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SC470ITSTRT Datasheet(PDF) 16 Page - Semtech Corporation

Part # SC470ITSTRT
Description  Synchronous Buck Controller for Dynamic Load-Voltage Applications
PDF  31 Pages
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Manufacturer  SEMTECH [Semtech Corporation]
Direct Link  http://www.semtech.com
Logo SEMTECH - Semtech Corporation

SC470ITSTRT Datasheet(HTML) 16 Page - Semtech Corporation

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 2005 Semtech Corp.
www.semtech.com
SC470
POWER MANAGEMENT
Application Information (Cont.)
Figure 7 below shows recommended components that
work well.
Figure 7: Recommended Component Values
Please see the example switching waveforms on Pages
25 and 26.
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 (For TSSOP-14)
θ
JA = 46°C/W (For MLPQ-16)
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
=
•
•
•
+
•
•
•
•
+
•
•
+
•
•
=
−
−
−
−
So for TSSOP-14,
C
8
.
93
100
088
.
0
85
T
J
°
=
•
+
=
And for MLPQ-16,
C
0
.
89
46
088
.
0
85
TJ
°
=
•
+
=
As can be seen, the heating effects due to internal power
dissipation are practically negligible, thus requiring no
special consideration thermally during layout.
Q3
R8
10k
C11
22n
R9
1k
VGATE
VPULLUP



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