LT1461-2.5 Linear Technology, LT1461-2.5 Datasheet - Page 7

• Manufacturer Part Number: LT1461-2.5
• Description: Micropower Precision Low Dropout Series Voltage Reference
• Manufacturer: Linear Technology

APPLICATIONS
Bypass and Load Capacitors
The LT1461 requires a capacitor on the input and on the
output for stability. The capacitor on the input is a supply
bypass capacitor and if the bypass capacitors from other
components are close (within 2 inches) they should be
sufficient. The output capacitor acts as frequency com-
pensation for the reference and cannot be omitted. For
light loads 1mA, a 1 F nonpolar output capacitor is
usually adequate, but for higher loads (up to 75mA), the
output capacitor should be 2 F or greater. Figures 1 and
2 show the transient response to a 1mA load step with a
1 F output capacitor and a 50mA load step with a 2 F
output capacitor.
Precision Regulator
The LT1461 will deliver 50mA with V
higher load current with higher V
typically 12ppm/mA, which means for a 50mA load step,
the output will change by only 1.5mV. Thermal regulation,
caused by die temperature gradients and created from
1mA/DIV
200mV/DIV
I
20mV/DIV
OUT
50mA/DIV
V
V
0mA
1mA
I
OUT
OUT
OUT
Figure 2. 50mA Load Step with C
Figure 1. 1mA Load Step with C
U
INFORMATION
U
IN
W
IN
. Load regulation is
= V
L
L
= 1 F
= 2 F
OUT
1461 F01
1461 F02
+ 2.5V and
U
load current or input voltage changes, is not measurable.
This often overlooked parameter must be added to normal
line and load regulation errors. The load regulation photo,
on the first page of this data sheet, shows the output
response to 200mW of instantaneous power dissipation
and the reference shows no sign of thermal errors. The
reference has thermal shutdown and will turn off if the
junction temperature exceeds 150 C.
Shutdown
The shutdown (Pin 3 low) serves to shut off load current
when the LT1461 is used as a regulator. The LT1461
operates normally with Pin 3 open or greater than or equal
to 2.4V. In shutdown, the reference draws a maximum
supply current of 35 A. Figure 3 shows the transient
response of shutdown while the part is delivering 25mA.
After shutdown, the reference powers up in about 200 s.
PC Board Layout
In 13- to 16-bit systems where initial accuracy and tem-
perature coefficient calibrations have been done, the me-
chanical and thermal stress on a PC board (in a card cage
for instance) can shift the output voltage and mask the true
temperature coefficient of a reference. In addition, the
mechanical stress of being soldered into a PC board can
cause the output voltage to shift from its ideal value.
Surface mount voltage references are the most suscep-
tible to PC board stress because of the small amount of
plastic used to hold the lead frame.
A simple way to improve the stress-related shifts is to
mount the reference near the short edge of the PC board,
or in a corner. The board edge acts as a stress boundary,
PIN 3
V
OUT
Figure 3. Shutdown While Delivering 25mA, R
5V
0V
0V
LT1461-2.5
1461 F03
L
= 100
7