LT6002 datasheet(16/26 Pages) AD

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LTC6255/LTC6256/LTC6257

Rev E

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TYPICAL APPLICATIONS

When VIN is at 0V, the op amp supply current is nominally

65µA, but the 450µV maximum input offset may appear

across R3 inducing a 4.5mA current in the LED. Some ap-

plicationswantaguaranteedzeroLEDcurrentatVIN= 0,and

this is the purpose of RUP. RUP forces 5µA reverse current

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 RSD ensures that the

LTC6255 output goes low when shutting down.

Figure 6. LTC6255 Applied as a LED Current Driver with 2µs Rise Time

Figure 7. Time Domain Response Showing 2µs Rise Time. Top Waveform

Is VIN. Middle Waveform Is the 10mA to 1A Step Measured at R3, then the

0mA to 1A Step Showing Extra 2.7µs Delay When Recovering From 0mA

2µs Rise Time Analog 1A Pulsed LED Current Driver

ILED = VIN • 200mA/V

ILED

625567 F06

LTC6255

220pF

Q1 TO Q3

MOSFETs

× 2N7000

LED

OSRAM

LRW5SM

* RSD GUARANTEES LED OFF WHEN OP AMP SHDN. OTHERWISE OPTIONAL.

** RUP FORCES LED COMPLETELY OFF WHEN VIN = 0. OTHERWISE OPTIONAL.

STANDBY SUPPLY CURRENT WITH VIN = 0: 65µA RUP OPEN

650µA RUP INSTALLED

10% TO 90% RISE TIME: 10mA TO 1A, 2µs

0mA TO 1A, ADD 2.7µs DELAY

VIN

9.76k

51Ω

200Ω

SHDN

RUP

1M**

240Ω

RSD

100k*

0.1Ω

100mW

625567 F07

10mA TO 1A

0mA TO 1A

(EXTRA DELAY)

VIN

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