SPX1117M3 Exar Corporation, SPX1117M3 Datasheet - Page 7

• Manufacturer Part Number: SPX1117M3
• Description: 800mA Low Dropout Voltage Regulator
• Manufacturer: Exar Corporation

Thermal Characteristics
The thermal resistance of SPX1117 (SOT-223
Package) is 15°C/W from junction to tab and 31
°C/W from tab to ambient for a total of 46 °C/W
from junction to ambient (Table 1). The SPX1117
features the internal thermal limiting to protect
the device during overload conditions. Special
care needs to be taken during continuous load
conditions such that the maximum junction tem-
perature does not exceed 125 °C. Thermal pro-
tection is activated at >155°C and deactiviated
at <140 °C.
Taking the FR-4 printed circuit board and 1/16
thick with 1 ounce copper foil as an experiment
(fig.13), the PCB material is effective at trans-
mitting heat with the tab attached to the pad area
and a ground plane layer on the backside of the
substrate. Refer to table 1 for the results of the
experiment.
The thermal interaction from other components
in the application can effect the thermal resis-
tance of the SPX1117. The actual thermal
resistance can be determined with experimenta-
tion.
SPX1117 power dissipation is calculated as
follows:
Maximum Junction Temperature range:
(junction-to-ambient)
Maximum junction temperature must not ex-
ceed the 125°C.
Date: 8/25/04
Figure 19. Substrate Layout for SOT-223
T
J
= T
A
(max) + P
P
D
= (V
IN
D
- V
* thermal resistance
OUT
)(I
SPX1117 800mA Low Dropout Voltage Regulator
OUT
)
50 X 50mm
35 X 17mm
16 X 10mm
7
APPLICATION INFORMATION: Continued
Ripple Rejection
Ripple rejection can be improved by adding a
capacitor between the ADJ pin and ground as
shown in Figure 23. When ADJ pin bypassing
is used, the value of the output capacitor re-
quired increases to its maximum. If the ADJ pin
is not bypassed, the value of the output capacitor
can be lowered to 10µF for an electrolytic alu-
minum capacitor or 2.2µF for a ceramic or solid
tantalum capacitor (Fig 22).
However the value of the ADJ-bypass capacitor
should be chosen with respect to the following
equation:
Where
If an ADJ-bypass capacitor is used, the ampli-
tude of the output ripple will be independent of
the output voltage. If an ADJ-bypass capacitor
is not used, the output ripple will be proportional
to the ratio of the output voltage to the reference
voltage:
Where M = multiplier for the ripple seen when
the ADJ pin is optimally bypassed.
Ripple rejection for the adjustable version is
shown in Figure 20.
(adj.), Iload=200mA
Figure 20. Ripple Rejection; Vin=3.3V, Vout=1.8V
-10
-20
-30
-40
-50
-60
-70
-80
-90
0
C = value of the capacitor in Farads
(select an equal or larger standard value),
F
R
10
R
1
= ripple frequency in Hz,
= value of resistor R1 in Ohms.
C = 1 / ( 6.28 * F
100
M = V
V
REF
1000
Frequency, Hz
OUT
=1.25V
© Copyright 2004 Sipex Corporation
/ V
10000
R
REF
* R
1
100000
)
1000000