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ML4632CS Scheda tecnica(PDF) 4 Page - Micro Linear Corporation |
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ML4632CS Scheda tecnica(HTML) 4 Page - Micro Linear Corporation |
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4 / 8 page  ML4632 4 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 20 Ω will be ILED (HIGH) = 1.2V/RTSET = 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 IOFF pin to VCC. The additional current will be equal to (VCC – 0.7V)/RIOFF. 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 DRV AMP TO DRIVER DRV (11) VREF (10) PTAT (12) R1 R2 Figure 1. Current for Programming Output Temperature Coefficient In this configuration the temperature coefficient is TC ppm C R RR and I VV R RR RTSET ILED LED HIGH =° + = + + (/ ) , . () 3300 1 12 10 2 2 12 The output current will be a linear function of temperature. A plot of ILED versus temperature for several values of the programming resistance, R1 and R2, in figure 2. 60 55 50 45 025 50 75 85 T(°C) R1 = 1 Ω R2 = 3R1 R1 = R2 R1 = 3R2 R2 = 0 Ω NOTE: R1 + R2 ≥ 10kΩ Figure 2. ILED vs T, RTSET = 20Ω |
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