Datasheet

LM27x7
HG
BOOT
ISEN
LG
PGND
FB
Vcc
SD
PWGD
FREQ
SS
SGND
EAO
PGND
+
+
Vin = 12V
Vo = 5V@1.8A
2 x 680uF
16V
26 m:
47uH 2.7A
53 m:
Rfb2
Rfb1
Cc1
Cc2 Rc1
Css
Rfadj
Cin
Cboot
Q1
Rc2 Cc3
10uF
16V
10uF
6.3V
+5V (low current source)
D1
D2
10k
1.37k
22n
56p
3.9n 61.9k
750
12n
267k
0.1u
2.2u
L1
Cinx
Cox
Co1,2
680uF
16V
1.54A
Cin1
LM27x7
HG
BOOT
ISEN
LG
PGND
FB
Vcc
SD
PWGD
FREQ
SS
SGND
EAO
PGND
+
+
+5V
Vin = 11 to 13V
Vo = 3.3V@3A
4.2uH, 5.5A
15 m:
Rfb1
Cc1
Cc2 Rc1
Rcs
Css
Rfadj
Cin
D1
Cboot
Q1/Q2
1uH, 6.4A
7.3 m:
Rc2
Cc3
2 x 680uF
16V 1.54A
LM78L05
Vin = 11 to 13V
To 2nd LM27x7
2.21k
0.1u
2k
2.2u
32.5k
12n
8.2p
1n
4.7n2.37k
52.3k
Lin
Co1,2
680uF
16V, 1.54A
Cin1
10uF
16V
Cinx
Rfb2
10k
10uF
25V
Cox
LM2727, LM2737
www.ti.com SNVS205D –AUGUST 2002–REVISED MARCH 2013
Figure 30. 12V Unregulated to 3.3V, 3A, 750kHz
This circuit shows the LM27x7 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 31. 12V to 5V, 1.8A, 100kHz
In situations where low cost is very important, the LM27x7 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 LM27x7 could come from a zener diode or
inexpensive regulator, such as the one shown in Figure 30. Because the LM27x7 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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Product Folder Links: LM2727 LM2737