LTC6257 Linear Technology, LTC6257 Datasheet - Page 17

• Manufacturer Part Number: LTC6257
• Description: 65uA Power Efficient Rail-to-Rail I/O Op Amps
• Manufacturer: Linear Technology

typicAl ApplicAtions
amp LED is quite bright even when driven at 1% or even
0.1% duty cycles and these constitute 10mA and 1mA
average current levels respectively, in which case the supply
current of the op amp becomes noticeable. The LTC6255
combines 6.5MHz of gain-bandwidth product and 1.8V/μs
slew rate on a supply current budget of only 65µA.
When V
65µA, but the 450µV maximum input offset may appear
across R3 inducing a 4.5mA current in the LED. Some ap-
plications want a guaranteed zero LED current at V
V
*R
*R
STANDBY SUPPLY CURRENT WITH V
10% TO 90% RISE TIME: 10mA TO 1A, 2µs
IN
SD
UP
Figure 6: LTC6255 Applied as a LED Current Driver with 2µs
Rise Time
5V
GUARANTEES LED OFF WHEN OP AMP SHDN. OTHERWISE OPTIONAL.
FORCES LED COMPLETELY OFF WHEN V
9.76k
200
R1
2µs Rise Time Analog 1A Pulsed LED Current Driver
R2
1M**
R
UP
IN
is at 0V, the op amp supply current is nominally
+
–
LTC6255
5V
0mA TO 1A, ADD 2.7µs DELAY
SHDN
220pF
C1
IN
= 0: 65µA R
I
LED
51
R4
650µA R
= V
IN
IN
= 0. OTHERWISE OPTIONAL.
• 200mA/V
R
100k*
UP
UP
SD
OPEN
INSTALLED
240
Q1
R5
Q2
I
LED
5V
IN
Q3
= 0, and
R3
0.1
100mW
Q1 TO Q3
MOSFETs
3 2N7000
LED
OSRAM
LRW5SM
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this is the purpose of R
through R5 creating a negative 1.2mV output offset at R3.
This guarantees a zero LED current, but note that the op
amp supply current rises from 65µA to a still respectable
650µA in this case due to internal protection circuitry for
the output stage. For reduced current, the LTC6255 can
be shut down, but the output becomes high impedance
and may leak high which will turn on the MOSFETs and
LED hard. Adding pull-down resistor R
LTC6255 output goes low when shutting down.
LTC6255/LTC6256/LTC6257
Figure 7: Time Domain Response Showing 2µs Rise Time.
Top Waveform Is V
Step Measured at R3, then the 0mA to 1A Step Showing
Extra 2.7µs Delay When Recovering From 0mA
10mA TO 1A
(EXTRA DELAY)
0mA TO 1A
IN
V
IN
. Middle Waveform Is the 10mA to 1A
UP
. R
UP
forces 5µA reverse current
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SD
ensures that the
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