SC411 Semtech Corporation, SC411 Datasheet - Page 13

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

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and,
From these values of tON we can calculate the nominal
switching frequency as follows:
and,
t
V
used to charge an internal 3.3pF capacitor to V
equations above refl ect this along with any internal com-
ponents or delays that infl uence t
select R
t
fSW_VBAT(MIN)
Now that we know t
the inductor. To do this we select an acceptable inductor
ripple current. The calculations below assume 50% of I
which will give us a starting place.
and,
For our example:
We will select an inductor value of 2.2μH to reduce the
ripple current, which can be calculated as follows:
© 2007 Semtech Corp.
t
POWER MANAGEMENT
ON
ON_VBAT(MIN)
ON_VBAT(MAX)
Application Information (Cont.)
BAT
is generated by a one-shot comparator that samples
I
L
via R
L
RIPPLE_VBAT(MIN)
L
VBAT(MIN)
VBAT(MAX)
VBAT(MIN)
tON
= 3.3 10
tON
= 266kHz and f
= 563ns and t
= 1MΩ:
f
f
, converting this to a current. This current is
SW
SW
= 1.3μH and L
= V
_
_
= V
VBAT
VBAT
= V
(
(
BAT(MIN)
MIN
MAX
BAT(MAX)
-12
ON
)
)
BAT(MIN)
=
=
we can calculate suitable values for
(
(
V
Rt
V
ON_VBAT(MAX)
BAT
BAT
ON
SW_VBAT(MAX)
V
VBAT(MAX)
(
+ 37 10
MIN
(
V
MAX
OUT
OUT
V
)
OUT
)
V
•
V
•
OUT
t
ON
OUT
t
ON
= 1.6μH
= 255ns
t
t
ON
0.5 I
_
0.5 I
ON_VBAT(MIN)
ON_VBAT(MAX)
= 235kHz
3
VBAT
_
VBAT
. For our example we
t
ON_VBAT(MIN)
(
MIN
V
(
MAX
BAT(MAX)
V
)
OUT
OUT
)
OUT
)
Hz
L
)
Hz
+ 50 10
H
H
OUT
A
P-P
. The
-9
OUT
s
13
and,
For our example:
I
From this we can calculate the minimum inductor current
rating for normal operation:
For our example:
I
Next we will calculate the maximum output capacitor
equivalent series resistance (ESR). This is determined by
calculating the remaining static and transient tolerance
allowances. Then the maximum ESR is the smaller of the
calculated static ESR (R
(R
Where ERR
the DC error. The DC error will be 1.2% plus the tolerance
of the feedback resistors, thus 2.2% total for 1% feedback
resistors.
For our example:
ERR
R
Where ERR
this calculation assumes that the worst case load tran-
sient is full load. For half of full load, divide the I
by 2.
RIPPLE_VBAT(MIN)
INDUCTOR(MIN)
ESR_ST(MAX)
ESR_TR(MAX)
I
RIPPLE_VBAT(MAX)
ST
= 48mV and ERR
I
R
R
= 19.8mΩ
INDUCT OR
):
= 7.1A
ESR
ESR
ST
TR
= 1.74A
_
is the transient output tolerance. Note that
_
is the static output tolerance and ERR
T R
ST
(
= V
(
(
MAX
MIN
MAX
(MIN)
)
)
)
=
BAT(MAX)
=
P-P
=
I
OUT
⎛
⎜ ⎜
⎝
(
ERR
and I
I
OUT
DC
(
ESR_ST(MAX)
(
ERR
MAX
I
RIPPLE
= 26.4mV, therefore,
+
ST
)
RIPPLE_VBAT(MAX)
I
+
V
RIPPLE
T R
−
OUT
_
I
RIPPLE
ERR
VBAT
−
) and transient ESR
ERR
_
VBAT
2
(
MAX
_
DC
VBAT
2
t
(
)
ON_VBAT(MAX)
DC
)
MAX
•
(
)
MAX
2
)
= 2.18A
⎞
⎟ ⎟
⎠
Ohms
)
L
Ohms
A
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(
MIN
SC411
)
P-P
A
OUT
P-P
term
DC
is