Datasheet

LTC6255/LTC6256/LTC6257
16
625567fb
Figure 5
–
+
V
OUT
625567 F05
LTC6255
4
1
C2
3
6
V
IN
C1
R3
V
+
V
+
R1
R2
0.1µF
SHDN
2
5
100k
100k
10µF
typicAl ApplicAtions
RC Component Equations
R2 =
1− 1− 4 Q
2
Gain + 1
[ ]
C2
C1






4 π Q f
O
C2
R3 =
1
4 π
2
R2 C1 C2 f
O
2
Gain =
R2
R1
R1=
R2
Gain
C1> 4 Q
2
Gain + 1
( )
C2
Maximum f
–3dB
= 100kHz and
Maximum Gain =
100kHz
f
–3dB
Table 1.
f
O
AND Q VALUES
2nd Order Lowpass
Butterworth f
O
= f
–3dB
Q = 0.707
Bessel f
O
= 1.274 • f
–3dB
Q = 0.577
4th Order Lowpass
Butterworth f
O
= f
–3dB
f
O
= f
–3dB
Q = 0.541
Q = 1.307
Bessel f
O
= 1.419 • f
–3dB
f
O
= 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.
Most LED applications pulse the LEDs for the best efficiency,
and many applications take advantage of control of both
pulse width and analog current amplitude.
In order to extend the circuit’s input range to accommodate
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
LTC6255 combines 6.5MHz of gain-bandwidth product and
1.8V/μs slew rate on a supply current budget of only 65µA.