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SC1470ITSTRT датащи(PDF) 11 Page - Semtech Corporation

номер детали SC1470ITSTRT
подробное описание детали  Synchronous Buck Pseudo Fixed Frequency Power Supply Controller
PDF  25 Pages
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производитель  SEMTECH [Semtech Corporation]
домашняя страница  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC1470ITSTRT датащи(HTML) 11 Page - Semtech Corporation

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 2005 Semtech Corp.
www.semtech.com
SC1470
POWER MANAGEMENT
V
BAT via RtON, converting this to a current. This current is
used to charge an internal 3.3pF capacitor to V
OUT. The
equations above reflect this along with any internal
components or delays that influence t
ON. For our example
we select R
tON = 1MΩ:
t
ON_VBAT(MIN) = 563ns and tON_VBAT(MAX) = 255ns
f
SW_VBAT(MIN) = 266kHz and fSW_VBAT(MAX) = 235kHz
Now that we know t
ON we can calculate suitable values
for the inductor. To do this we select an acceptable
inductor ripple current. The calculations below assume
50% of I
OUT which will give us a starting place.
() ()H
I
5
.
0
t
V
V
L
OUT
)
MIN
(
VBAT
_
ON
OUT
)
MIN
(
BAT
)
MIN
(
VBAT
•
•
−
=
and
() () H
I
5
.
0
t
V
V
L
OUT
)
MAX
(
VBAT
_
ON
OUT
)
MAX
(
BAT
)
MAX
(
VBAT
•
•
−
=
For our example:
L
VBAT(MIN) = 1.3µH and LVBAT(MAX) = 1.6µH
We will select an inductor value of 2.2µH to reduce the
ripple current, which can be calculated as follows:
()
P
P
)
MIN
(
VBAT
_
ON
OUT
)
MIN
(
BAT
)
MIN
(
VBAT
_
RIPPLE
A
L
t
V
V
I
−
•
−
=
and
()
P
P
)
MAX
(
VBAT
_
ON
OUT
)
MAX
(
BAT
)
MAX
(
VBAT
_
RIPPLE
A
L
t
V
V
I
−
•
−
=
For our example:
I
RIPPLE_VBAT(MIN) = 1.74AP-P and IRIPPLE_VBAT(MAX) = 2.18AP-P
From this we can calculate the minimum inductor
current rating for normal operation:
)
MIN
(
)
MAX
(
VBAT
_
RIPPLE
)
MAX
(
OUT
)
MIN
(
INDUCTOR
A
2
I
I
I
+
=
For our example:
I
INDUCTOR(MIN) = 7.1A(MIN)
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
ESR_ST(MAX)) and transient ESR
(R
ESR_TR(MAX)):
()
Ohms
I
2
ERR
ERR
R
)
MAX
(
VBAT
_
RIPPLE
DC
ST
)
MAX
(
ST
_
ESR
•
−
=
Where ERR
ST is the static output tolerance and ERRDC is
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
ST = 48mV and ERRDC = 24mV, therefore
R
ESR_ST(MAX) = 22mΩ
()
Ohms
2
I
I
ERR
ERR
R
)
MAX
(
VBAT
_
RIPPLE
OUT
DC
TR
)
MAX
(
TR
_
ESR


+
−
=
Where ERR
TR is the transient output tolerance. Note that
this calculation assumes that the worst case load
transient is full load. For half of full load, divide the I
OUT
term by 2.
For our example:
ERR
TR = 96mV and ERRDC = 24mV, therefore
R
ESR_TR(MAX) = 10.2mΩ for a full 6A load transient
We will select a value of 12.5m
Ω maximum for our
design, which would be achieved by using two 25m
Ω
output capacitors in parallel.
Note that for constant-on converters there is a minimum
ESR requirement for stability which can be calculated as
follows:



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