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Applications Description
•
Output Voltage Adjustment
Like most regulators, RS1117 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 RS1117 series, the voltage divider is provided
internally.
•
Thermal Protection
RS1117 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 RS1117 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
RS1117 series are protected against overload conditions. Current protection is triggered at typically 1.5A.
•
Stability And Load Regulation
RS1117 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
= 100 uF or more is
typical for high current regulator design.
RS1117 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 RS1117
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.
V
IN
V
IN
In
ADJ
Out
V
REF
I
ADJ
10uA
R2
R1
V
OUT
V
OUT
=V
REF
(1+R
2
/R
1
) + I
ADJ
R
2
In
Adj
Out
R
P
Parasitic
Line Resistance
R1
Connect
R1 to Case
RL
R2
Connect
R2 to Load
•
Thermal Consideration
The RS1117 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.
Junction-to-case thermal resistance is specified from the IC junction to the bottom of the case directly below the die. The bonding
wires are appending paths. The former is the lowest resistance path. Proper mounting is required to ensure the best possible thermal
flow this area of the package to the heat sink. Thermal compound at the case-to-heat-sink interface is strongly recommended. The
case of all devices in this series is electrically connected to the output. Therefore, if the case of the device must be electrically
isolated, a thermally conductive spacer can be used, as long its thermal resistance is considered.
•
Protection Diode
(The figure is shown as Regulator with Reverse Diode Protection in advanced applications)
In general operation, RS1117 series don’t need any protection diodes. From the cross-section structure of RS1117 series, the
output pin is connected to P+ substrate, and the input pin is connected to N- well. There is a parasitic reverse diode between them. It
can handle microsecond surge currents of 5A to 10A. Even with large output capacitance, it is very difficult to get those values of
surge currents in normal operation. Only with high value output capacitors, such as 1000uF. And with the input pin instantaneously
shorted to ground, can damage occur. A crowbar circuit at the input of the RS1117 series can generate those kinds of currents, and a
diode from output to input is recommended. Normal power supply cycling or even plugging and unplugging in the system will not
generate currents large enough to do any damage.
DS-RS1117-04
Dec, 2007
www.Orister.com
ORISTER [ ORISTER CORPORATION ]
