sc483itstrt Semtech Corporation, sc483itstrt Datasheet - Page 12

• Manufacturer Part Number: sc483itstrt
• Description: Sc483 Dual Synchronous Buck Pseudo-fixed-frequency Power-supply Controller
• Manufacturer: Semtech Corporation

t
V
used to charge an internal 3.3pF capacitor to V
equations above reflect this along with any internal
components or delays that influence t
we select R
t
f
Now that we know t
for the inductor. To do this we select an acceptable
inductor ripple current. The calculations below assume
50% of I
and
For our example:
L
We will select an inductor value of 2.2µH to reduce the
ripple current, which can be calculated as follows:
and
For our example:
I
L
L
I
I
POWER MANAGEMENT
f
RIPPLE_VBAT(MIN)
ON
ON_VBAT(MIN)
SW_VBAT(MIN)
RIPPLE
RIPPLE
SW
VBAT(MIN)
BAT
VBAT
VBAT
2005 Semtech Corp.
is generated by a one-shot comparator that samples
_
via R
VBAT
(
(
_
_
MIN
MAX
VBAT
VBAT
(
)
= 1.3µH and L
MAX
OUT
)
tON
(
(
= 563ns and t
= 266kHz and f
MIN
MAX
)
, converting this to a current. This current is
which will give us a starting place.
tON
V
V
= 1.74A
)
BAT
)
BAT
= 1M :
V
(
MIN
(
BAT
V
MAX
V
BAT
BAT
)
(
MAX
)
ON
P-P
(
MIN
(
MAX
V
)
V
VBAT(MAX)
we can calculate suitable values
OUT
and I
)
V
OUT
)
OUT
t
ON_VBAT(MAX)
ON
V
SW_VBAT(MAX)
V
OUT
_
RIPPLE_VBAT(MAX)
t
OUT
VBAT
ON
= 1.6µH
0
t
ON
5 .
0
_
(
5 .
VBAT
MAX
_
t
VBAT
ON
t
= 255ns
I
ON
OUT
)
(
I
= 235kHz
_
MIN
OUT
ON
(
Hz
VBAT
_
MAX
L
VBAT
. For our example
)
L
H
= 2.18A
)
(
MIN
H
(
MAX
)
A
)
A
P
P
P
OUT
P-P
P
. The
12
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% plus the tolerance
of the feedback resistors, thus 2% total for 1%
feedback resistors.
For our example:
ERR
R
Where ERR
this calculation assumes that the worst case load
transient is full load. For half of full load, divide the I
term by 2.
For our example:
ERR
R
I
R
R
INDUCTOR(MIN)
INDUCTOR
ESR_TR(MAX)
ESR_ST(MAX)
ESR
ESR
ESR_TR(MAX)
ST
TR
_
_
ST
TR
= 48mV and ERR
= 96mV and ERR
(
(
(
MAX
MIN
MAX
= 10.2m
= 22m
)
):
)
= 7.1A
)
TR
ST
I
is the transient output tolerance. Note that
is the static output tolerance and ERR
OUT
ERR
I
OUT
(
ERR
MAX
(MIN)
I
RIPPLE
ST
)
I
for a full 6A load transient
RIPPLE
TR
_
I
RIPPLE
ERR
VBAT
DC
DC
ERR
ESR_ST(MAX)
= 24mV, therefore
_
= 24mV, therefore
(
VBAT
2
MAX
_
DC
VBAT
2
)
(
DC
MAX
(
MAX
2
)
) and transient ESR
Ohms
)
Ohms
A
(
MIN
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)
SC483
DC
OUT
is