LT1118 Linear Technology, LT1118 Datasheet - Page 5

• Manufacturer Part Number: LT1118
• Description: Low IQ/ Low Dropout/ 800mA/ Source and Sink Regulators Adjustable and Fixed 2.5V/ 2.85V/ 5V Output
• Manufacturer: Linear Technology

IN: Input Supply Pin. This pin should be decoupled with a
1 F or larger low ESR capacitor. The two IN pins on the
SO-8 package must be directly connected on the printed
circuit board to prevent voltage drops between the two
inputs. When used as a SCSI active termination, IN con-
nects to term power. When used as a supply splitter, IN is
also the positive supply output.
GND: Ground Pin. The multiple GND pins on the SO-8
package are internally connected, but lowest load regula-
tion errors will result if these pins are tightly connected on
the printed circuit board. This will also aid heat dissipation
at high power levels.
EN: TTL/CMOS Logic Input. A high level allows normal
operation. A low level reduces supply current to zero. This
pin is internally connected to V
devices.
Output Capacitor
The LT1118 family of voltage regulators require an output
capacitor for stability. The regulators are stable with
output capacitors larger than 0.2 F. Load transient re-
sponse is very sensitive to output capacitor value and
dielectric choice. Low ESR ceramic capacitors are the best
choice for fast settling to load transients. Output capaci-
tors between 0.2 F and 1 F give good performance in
most applications. Larger tantalum output capacitors may
be used to reduce load transient amplitudes. Larger ca-
pacitors should be paralleled with a 0.1 F ceramic capaci-
tor to maintain quick settling time performance and to
insure stability of the regulator control loop. Capacitors
larger than 10 F may be safely used with the regulators,
but little transient settling performance improvement re-
sults from their use.
PIN
OPERATIO
U
FUNCTIONS
U
U
U
IN
on 3-lead ST packaged
OUT: Regulated Output Voltage. Output can source or sink
current. Current limit for sourcing and sinking current is
provided to protect the device from fault conditons. The
output must have a low ESR output filter capacitor. C
may be needed in parallel with C
C
FB: Feedback Pin. The feedback pin is used to program the
output voltage of the adjustable S8 part. The output
voltage is determined by the selection of feedback resis-
tors defined by the following formula:
Worst case transient response of the LT1118 regulators
occurs at zero output current. In order to obtain the fastest
transient settling performance, the regulator output may
be preloaded with a small current. The preload current
reduces the regulator output impedance, reducing output
voltage settling time.
Thermal Considerations
The LT1118 regulators are packaged in plastic SOT-223
and fused lead S8 packages. These small footprint pack-
ages are unable to convectively dissipate the heat gener-
ated when the regulator is operating at high current levels.
In order to control die operating temperatures, the PC
board layout should allow for maximum possible copper
area at the ground pins of the LT1118. The ground pins
and the tab on the SOT-223 package are electrically and
V
OUT
1 F
IN
0.22 F to guarantee stability. A 0.1 F ceramic capacitor
V
OUT
is too high.
= V
FB
IN
EN
LT1118-2.85/LT1118-5
(1 + R
LT1118
GND
LT1118/LT1118-2.5
LT1118 Adjustable
OUT
1
/R
FB
2
) + I
R
R
1
2
FB
R
OUT
1
1 F
if the ESR of the main
V
OUT
= V
FB
1 +
R
R
1118 TA06
1
2
+ I
FB
5
OUT
R
1