ML4632
FUNCTIONAL DESCRIPTION
The ML4632 accepts ECL and TTL input signals and
generates a high speed, high accuracy output current
which is independent of supply voltage variations. The
output current is programmable from 20mA to 100mA.
A temperature coefficient can be programmed into the
output current and a peaking circuit can be added with
a few external components.
The input of the LED driver accepts both ECL and TTL
signals. The ECL input stage is a standard NPN differential
pair with a common mode range of between 3V and 4.5V
with a +5V supply. A bias voltage VBIAS is available for
biasing either ECL input for single-ended operation. The
TTL input has a standard switching range of between 0.8V
and 2.0V. These inputs are ANDed so that the extra input
can be used as an enable.
Output current to the LED is set by connecting the
appropriate resistance from RTSET to ground. With the
VREF and DRV pins tied together, the high level output
voltage at RTSET will be 1.2V. The current through the
LED. The output current with RTSET set to 20Ω will be
I
LED
(HIGH) = 1.2V/R
TSET
= 1.2V/20Ω = 60mA.
The low level output current is set internally by a resistor
at approximately 0.7mA. This current prebiases the LED
and results in faster optical rise times. The value of this
current can be increased by connecting a resistor from the
I
OFF
pin to V
CC
. The additional current will be equal to
(V
CC
– 0.7V)/R
IOFF
.
The voltage input at the DRV pin appears across the
RTSET pin when the LED is turned on. The current in
RTSET is directed through the LED. Therefore the voltage
set at DRV along with the RTSET resistor sets current
through the LED.
A temperature coefficient of between 0ppm/°C and
3300ppm/°C can be programmed into the high level
output current to compensate for the drop in LED optical
output power at high temperatures. This is accomplished
by driving the DRV pin from a resistor divider between the
VREF and PTAT pins.
When DRV is tied directly to PTAT, the peak voltage at
RTSET will be 1.0V at 25°C and have a 3300ppm/°C
temperature coefficient. At 85°C, PTAT is 1.2V and equal
to VREF. An arbitrary temperature coefficient less than
3300 ppm/°C can be set by using a resistor divider
between PTAT and VREF to set the voltage at DRV, as
shown in figure 1.
REF
PTAT
PTAT
(12)
R2
VREF
(10)
R1
DRV
(11)
DRV
AMP
TO
DRIVER
Figure 1. Current for Programming
Output Temperature Coefficient
In this configuration the temperature coefficient is
TC
ILED
=
(3300ppm /
°
C)
R1 , and
R1
+
R2
I
LED (HIGH)
1V
+
0.2V
R2
R1
+
R2
=
RTSET
The output current will be a linear function of temperature.
A plot of I
LED
versus temperature for several values of the
programming resistance, R1 and R2, in figure 2.
60
R1 = 1Ω
R2 = 3R1
R1 = R2
55
I
LED
(mA)
R1 = 3R2
50
R2 = 0Ω
NOTE: R
1
+ R
2
≥
10kΩ
45
0
25
50
T(°C)
75 85
Figure 2. I
LED
vs T, R
TSET
= 20Ω
4
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
