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SC411 Scheda tecnica(PDF) 14 Page - Semtech Corporation

Il numero della parte SC411
Spiegazioni elettronici  Synchronous Buck Pseudo-Fixed Frequency Power Supply Controller
PDF  27 Pages
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Produttore elettronici  SEMTECH [Semtech Corporation]
Homepage  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC411 Scheda tecnica(HTML) 14 Page - Semtech Corporation

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© 2007 Semtech Corp.
www.semtech.com
SC411
POWER MANAGEMENT
Application Information (Cont.)
For our example:
ERR
TR = 96mV and ERRDC = 26.4mV, therefore,
R
ESR_TR(MAX) = 9.8mΩ for a full 6A load transient
We will select a value of 12.5mΩ maximum for our de-
sign, which would be achieved by using two 25mΩ output
capacitors in parallel.
Note that for constant-on converters there is a minimum
ESR requirement for stability which can be calculated as
follows:
This criteria should be checked once the output
capacitance has been determined.
Now that we know the output ESR we can calculate the
output ripple voltage:
and,
For our example:
V
RIPPLE_VBAT(MAX) = 27mVP-P and VRIPPLE_VBAT(MIN) = 22mVP-P
Note that in order for the device to regulate in a controlled
manner, the ripple content at the feedback pin, V
FB, should
be approximately 15mV
P-P at minimum VBAT, and worst
case no smaller than 10mV
P-P.
If V
RIPPLE_VBAT(MIN) is less than
15mV
P-P the above component values should be revisited
in order to improve this. Quite often a small capacitor,
C
TOP, is required in parallel with the top feedback resis-
tor, R
TOP, in order to ensure that VFB is large enough. CTOP
should not be greater than 100pF. The value of C
TOP can
be calculated as follows, where R
BOT is the bottom feed-
back resistor.
SW
OUT
)
MIN
(
ESR
f
C
2
3
R
•
•
π
•
=
Firstly calculating the value of Z
TOP required:
()Ohms
015
.
0
V
015
.
0
R
Z
)
MIN
(
VBAT
_
RIPPLE
BOT
TOP
−
•
=
Secondly calculating the value of C
TOP required to achieve
this:
For our example we will use R
TOP = 20.0kΩ and RBOT =
14.3kΩ, therefore,
Z
TOP = 6.67kΩ and CTOP = 60pF
We will select a value of C
TOP = 56pF. Calculating the
value of VFB based upon the selected C
TOP:
P
P
TOP
)
MIN
(
VBAT
_
SW
TOP
BOT
BOT
)
MIN
(
VBAT
_
RIPPLE
)
MIN
(
VBAT
_
FB
V
C
f
2
R
1
1
R
R
V
V
−
⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎝
⎛
•
•
π
•
+
+
•
=
For our example:
V
FB_VBAT(MIN) = 14.8mVP-P - good
Next we need to calculate the minimum output capaci-
tance required to ensure that the output voltage does not
exceed the transient maximum limit, POSLIM
TR, starting
from the actual static maximum, V
OUT_ST_POS, when a load
release occurs:
For our example:
V
OUT_ST_POS = 1.226V
P
P
)
MAX
(
VBAT
_
RIPPLE
ESR
)
MAX
(
VBAT
_
RIPPLE
V
I
R
V
−
•
=
P
P
)
MIN
(
VBAT
_
RIPPLE
ESR
)
MIN
(
VBAT
_
RIPPLE
V
I
R
V
−
•
=
F
f
2
R
1
Z
1
C
)
MIN
(
VBAT
_
SW
TOP
TOP
TOP
•
π
•
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
−
=
V
ERR
V
V
DC
OUT
POS
_
ST
_
OUT
+
=
V
TOL
V
POSLIM
TR
OUT
TR
•
=



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