SC411 Semtech Corporation, SC411 Datasheet - Page 14

• Manufacturer Part Number: SC411
• Description: Synchronous Buck Pseudo-Fixed Frequency Power Supply Controller
• Manufacturer: Semtech Corporation

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For our example:
ERR
R
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
Note that in order for the device to regulate in a controlled
manner, the ripple content at the feedback pin, V
be approximately 15mV
case no smaller than 10mV
15mV
in order to improve this. Quite often a small capacitor,
C
tor, R
should not be greater than 100pF. The value of C
be calculated as follows, where R
back resistor.
© 2007 Semtech Corp.
POWER MANAGEMENT
Application Information (Cont.)
RIPPLE_VBAT(MAX)
TOP
ESR_TR(MAX)
, is required in parallel with the top feedback resis-
TR
TOP
P-P
= 96mV and ERR
, in order to ensure that V
the above component values should be revisited
= 9.8mΩ for a full 6A load transient
V
V
RIPPLE
RIPPLE
= 27mV
R
_
_
ESR
VBAT
VBAT
(
MIN
(
(
MAX
MIN
P-P
)
=
)
)
DC
=
and V
=
2
P-P
R
R
•
= 26.4mV, therefore,
ESR
π
ESR
P-P
at minimum V
•
. If V
RIPPLE_VBAT(MIN)
•
C
•
I
3
I
RIPPLE
OUT
RIPPLE
FB
•
RIPPLE_VBAT(MIN)
BOT
_
f
_
is large enough. C
VBAT
SW
VBAT
is the bottom feed-
(
MIN
(
MAX
= 22mV
)
V
)
V
BAT
P
−
P
P
−
, and worst
P
is less than
FB
P-P
, should
TOP
can
TOP
14
Firstly calculating the value of Z
Secondly calculating the value of C
this:
For our example we will use R
14.3kΩ, therefore,
Z
We will select a value of C
value of VFB based upon the selected C
For our example:
V
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
from the actual static maximum, V
release occurs:
For our example:
V
POSLIM
V
TOP
FB_VBAT(MIN)
OUT_ST_POS
FB
_
VBAT
= 6.67kΩ and C
(
MIN
T R
Z
)
=
= 1.226V
T OP
=
= 14.8mV
V
RIPPLE
V
=
OUT
C
. 0
R
V
_
T OP
VBAT
015
OUT
•
BOT
TOL
(
_
MIN
=
ST
•
P-P
2
TOP
)
_
•
(
T R
POS
V
⎛
⎜ ⎜
⎝
•
⎛
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎝
- good
Z
RIPPLE
= 60pF
R
π
V
T OP
1
BOT
=
•
TOP
f
V
SW
+
_
−
OUT
VBAT
R
_
= 56pF. Calculating the
R
VBAT
TOP
1
T OP
T OP
+
1
(
MIN
TOP
ERR
(
= 20.0kΩ and R
+
MIN
)
⎞
⎟ ⎟
⎠
2
−
required:
TOP
)
•
OUT_ST_POS
R
. 0
F
π
DC
BOT
015
•
required to achieve
f
V
SW
1
TOP
_
)
VBAT
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Ohms
:
, when a load
SC411
(
MIN
)
TR
•
, starting
C
BOT
T OP
=
⎞
⎟
⎟
⎟
⎟
⎟
⎟
⎟
⎠
V
P
−
P