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參數(shù)資料
| 型號： | SC486 |
| 廠商： | Semtech Corporation |
| 英文描述： | Complete DDR1/2/3 Memory Power Supply |
| 中文描述： | 完全DDR1/2/3內(nèi)存供電 |
| 文件頁數(shù)： | 18/26頁 |
| 文件大小： | 382K |
| 代理商： | SC486 |
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18
2006 Semtech Corp.
www.semtech.com
SC486
POWER MANAGEMENT
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
= thermal impedance junction to ambient from
absolute maximum ratings (°C/W)
The power dissipation may be calculated as follows,
assuming that VTT spends 50% of its time sourcing
current and 50% sinking:
(
)
W
ITT
VTT
VTTIN
D
mA
1
VBST
f
Q
V
I
VDDP
I
VCCA
P
+
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 (Hz)
VBST = boost pin voltage during t
ON
(V)
D = duty cycle
VTTIN = input voltage for VTT LDO (V)
ITT = maximum VTT current (A)
Inserting the following values for VBAT
condition (since
this is the worst case condition for power dissipation in
the controller) as an example):
T
A
= 85°C
θ
= 29°C/W
VCCA = VDDP = 5V
I
VCCA
= 2500μA (data sheet maximum)
I
VDDP
= 150μA (data sheet maximum)
V
g
Q
= 60nC
f = 366kHz
VBAT
(MIN)
= 8V
VBST
(MIN)
= VBAT
(MIN)
+VDDP = 13V
D
VDDQ = VTTIN = 1.8V
VTT = 0.9V
ITT = 1.2A
gives us:
(
8
)
W
206
.
2
9
225
.
mA
1
13
e
=
366
e
60
5
e
150
5
e
2500
5
P
+
3
9
6
6
D
+
+
+
=
and therefore:
=
C
120
29
206
.
+
85
T
J
°
=
As can be seen, the heating effects due to internal power
dissipation are dominated by the VTT LDO, but they can
be managed comfortably by the MLPQ-24 package which
is heatsunk to the ground plane using 4 vias from its
thermal pad.
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