LT6002 datasheet(15/26 Pages) AD

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LTC6255/LTC6256/LTC6257

Rev E

For more information www.analog.com

Figure 5

VOUT

625567 F05

LTC6255

VIN

V+

0.1µF

SHDN

100k

10µF

TYPICAL APPLICATIONS

RC Component Equations

R2 =

1− 1− 4 Q2 Gain+1

[

]

⎛

⎝

⎜

⎞

⎠

⎟

4 π Q fO C2

R3 =

4 π2 R2 C1 C2 fO

Gain =

R1=

Gain

C1> 4 Q2 Gain+1

(

)C2

Maximum f–3dB = 100kHz and

Maximum Gain =

100kHz

f–3dB

Table 1.

fO AND Q VALUES

2nd Order Lowpass

Butterworth

fO = f–3dB

Q = 0.707

Bessel

fO = 1.274 • f–3dB

Q = 0.577

4th Order Lowpass

Butterworth

fO = f–3dB

Q = 0.541

Q = 1.307

Bessel

fO = 1.419 • f–3dB

fO = 1.591 • f–3dB

Q = 0.522

Q = 0.806

2µs Rise Time Analog 1A Pulsed LED Current Driver

Figure 6 shows the LTC6255 applied as a fast, efficient

analog LED current driver. High power LEDs are used in

applications ranging from brake lights to video projectors.

MostLEDapplicationspulsetheLEDsforthebestefficiency,

and many applications take advantage of control of both

pulse width and analog current amplitude.

Inordertoextendthecircuit’sinputrangetoaccommodate

5V output DACs, the input voltage is initially divided by

50 through the R1:R2 divider. The reduced step is applied

to the LTC6255 non inverting input, and LTC6255 output

rises until MOSFETs Q1 through Q3 begin to turn on,

increasing the current in their drains and therefore the

LED. The amount of current is sensed on R3, and fed back

to the LTC6255 inverting input through R5. The loop is

compensated by R5 and C1, with R4 distancing the gate

capacitance from the op amp output for the best time

domain response. 10% to 90% rise time was measured

at 2µs on a 10mA to 1A pulse. Starting at 0 current there

is an additional delay of 2.7µs.

It may seem strange to use a micropower op amp in a

high current LED application, but it can be justified by the

low duty cycles encountered in LED drive applications. A

one 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

LTC6255combines6.5MHzofgain-bandwidthproductand

1.8V/μs slew rate on a supply current budget of only 65µA.

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