LTC6257 Linear Technology, LTC6257 Datasheet - Page 16

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

LTC6255/LTC6256/LTC6257
typicAl ApplicAtions
RC Component Equations
Maximum f

R
R
R
Maximum Gain
Gain
C
1
1 4
2
3
V
=
>
=
=
IN
=
Gain
1
4
R
−
R
R
Q Gain
2
π
R1
2
1
2
2
(

 
–3dB
R C C f
1 4
2 1 2
−
4
R2
1
C1
= 100kHz and
π
R3
=
+
Q Gain
V
100
1 2
Q f C
+
2
)
f
3 –
C
[
O
Figure 5
10µF
kHz
dB
100k
100k
C2
O
2
2
+
4
3
1
]
–
+
C
C
LTC6255
2
1
V
+
2
6

 
0.1µF
SHDN
625567 F05
5
1
V
OUT
Table 1.
f
2nd Order Lowpass
Butterworth
Bessel
4th Order Lowpass
Butterworth
Bessel
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
O
AND Q VALUES
f
f
f
f
f
f
O
O
O
O
O
O
= f
= 1.274 • f
= f
= f
= 1.419 • f
= 1.591 • f
–3dB
–3dB
–3dB
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–3dB
–3dB
–3dB
Q = 0.707
Q = 0.577
Q = 0.541
Q = 1.307
Q = 0.522
Q = 0.806
625567f