SPX1117T Exar Corporation, SPX1117T Datasheet - Page 7

• Manufacturer Part Number: SPX1117T
• 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
SPX1117 features the internal thermal limiting
to protect the device during overload condi-
tions. Special care needs to be taken during
continuous load conditions such that the
maximum junction temperature does not
exceed 125 C. Thermal protection is activated
at >144 C and deactiviated at <137 C.
Taking the FR-4 printed circuit board and 1/16
thick with 1 ounce copper foil as an experi-
ment (fig.13), the PCB material is effective at
transmitting 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 compo-
nents in the application can effect the thermal
resistance of the SPX1117. The actual thermal
resistance can be determined with experimen-
tation.
SPX1117 power dissipation is calculated as
follows:
Maximum Junction Temperature range:
(junction-to-ambient)
Maximum junction temperature must not
exceed the 125 C.
Rev. 3/4/02
Figure 19. Substrate Layout for SOT-223
C/W from junction to ambient (Table 1). The
T
J
= T
A
(max) + P
P
D
= (V
IN
D
- V
* thermal resistance
OUT
)(I
OUT
)
SPX1117 800mA Low Dropout Voltage Regulator
7
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
required increases to its maximum. If the ADJ
pin is not bypassed, the value of the output
capacitor can be lowered to 10 F for an
electrolytic aluminum capacitor or 2.2 F for a
solid tantalum capacitor (Fig 22).
However the value of the ADJ-bypass capaci-
tor should be chosen with respect to the
following equation:
Where
If an ADJ-bypass capacitor is used, the
amplitude of the output ripple will be indepen-
dent 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
C = value of the capacitor in Farads
(select an equal or larger standard value),
F
R
R
1
= ripple frequency in Hz,
= value of resistor R1 in Ohms.
C = 1 / ( 6.28 * F
M = V
V
REF
OUT
=1.25V
© Copyright 2002 Sipex Corporation
/ V
R
REF
* R
1
)