Datasheet

LT1790
20
1790fb
APPLICATIONS INFORMATION
Figure 6. LT1790-2.5 Sinking – 4mA to –5mA
Positive or Negative Operation
Series operation is ideal for extending battery life. If an
LT1790 is operated in series mode it does not require an
external current setting resistor. The specifi cations guar-
antee that the LT1790 family operates to 18V. When the
circuitry being regulated does not demand current, the
series connected LT1790 consumes only a few hundred
μW, yet the same connection can sink or source 5mA of
load current when demanded. A typical series connection
is shown on the front page of this data sheet.
The circuit in Figure 7 shows the connection for a –2.5V
reference, although any LT1790 voltage option can be
confi gured this way to make a negative reference. The
LT1790 can be used as very stable negative references,
however, they require a positive voltage applied to Pin 4
to bias internal circuitry. This voltage must be current
limited with R1 to keep the output PNP transistor from
turning on and driving the grounded output. C1 provides
stability during load transients. This connection maintains
nearly the same accuracy and temperature coeffi cient of
the positive connected LT1790.
Long-Term Drift
Long-term drift cannot be extrapolated from accelerated
high temperature testing. This erroneous technique gives
drift numbers that are widely optimistic. The only way
long-term drift can be determined is to measure it over
the time interval of interest. The LT1790S6 drift data was
taken on over 100 parts that were soldered into PC boards
similar to a real world application. The boards were then
placed into a constant temperature oven with T
A
= 30°C,
their outputs scanned regularly and measured with an 8.5
digit DVM. Long-term drift curves are shown in the Typical
Performance Characteristics section.
LT1790-2.5
C
L
1μF
C1
0.1μF
1790 F07
4
1, 2
6
R1
10k
V
OUT
= –2.5V
V
EE
3V
R
L
=
V
EE
– V
OUT
125μA
Figure 7. Using the LT1790-2.5 to Build a –2.5V Reference
1790 F06
V
GEN
V
OUT
(AC COUPLED)
6V
4V
2V
0V
8V