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SC1470ITSTRT датащи(PDF) 13 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) 13 Page - Semtech Corporation

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 2005 Semtech Corp.
www.semtech.com
SC1470
POWER MANAGEMENT
660µF may be used. Alternatively, one 15m
Ω or 12mΩ
220µF, 330µF or 470µF capacitor may be used (with
the appropriate change to the calculation for C
TOP),
depending upon the load transient requirements.
Next we calculate the RMS input ripple current, which is
largest at the minimum battery voltage:
()
RMS
MIN
_
BAT
OUT
OUT
)
MIN
(
BAT
OUT
)
RMS
(
IN
A
V
I
V
V
V
I
•
−
•
=
For our example:
I
IN(RMS) = 2.14ARMS
Input capacitors should be selected with sufficient ripple
current rating for this RMS current, for example a 10µF,
1210 size, 25V ceramic capacitor can handle
approximately 3A
RMS. Refer to manufacturer’s data
sheets.
Finally, we calculate the current limit resistor value. As
described in the current limit section, the current limit
looks at the “valley current”, which is the average output
current minus half the ripple current. We use the
maximum room temperature specification for MOSFET
R
DS(ON) at VGS = 4.5V for purposes of this calculation:
A
2
I
I
I
)
MIN
(
VBAT
_
RIPPLE
OUT
VALLEY
−
=
The ripple at low battery voltage is used because we want
to make sure that current limit does not occur under
normal operating conditions.
()
Ohms
10
10
4
.
1
R
2
.
1
I
R
6
)
ON
(
DS
VALLEY
ILIM
−
•
•
•
•
=
For our example:
I
VALLEY = 5.13A, RDS(ON) = 9mΩ and RILIM = 7.76kΩ
We select the next lowest 1% resistor value: 7.68k
Ω
Thermal Considerations
The junction temperature of the device may be
calculated as follows:
C
P
T
T
JA
D
A
J
°
θ
•
+
=
Where:
T
A = ambient temperature (°C)
P
D = power dissipation in (W)
θ
JA = thermal impedance junction to ambient from
absolute maximum ratings (°C/W)
The power dissipation may be calculated as follows:
W
D
mA
1
VBST
f
Q
V
I
VDDP
I
VCCA
P
g
g
VDDP
VCCA
D
•
•
+
•
•
+
•
+
•
=
Where:
VCCA = chip supply voltage (V)
I
VCCA = operating current (A)
VDDP = gate drive supply voltage (V)
I
VDDP = gate drive operating current (A)
V
g = gate drive voltage, typically 5V (V)
Q
g = FET gate charge, from the FET datasheet (C)
f = switching frequency (kHz)
VBST = boost pin voltage during t
ON (V)
D = duty cycle
Inserting the following values for VBAT
(MIN) condition (since
this is the worst case condition for power dissipation in
the controller) as an example (VOUT = 1.2V):
T
A = 85°C
θ
JA = 100°C/W
VCCA = VDDP = 5V
I
VCCA = 1100µA (data sheet maximum)
I
VDDP = 150µA (data sheet maximum)
V
g = 5V
Q
g = 60nC
f = 266kHz
VBAT
(MIN) = 8V
VBST
(MIN) = VBAT(MIN)+VDDP = 13V
D
(MIN) = 1.2/8 = 0.15
gives us:
W
088
.
0
15
.
0
10
1
13
10
266
10
60
5
10
150
5
10
1100
5
P
3
3
9
6
6
D
=
•
•
•
+
•
•
•
•
+
•
•
+
•
•
=
−
−
−
−
and:
C
8
.
93
100
088
.
0
85
T
J
°
=
•
+
=



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