LT1790A LINER [Linear Technology], LT1790A Datasheet - Page 19

• Manufacturer Part Number: LT1790A
• Description: Micropower SOT-23 Low Dropout Reference Family
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
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 specifications 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
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
configured 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
W, yet the same connection can sink or source 5mA of
Figure 7. Using the LT1790-2.5 to Build a –2.5V Reference
V
GEN
8V
6V
4V
2V
0V
Figure 6. LT1790-2.5 Sinking – 4mA to – 5mA
R
L
=
V
6
EE
125 A
– V
U
LT1790-2.5
OUT
V
EE
1, 2
4
INFORMATION
U
10k
R1
C1
0.1 F
C
1 F
1790 F07
W
L
V
OUT
3V
= –2.5V
1790 F06
V
(AC Coupled)
U
OUT
stability during load transients. This connection main-
tains nearly the same accuracy and temperature coeffi-
cient of the positive connected LT1790.
Long-Term Drift
Long-term drift cannot be extrapolated from acceler-
ated high temperature testing. This erroneous tech-
nique gives drift numbers that are widely optimistic. The
only way long-term drift can be determined is to mea-
sure 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
measured with an 8.5 digit DVM. Long-term drift curves
are shown in the Typical Performance Characteristics.
Hysteresis
Hysteresis data shown in Figures 8 and 9 represent the
worst-case data taken on parts from 0 C to 70 C and from
– 40 C to 85 C. Units were cycled several times over these
temperature ranges and the largest change is shown. As
expected, the parts cycled over the higher temperature
range have higher hysteresis than those cycled over the
lower range.
When an LT1790 is IR reflow soldered onto a PC board, the
output shift is typically just 150ppm (0.015%).
Higher Input Voltage
The circuit in Figure 10 shows an easy way to increase the
input voltage range of the LT1790. The zener diode can be
anywhere from 6V to 18V. For equal power sharing be-
tween R1 and the zener (at 30V), the 18V option is better.
The circuit can tolerate much higher voltages for short
periods and is suitable for transient protection.
Assuming 80 A max supply current for the LT1790, a
25 A load, 120mV max dropout and a 4V to 30V input
specification, the largest that R1 can be is (4V – 3.3V –
120mV)/(80 A + 25 A) = 5.5k. Furthermore, assuming
220mW of dissipation in the 18V SOT-23 zener, this gives
a max current of (220mW)/(18V) = 12.2mA. So the
smallest that R1 should be is (30V – 18V)/12.2mA = 1k,
rated at 150mW.
A
= 30 C, their outputs scanned regularly and
LT1790
19
1790fa