LM3670_15 Datasheet, PDF (16/25 Pages) Texas Instruments – LM3670 Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits

LM3670
SNVS250E – NOVEMBER 2004 – REVISED FEBRUARY 2013
VOUT (V)
0.7
0.8
0.9
1.0
1.1
1.2
1.24
1.24
1.5
1.6
1.7
1.875
2.5
Table 3. Adjustable LM3670 Configurations for Various V_OUT
R1 (KΩ)
R2 (KΩ)
C1 (pF)
C2 (pF)
L (µH)
CIN (µF)
80.6
200
200
150
4.7
4.7
120
200
130
none
4.7
4.7
160
200
100
none
4.7
4.7
200
200
82
none
4.7
4.7
240
200
68
none
4.7
4.7
280
200
56
none
4.7
4.7
300
200
56
none
4.7
4.7
221
150
75
120
4.7
4.7
402
200
39
none
10
4.7
442
200
39
none
10
4.7
487
200
33
none
10
4.7
549
200
30
none
10
4.7
806
200
22
82
10
4.7
(1) (10 || 4.7)
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COUT (µF)
10
10
10
10
10
10
10
10
10
10
10
14.7 ⁽¹⁾
22
Board Layout Considerations
PC board layout is an important part of DC-DC converter design. Poor board layout can disrupt the performance
of a DC-DC converter and surrounding circuitry by contributing to EMI, ground bounce, and resistive voltage loss
in the traces. These can send erroneous signals to the DC-DC converter IC, resulting in poor regulation or
instability.
The light shaded area is the top surface ground. C_OUT, C_IN, Feedback R
and C grounds all come to this area which is as far away from the SW pin
as possible to avoid the noise created at the SW pin.
Note that the top and bottom GND sides are kept away from the SW pin to
avoid picking up noise from the SW pin which swings from GND to V_IN.
EN
POST
PIN
EN post pin is connected to EN with a bottom side trace to
maintain unbroken ground plane on top of board
GND
As many through holes
as possible here to
connect the top and
bottom ground planes
G
EN
ND
Bottom surface - the darker
R2_fb
C2_fb
shaded area is all GND EXCEPT
for area around SW to avoid
C_OUT
FB
S
picking up switch noise.
W
R1_fb
C1_fb
EN,GND,V_IN,FB,SW are
the pads for the SOT-23-5
package
V
IN
C_IN
The V_IN, SW, V_OUT traces,
C_IN, C_OUT traces & pads
should be thick - they are
high current paths
SW node is switching
between V_IN and GND at
1 MHz - VERY NOISY! -
keep all GNDs and GND
planes away!
V
OUT
If possible put the feedback Rs and Cs on the back side so the C_OUT
GND can move closer to the IC GND
L1
Figure 25. Board Layout Design Rules for the LM3670
16
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