A
LM78L05
V
IN
= 11 to 13V
To 2nd LM27x7
2.21k
0.1P
2k
2.2P
32.5k
12n
8.2p
1n
4.7n2.37k
52.3k
Co1,2
680 PF
16V, 1.54A
10 PF
16V
10k
10 PF
25V
Cox
I
SEN
V
CC
R
fb2
R
fb1
R
C1
R
cs
C
ss
C
IN
C
BOOT
C
IN1
C
INx
L
IN
C
C2
R
C2
C
C3
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.
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