
8
2006 Semtech Corp.
www.semtech.com
SC1452
POWER MANAGEMENT
Applications Information (Cont.)
less
than 150ms, C
can be calculated using the
equation above, or read from the chart below.
0.01
0.1
1
10
100
1000
0.1
1
10
100
1000
C
DLYB
(nF)
t
R
t
RSTB
= 150ms max.
Component Selection
Output capacitor - Semtech recommends a minimum
capacitance of 1μF at the output with an equivalent
series resistance (ESR) of < 1
over temperature. The
SC1452 has been designed to be used with ceramic
capacitors, but does not have to be used with ceramic
capacitors, allowing the designer a choice. Increasing the
bulk capacitance will further reduce output noise and
improve the overall transient response.
Input capacitor - Semtech recommends the use of a 1μF
ceramic capacitor at the input. This allows for the device
being some distance from any bulk capacitance on the
rail. Additionally, input droop due to load transients is
reduced, improving overall load transient response.
Bypass capacitor - Semtech recommends the use of a
10nF ceramic capacitor to bypass the bandgap
reference. Increasing this capacitor to 100nF will
further improve power supply rejection. C
BYP
may be
omitted if low noise operation is not required.
Thermal Considerations
The worst-case power dissipation for this part is given
by:
(1)
I
V
+
For all practical purposes, equation (1) can be reduced
to the following expression:
)
MIN
(
OUTB
)
MAX
(
IN
V
V
+
(2)
Looking at a typical application:
V
IN(MAX)
= 4.2V
V
OUTA
V
OUTB
= 3.3V - 2% (worst case) = 3.234V
I
OUTA
= I
OUTB
= 150mA
T
Inserting these values into equation (2) above gives us:
Using this figure, we can calculate the maximum thermal
impedance allowable to maintain T
J
≤
125°C:
(
P
)
MAX
(
D
=
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 SC1452 is
113°C/W. Thus no additional heatsinking is required for
the above conditions. The junction temperature can be
further reduced by using larger trace widths and
connecting pcb copper area to the GND pin (pin 3), which
connects 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, output and bypass capacitors close
to the device for optimal transient response and device
behaviour.
(
)
)
MAX
(
Q
)
MAX
(
IN
)
MAX
(
OUTA
I
)
MIN
(
OUTA
)
MAX
(
IN
)
MAX
(
D
V
V
P
=
)
MAX
(
OUTB
I
MAX
OUTA
MIN
OUTA
MAX
IN
MAX
D
(
2
)
(
2
)
W
334
.
145
.
189
.
15
.
234
.
15
.
94
.
+
P
)
MAX
(
D
=
=
+
=
)
(
)
W
/
C
120
334
°
.
85
125
T
T
)
MAX
(
A
)
MAX
(
)
MAX
(
JA
=
=
θ