SC1456 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
Manufacturer
SC1456 Datasheet PDF : 10 Pages
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POWER MANAGEMENT
Applications Information (Cont.)
VIN1
VIN2
(OR VIN1)
C1
C2
1uF
1uF
U1
10 IN1
1 EN1
5 IN2
6 EN2
SC1456
OUT1 9
OUT2 4
SET1 3
SET2 8
GND GND
2
7
SC1456
VOUT1 (SEE EQUATION)
VOUT2 = PRESET
R1
C3
C4
2.2uF
1uF
R2
Figure 1: Output 1 Set Externally and Output 2 Set Internally
Thermal Considerations
The worst-case power dissipation for this part is given
by:
P = (V − V )• I + V • I D(MAX)
IN1(MAX )
OUT1(MIN)
OUT1(MAX )
IN1(MAX ) Q1(MAX)
( ) + V − V • I IN2(MAX)
OUT 2( MIN )
OUT 2(MAX)
+ V • I IN2(MAX) Q2(MAX)
For all practical purposes, it can be reduced to:
( ) P = V − V • I D(MAX)
IN1( MAX )
OUT1(MIN)
OUT1(MAX )
( ) + V − V • I IN2(MAX)
OUT 2( MIN )
OUT 2(MAX )
Looking at a typical application:
VIN1(MAX) = VIN2(MAX) = 4.2V
VOUT1(MIN) = VOUT2(MIN) = (2.8V - 3.5%) = 2.702V worst-case
IOUT1 = IOUT2 = 100mA
TA = 85°C
This gives us:
With the standard MSOP-10 Land Pattern shown at the
end of this datasheet, and minimum trace widths, the
thermal impedance junction to ambient for SC1456 is
113°C/W. Thus with no additional heatsinking,
TJ(MAX) = 119°C.
The junction temperature can be reduced further by the
use of larger trace widths, and connecting pcb copper
area to the GND pins (pins 2 and 7), which connect
directly to the device substrate. Lower junction
temperatures improve overall output voltage accuracy.
Layout Considerations
While layout for linear devices is generally not as critical
as for a switching application, careful attention to detail
will ensure reliable operation.
1) Attaching the part to a larger copper footprint will
enable better heat transfer from the device, especially
on PCBs where there are internal ground and power
planes.
2) Place the input and output capacitors close to the
device for optimal transient response and device
behavior.
PD(MAX) = 2 • (4.2 − 2.702)• 0.100 = 300mW
Using this figure, we can calculate the maximum thermal
impedance allowable to maintain TJ ≤ 125°C:
3) Connect all ground connections directly to the ground
plane. If there is no ground plane, connect to a common
local ground point before connecting to board ground.
( ) ( ) θ = (J−A)(MAX)
T − T J(MAX)
A(MAX )
PD( MAX )
=
125 − 85
0.300
= 133°C / W
2005 Semtech Corp.
9
www.semtech.com
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