SC486 Semtech Corporation, SC486 Datasheet - Page 18

• Manufacturer Part Number: SC486
• Description: Complete DDR1/2/3 Memory Power Supply
• Manufacturer: Semtech Corporation

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Thermal Considerations
The junction temperature of the device may be calculated
as follows:
Where:
T
P
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:
Where:
VCCA = chip supply voltage (V)
I
VDDP = gate drive supply voltage (V)
I
V
Q
f = switching frequency (Hz)
VBST = boost pin voltage during t
D = duty cycle
VTTIN = input voltage for VTT LDO (V)
ITT = maximum VTT current (A)
P
POWER MANAGEMENT
T
VCCA
VDDP
A
JA
D
g
g
J
D
2006 Semtech Corp.
V
= ambient temperature (°C)
= gate drive voltage, typically 5V (V)
= power dissipation in (W)
VTTIN
= FET gate charge, from the FET datasheet (C)
= thermal impedance junction to ambient from
g
= operating current (A)
= gate drive operating current (A)
T
VCCA
A
Q
g
P
D
f
VTT
I
VCCA
VBST
JA
ITT
VDDP
C
1
mA
W
I
D
VDDP
ON
(V)
18
Inserting the following values for VBAT
this is the worst case condition for power dissipation in
the controller) as an example):
T
VCCA = VDDP = 5V
I
I
V
Q
f = 366kHz
VBAT
VBST
D
VDDQ = VTTIN = 1.8V
VTT = 0.9V
ITT = 1.2A
gives us:
and therefore:
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.
P
T
VCCA
VDDP
A
JA
g
g
1(MIN)
D
J
5
= 85°C
= 5V
1
= 60nC
= 29°C/W
8 .
= 2500µA (data sheet maximum)
= 150µA (data sheet maximum)
(MIN)
85
(MIN)
5
60
= 1.8/8 = 0.225
e
0
2500
= 8V
= VBAT
9 .
. 1
9
206
366
e
1
2 .
6
(MIN)
e
29
3
5
+VDDP = 13V
. 1
206
13
150
120
1
e
C
mA
W
6
. 0
225
(MIN)
condition (since
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SC486