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Detail Description
Output Voltage Adjustment
Like most regulators, RS1117A series regulate the output by
comparing the output voltage to an internally generated
reference voltage. On the adjustable version, the V
REF
is
available externally as 1.25V between V
OUT
and ADJ. The voltage
ratio formed by R
1
and R
2
should be set to conduct 10mA
(minimum output load). The output voltage is given by the
following equation: V
OUT
=V
REF
(1+R
2
/R
1
) + I
ADJ
R
2
On fixed versions of RS1117A series, the voltage divider is
provided internally.
Thermal Protection
RS1117A series have thermal protection which limits junction
temperature to 150°C. However, device functionality is only
guaranteed to a maximum junction temperature of +125°C.
The power dissipation and junction temperature for RS1117A in all packages given by P
D
=(V
IN
‐V
OUT
)I
OUT
,
T
JUNCTION
=T
AMBIENT
+(P
D
xθ
JA
), Note: T
JUNCTION
must not exceed 125°C
Current Limit Protection
RS1117A series are protected against overload conditions. Current protection is triggered at typically 1.5A.
Stability and Load Regulation
RS1117A series require a capacitor from V
OUT
to GND to provide
compensation feedback to the internal gain stage. This is to
ensure stability at the output terminal. Typically, a 47uF
tantalum or 100uF aluminum electrolytic is sufficient.
Note : It is important that the ESR for this capacitor does not
exceed 0.5Ω.
The output capacitor does not have a theoretical upper limit
and increasing its value will increase stability. C
OUT
=100uF or
more is typical for high current regulator design.
In
V
IN
Out
R
P
Parasitic
Line Resistance
V
IN
In
ADJ
V
REF
I
ADJ
10uA
R2
R1
Out
V
OUT
V
OUT
=V
REF
(1+R
2
/R
1
) + I
ADJ
R
2
Adj
R1
Connect
R1 to Case
RL
R2
Connect
R2 to Load
RS1117A series load regulation are limited by the resistance of
the wire connecting it to the load(R
P
). For the adjustable version, the best load regulation is accomplished when the top of
the resistor divider(R
1
) is connected directly to the output pin of the RS1117A series. When so connected, R
P
is not
multiplied by the divider ratio. For fixed output versions, the top of R
1
is internally connected to the output and ground pin
can be connected to low side of the load as a negative side sense if, so desired.
Thermal Consideration
The RS1117A series contain thermal limiting circuitry designed to protect itself for over‐temperature conditions. Even for
normal load conditions, maximum junction temperature ratings must not be exceeded. As mention in thermal protection
section, we need to consider all sources of thermal resistance between junction and ambient. It contains junction‐to‐case,
case‐to‐heat‐sink interface and heat sink resistance itself. An additional heat sink is applied externally sometimes. It can
increase the maximum power dissipation. For example, the equivalent junction temperature of 300mA output current is
115°C without external heat sink. Under the same junction temperature IC can operates 500mA with an adequate heat sink.
Therefore, to attach an extra heat sink is recommended.
DS‐RS1117A‐16
September, 2009
www.Orister.com
ORISTER [ ORISTER CORPORATION ]
