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參數(shù)資料
| 型號(hào)： | SC483 |
| 廠商： | Semtech Corporation |
| 英文描述： | Dual Synchronous Buck Pseudo Fixed Frequency Power Supply Controller |
| 中文描述： | 雙同步降壓偽固定頻率電源控制器 |
| 文件頁(yè)數(shù)： | 14/27頁(yè) |
| 文件大小： | 478K |
| 代理商： | SC483 |
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14
2005 Semtech Corp.
www.semtech.com
SC483
POWER MANAGEMENT
For our example:
C
OUT(MIN)
= 610μF.
We will select 440μF, using two 220μF, 25m
capacitors in parallel. For smaller load release overshoot,
660μF may be used.
Next we calculate the RMS input ripple current, which is
largest at the minimum battery voltage:
(
)
RMS
MIN
_
BAT
OUT
I
OUT
)
MIN
(
BAT
OUT
)
RMS
(
IN
I
A
V
V
V
V
=
For our example:
I
IN(RMS)
= 2.14A
RMS
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
. Refer to manufacturer’s data
sheets and derate appropriately.
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 V
GS
= 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
R
2
I
R
6
)
ON
(
DS
VALLEY
ILIM
=
For our example:
I
VALLEY
= 5.13A, R
DS(ON)
= 9m
and R
ILIM
= 7.76k
We select the next lowest 1% resistor value: 7.68k
Thermal Considerations
The junction temperature of the device may be calcu-
lated as follows:
C
P
T
T
JA
D
A
J
°
θ
+
=
Where:
T
A
= ambient temperature (°C)
P
D
= power dissipation in (W)
θ
= 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
D
mA
1
VBST
f
Q
V
I
VDDP
I
VCCA
2
P
+
2
2
2
g
g
1
1
1
g
g
VDDP
VCCA
D
+
+
+
+
=
Where:
VCCA = chip supply voltage (V)
I
= operating current (A)
VDDP = gate drive supply voltage (V)
I
VDDP
= gate drive operating current (A)
V
g
Q
= FET gate charge, from the FET datasheet (C)
f
= switching frequency (kHz)
VBST = boost pin voltage during t
ON
(V)
D
x
= duty cycle
Inserting the following values for VBAT
condition (since
this is the worst case condition for power dissipation in
the controller) as an example (OUT1 = 1.5V, OUT2 = 1.2V):
T
A
= 85°C
θ
= 84°C/W
VCCA = VDDP = 5V
I
VCCA
= 1100μA (data sheet maximum)
I
VDDP
= 150μA (data sheet maximum)
V
g
Q
= 60nC
f
1
= 250kHz
f
= 300kHz
VBAT
(MIN)
= 8V
VBST
(MIN)
= VBAT
(MIN)
+VDDP = 13V
D
1(MIN)
D
2(MIN)
= 1.2/8 = 0.15
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相關(guān)代理商/技術(shù)參數(shù) |
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| SC486EVB | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:Complete DDR1/2/3 Memory Power Supply |
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