Current sources for fiber-optic lasers:
a compendium of pleasant current events
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The preceding circuit uses Q1's linear regulation to close the feedback loop. This approach offers simplicity at the expense of efficiency. Q1's power dissipation can approach 1W under some conditions. Many applications permit this amount of power, but some situations require minimizing heat. Replacing Q1 with a step-down switching regulator minimizes the heat (Figure 2). The switched-mode power delivery eliminates almost all of the transistor's heat.
Three additional features allow the circuit in Figure 3 to operate in a fully protected and self-contained fashion. The circuit monitors its power supply and "self-enables" when the supply is within limits, eliminating the enable port and external watchdog in the previous examples. A settable current clamp and open-laser protection prevent laser damage. The self-enable uses an LT1431 shunt regulator. This regulator has the highly desirable property of maintaining a predictable open-collector output when operating below its minimum supply voltage. At initial turn-on, supply voltage is 1V, the LT1431's output does not switch, and current flows to Q3's base. Q3 turns on, preventing Q1's base from receiving bias. Additionally, this action pulls down the circuit's current-programming input, which drives IC1's – input. This arrangement ensures that the laser cannot receive current until Q3 turns off. Also, when Q3 turns off, IC1's output cleanly ramps up to the desired programmed current. The resistor values at the LT1431's reference input dictates that the device goes low when VSUPPLY passes through 4V. This 4V potential ensures proper circuit operation.
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pdf 版 : http://cds.linear.com/docs/Application%20Note/an90f.pdf
Dc current is the power source for a large group of fiber-optic lasers. A current source with modulation farther along the signal path supplies laser drive. The current source, although conceptually simple, constitutes a tricky design problem. You have to consider a number of practical requirements for a fiber-optic current source, and failure to do so can destroy lasers or optical components.
A laser current source is deceptively simple in concept. Inputs include a current-output programming port, an output-current clamp, and an enable command. Laser current is the sole output. In practice, however, a laser current source must meet a number of practical and subtle requirements. The key to a successful design is a thorough understanding of individual system requirements. Various approaches suit different sets of freedoms and constraints, although all must address some basic concerns.
Performance and protection are the basic concerns for laser current sources. Performance issues include the current source's magnitude and stability under all conditions, output-connection restrictions, voltage compliance, efficiency, programming interface, and power requirements. Protection features are necessary to prevent laser and optical component damage. The laser, which is an expensive and delicate device, must have protection under all conditions, including supply ramp-up and -down, improper control-input commands, open or intermittent load connections, and "hot plugging."
Laser-performance issues
The performance issues for a laser current source are as follows:
- Required power supply: The first step is to define the available power supply. A single-rail 5V supply is currently the most common and desirable. You must account for supply tolerances, which are typically ±5%. System-distribution-voltage drops may result in surprisingly low rail voltages at the point of load. Split rails are occasionally available, although they are relatively rare. Additionally, split-rail operation can complicate laser protection, particularly during supply sequencing.
- Output-current capability: Low-power lasers operate on less than 250 mA. Higher power types can require as much as 2.5A.
- Output-voltage compliance: The current source's output-voltage compliance must be able to accommodate the laser's forward-junction drop and any additional drops in the drive path. Typically, voltage compliance of 2.5V is adequate.
- Efficiency: Heat buildup in fiber-optic systems is often a concern due to space limitations. Accordingly, current-source efficiency can be an issue. Linear regulation is often adequate at low current. Switching-regulator-based approaches may be necessary at high current levels.
- Laser connection: In some cases, the laser may float off ground. Other applications require grounded-anode or -cathode operation. Grounding the anode seemingly mandates a negative supply, but you can retain single-rail operation by using switching-regulator techniques.
- Output-current programming: A programming-port voltage sets the output current. You can derive the voltage from a potentiometer, DAC, or filtered PWM. Typically, a range of 0 to 2.5V corresponds to a current range of 0 to 250 mA or 0 to 2.5A. Setpoint accuracy is usually within 0.5%, although you can readily achieve better tolerances.
- Stability: The current source needs good regulation against line, load, and temperature changes. Line- and load-induced variations should be within 0.05% with typical temperature drifts of 0.01%. Judicious component choice can considerably improve these figures.
- Noise: You must minimize current-source noise, which can modulate laser output. Typically, noise bandwidth to 100 MHz is of interest. A linearly regulated current source has inherently low noise and usually presents no problems. Switching-regulator-based current sources require special techniques to maintain low noise.
- Transient response: The current source does not need a fast transient response, but you cannot under any circumstances let the output overshoot the programmed current. Such overshoots can damage the laser or associated optical components.
The specifics of laser protection are as follows:
- Overshoot: Because outputs that overshoot the nominal programmed current can be destructive, you must account for any possible combination of improper control-input or power-supply turn-on and -off characteristics. Also, any spurious laser current under any condition is impermissible. Portions of the current-source circuitry may have undesired and unpredictable responses during supply ramp-up and -down, complicating the design.
- Enable: An enable line allows for shutting off the current-source output. You can also use the enable line to hold off the current-output during supply ramp-up to prevent undesired outputs. This use of the enable line can be tricky because the power supply for the enable-signal circuitry may be the same supply that runs the laser. The enable signal must reliably operate independently of the power supply's turn-on profile. Optionally, the enable function can be self-contained within the current source, eliminating the necessity of generating this signal.
- Output-current clamp: The output-current clamp sets the maximum output current, overriding the output-current programming command. A potentiometer, DAC, or filtered PWM signal can set this voltage-controlled input.
- Open-laser protection: An unprotected current source's output rises to maximum voltage if you disconnect the load. This circumstance can lead to hot-plugging the laser, a potentially destructive event. Intermittent laser connections can produce similar undesirable results. The current-source output should latch off if the load disconnects. Recycling the power clears the latch but only if the load has been established.
The preceding discussion dictates considerable care when designing laser-current sources. The delicate and expensive load, combined with the noted uncertainties, should promote an aura of thoughtful caution (see sidebar "Simulating the laser load"). The following circuit examples maintain this outlook and present practical, usable circuits.
A basic laser current source supplies as much as 250 mA via Q1 (Figure 1). This circuit requires that both laser terminals float. The amplifier controls laser current by maintaining the 1Ω shunt voltage at a potential that the programming input dictates. Local compensation at the amplifier stabilizes the loop, and the 0.1-μF capacitor filters input commands, ensuring that the loop never limits slew. This precaution prevents overshoot due to programming-input dynamics. The enable input turns off the current source by simultaneously grounding Q1's base and starving the amplifier's + input while biasing the – input high. This combination also ensures that the amplifier smoothly ramps to the desired output current when the enable switches low. An external watchdog circuit that switches after the power supply is within operating limits must drive the enable input. Because the external circuitry may operate from the same supply as the current source, the enable threshold in this circuit is 1V. The 1V threshold ensures that the enable input dominates the current source's output at low supply voltages during power turn-on. This feature prevents spurious outputs due to unpredictable amplifier behavior below the minimum supply voltage.
The preceding circuit uses Q1's linear regulation to close the feedback loop. This approach offers simplicity at the expense of efficiency. Q1's power dissipation can approach 1W under some conditions. Many applications permit this amount of power, but some situations require minimizing heat. Replacing Q1 with a step-down switching regulator minimizes the heat (Figure 2). The switched-mode power delivery eliminates almost all of the transistor's heat.
This circuit is similar to Figure 1's linear approach, except for the addition of the LTC1504 switching regulator. It is useful to liken the switching regulator's input, VCC; feedback, FB; and output, VSW; to the collector, base, and emitter, respectively, of Q1 in Figure 1. This analogy reveals that two circuits have similar operating characteristics with the switched-mode version enhancing efficiency. The regulator's output LC filter introduces phase shift, necessitating attention to loop compensation. The amplifier's local roll-off is similar to that in Figure 2, although good loop damping requires two phase-leading ac feedback elements, the 0.01- and 0.033-μF capacitors. In all other respects, including enable and programming-input considerations, Figure 2's circuit is identical to that in Figure 1.
Grounded-cathode operation
It is sometimes necessary to tie the laser's cathode to ground. A new circuit operates from a single supply and features grounded-cathode operation (Figure 3). This circuit is reminiscent of Figure 1 with a notable exception. In this case, differential amplifier IC3 senses the voltage across a shunt in the laser anode, permitting cathode grounding. IC2's gain-scaled output feeds back to IC1 for loop closure. Loop-compensation and enable-input considerations are similar to the circuits in Figure 1 and Figure 2, and, as before, you can replace Q1 with a switching regulator.
Three additional features allow the circuit in Figure 3 to operate in a fully protected and self-contained fashion. The circuit monitors its power supply and "self-enables" when the supply is within limits, eliminating the enable port and external watchdog in the previous examples. A settable current clamp and open-laser protection prevent laser damage. The self-enable uses an LT1431 shunt regulator. This regulator has the highly desirable property of maintaining a predictable open-collector output when operating below its minimum supply voltage. At initial turn-on, supply voltage is 1V, the LT1431's output does not switch, and current flows to Q3's base. Q3 turns on, preventing Q1's base from receiving bias. Additionally, this action pulls down the circuit's current-programming input, which drives IC1's – input. This arrangement ensures that the laser cannot receive current until Q3 turns off. Also, when Q3 turns off, IC1's output cleanly ramps up to the desired programmed current. The resistor values at the LT1431's reference input dictates that the device goes low when VSUPPLY passes through 4V. This 4V potential ensures proper circuit operation.
Supply start-up waveforms detail these self-enable features (Figure 4). Trace A, the nominal 5V rail, ramps for 3 msec before arriving at 5V. During this interval, the LT1431 (Trace B) follows the ramp, biasing Q3 on. The circuit does not control IC1's output (Trace C) during this period. Q1's emitter (Trace D), however, is cut off due to Q3's conduction and cannot pass the disturbance. As a result, the laser conducts no current (Trace E) during this time. When the supply (Trace A) ramps beyond 4V (just before the photo's fourth vertical division), the LT1431 switches low (Trace B), Q3 switches off, and the circuit self-enables. IC1's output (Trace C) ramps up, and Q1's emitter (Trace D) and the laser current (Trace E) are slaves to the output's movement. This action prevents any undesired current in the laser during supply turn-on, regardless of unpredictable circuit behavior at low supply voltages.
Programming-input changes
You must control laser current in situations other than supply turn-on. Response to programming-input changes must be similarly well-behaved. The laser-current response (Trace B) to a programming input step shows clean damping with no hint of overshoot (Trace A, Figure 5). The circuit in Figure 3 also includes open-laser protection. If the current source operates into an open load with no laser, the source produces maximum voltage at the laser output terminals. This circumstance can lead to hot-plugging the laser, a potentially destructive event. Intermittent laser connections can produce similar undesirable results. The LTC1696 overvoltage-protection controller guards against open-laser operation. This device's output latches high when its feedback input, FB, exceeds 0.88V. This circuit biases the FB pin so that a laser output voltage greater than 2.5V forces the LTC1696 high, triggering the SCR to shunt current away from the laser. The 470Ω resistor supplies SCR holding current, and the diodes ensure that no current flows in the output.
Figure 6 details events with a properly connected laser at supply turn-on. Trace A is the supply; Trace B, the laser voltage; Trace C, the LTC1696 output, and Trace D, the laser current. The waveforms show laser voltage, Trace B, rising to about 2V at supply turn-on, Trace A. Under these normal conditions, the LTC1696 output, Trace C, stays low, and laser current, Trace D, rises to the programmed value.
Figure 7 shows what happens when the circuit turns into an open-laser connection. Trace assignments are identical to those in Figure 6. At supply turn-on, Trace A, the laser voltage, Trace B, transitions beyond the 2.5V open-laser threshold. The LTC1696 output, Trace C, goes high; the SCR latches; and no current flows in the shunted laser line, Trace D. Once this event occurs, you must recycle the power to reset the LTC1696-SCR latch. If the laser has an improper connection, the circuit repeats its protective action. Open-laser protection is not restricted to turn-on. It also occurs if laser connection is lost at any time during normal circuit operation.
A final protection feature in Figure 3 is a current clamp that prevents the transmission of uncontrolled programming inputs by clamping them to a settable level. IC2, Q2, and associated components form the clamp. Normally, IC2's + input is above the circuit's programming input (Q2's emitter voltage), IC2's output is high, and Q2 is off. If the programming input exceeds IC2's + input level, IC2 swings low, Q2 comes on, and the amplifier feedback drives Q2's emitter to the clamp-adjust wiper potential. This action clamps IC1's input to the clamp-adjust setting, which prevents laser-current overdrive. Clamp action need not be fast to be effective because of IC1's 10-kΩ, 0.02-µF input filter. Figure 8's traces show clamp response to programming-input overdrive. When the programming input, Trace A, exceeds the clamp's preset level, Q2's emitter Trace B, does the same, causing IC2's output, Trace C, to swing down. IC2 feedback controls Q2's emitter to the clamp level, arresting the voltage applied to the 10-kΩ, 0.02-µF filter. The filter bandlimits the abrupt clamp operation, resulting in a smooth corner at IC1's positive input, Trace D. IC1's clamped input dictates a similarly shaped and clamped laser current, Trace E. The clamp remains active until the programming input falls below the clamp-adjust setting.
Grounded-cathode source provides 2.5A
The circuit in Figure 9, which stems from those in Figure 2 and Figure 3, provides as much as 2.5A to a grounded-cathode laser. IC1 is the control amplifier, the LT1506 switching regulator efficiently delivers output current, and IC2 senses laser current via a 0.1Ω shunt resistor. Loop operation is similar to that in Figure 2 and Figure 3 with IC2 providing dc feedback to IC1. Frequency compensation differs from that in the previous figures. The circuit achieves stable loop operation using a local roll-off at IC1, augmented by two lead networks associated with L1. Feeding back a lightly filtered (1-kΩ, 0.47-µF) version of the LT1506's VSW pin's output activity provides midband lead. High-frequency lead compensation, arriving via the 330Ω, 0.05-µF pair, optimizes edge response. The circuit in Figure 9 uses the externally controlled enable function, although you can also use Figure 3's self-enable feature. Similarly, you can also employ Figure 3's current clamp and open-laser protection in this circuit.
This circuit's switched-mode energy delivery provides high efficiency at high power, but output noise may be an issue. Residual harmonic content related to switching-regulator operation appears in the laser current. The resultant low-level modulation of laser output may be troublesome in some applications. Approximately 800 µA p-p of switching-regulator-related noise (0.05%) appears in the 2A laser-current output. This disturbance comprises fundamental ripple and switching-transition-related harmonics.
Lower the noise to 0.001%
A 0.05% noise content suits many optical-system applications. More stringent requirements benefit from extremely low-noise content. A grounded-cathode, 2A circuit uses special switching- regulator techniques to attain only 20 µA p-p noise, which is approximately 0.001% (Figure 10a). The circuit substantially reduces the noise by limiting edge-switching speed in the regulator's power stage (Reference 1). The LT1683 pulse-width modulator controls the voltage and current rise times in switches Q1 and Q2. The LT1683's output stage operates Q1 and Q2 in local loops that sense and control their edge times. The circuit feeds back transistor-voltage information using the 4.7-pF capacitors and derives and feeds back current status from the 0.033Ω shunt resistor. This arrangement permits the PWM-control IC to fix transistor-switching times, regardless of power-supply or load changes. The RVSL and RCSL resistors associated with the LT1683 controller set the transition rates. In practice, you set these resistor values by adjusting them to minimize output noise. The remainder of the circuit forms a grounded-cathode laser-current source.
Q1 and Q2 drive T1, and LC sections filter the rectified output. Because T1's secondary winding floats, the laser cathode and the 0.1Ω shunt resistor are at circuit ground. The shunt returns to T1's secondary center tap, completing a laser-current-flow path. This arrangement produces a negative voltage that corresponds to laser current at the shunt's ungrounded end. The circuit resistively sums this potential at IC1 with the positive-voltage current-programming-input information. IC1's output controls the LT1683's pulse-width drives to Q1 and Q2 via Q3, closing a loop to set laser current. The circuit sets loop compensation using bandlimiting at IC1 and Q3's collector, aided by a single-lead network arriving from the L1-L2 junction.
Some circuit details merit attention. An LT1054-based voltage multiplier feeds the LT1683's supply-input pins. This boosted voltage provides enough gate drive to ensure Q1-Q2 saturation. Damper networks across T1's rectifiers minimize diode-switching-related events in the output current. This circuit is compatible with the self-enable and laser-protection features of the previous circuits. Appropriate connection points appear in the figure.
The speed-controlled switching times result in a spectacular decrease in noise to just 20 µA p-p, or approximately 0.001% of the 2A-dc laser current (Figure 10b). Fundamental ripple residue and switching artifacts are visible against the measurement noise floor. Reliable wideband current-noise measurement at these levels requires special techniques (see sidebar "Verifying switching-regulator-related noise").
Ground the anode, and keep noise low
The circuit in Figure 11 is similar to that in Figure 10 and uses edge-time control to achieve an exceptionally low-noise output of 0.0025%. This circuit is intended for lower power lasers that require grounded-anode operation. The LT1533, a version of the previous circuit's LT1683, has internal power switches. These switches drive T1. T1's rectified and filtered secondary produces a negative output that biases the laser. The laser's anode connects to ground, and the 1Ω shunt resistor completes the current path to T1's secondary winding. This configuration makes T1's center tap voltage positive and proportional to laser current. IC1 compares this voltage to the current programming input. IC1 biases Q2, closing a loop around the LT1533. Local bandwidth limiting at IC1 and Q2's collector damping and feedback capacitors provide loop compensation.
This circuit's 2.5 µA p-p noise qualifies it for the most demanding applications. Residual switching-related noise approaches the measurement noise floor. The enable function operates as previously described. Additionally, this circuit is compatible with Figure 3's self-enable and laser-protection accessory circuits. The figure shows the changes that the grounded-anode operation necessitates.
Float the output
Figure 12 retains Figure 11's low noise but also has a fully floating output. You can ground either laser terminal without affecting circuit operation. The circuit realizes this feature by using feedback to control the transformer's primary current and by relying on interwinding coupling to maintain regulation (references 2, 3, and 4). This coupling varies slightly with operating point, limiting the output-current regulation to approximately 1%.
The schematic shows the LT1533 low-noise switching regulator driving T1. The circuit forces the LT1533 to run at a 50% duty cycle by grounding the duty pin. The LT1533 retains its controlled edge-time characteristics. Current flows through Q1 and the 0.1Ω shunt resistor into T1's primary winding. The LT1533's open-collector power outputs alternately chop primary current to ground. Q1's bias sets the primary-current magnitude and, hence, the 0.1Ω shunt voltage. IC1's output, which represents the difference between the output-current-programming input and IC2's amplified version of the shunt voltage, sets Q1's bias. This loop enforces a shunt voltage that's proportional to the value of the current-programming input. In this way, the current-programming input sets T1's primary current, which determines T1's secondary current through the laser. Differential amplifier IC2's gain-setting resistor calibrates the current-programming input's scaling.
The primary-side feedback's lack of global feedback mandates a current-regulation compromise. A plot of laser current versus programming input voltage shows 1% conformance over nearly the entire range. Some error below 10 mA exists due to nonideal transformer behavior, but this error is normally insignificant because it is below the typical laser-threshold current. Line regulation, which the sensing scheme also degrades, still is approximately 0.05%/V. Similarly, load regulation, over a 1 to 1.8V compliance voltage, is typically 2%.
This circuit's floating output complicates including the laser-protection and self-enable features of Figure 3. Biasing the LTC1696 from T1's center tap accomplishes open-laser protection. If the laser opens, the loop forces a marked rise at T1's center tap, latching the LTC1696's output high. This action skews IC1's inputs, sending its output low and shutting off Q1. All T1 drive ceases. Because the LTC1696 output latches, you must recycle the power to reset the circuit. If you haven't connected the laser, the latch acts again, protecting the laser from hot-plugging or intermittent connections. You can add the self-enable and current-clamp options in accordance with the notations on the schematic.
Connect anode to positive supply
Figure 13a's current source is useful for applications that commit the laser anode to the power supply. IC1A senses Q1's emitter and closes a loop that forces constant current in the laser. Local compensation at IC1 and input bandlimiting stabilize the loop. This circuit also includes an inherent self-enable feature. The LT1635 operates at supply voltages as low as 1.2V. At voltages higher than 1.2V, the LT1635's comparator-configured section, IC1B, holds off circuit output until the supply voltage reaches 2V. Below 1.2V supply, Q1's base biasing prevents unwanted outputs. The slew-retarded input and loop compensation yield a clean dynamic response with no overshoot. As the figure shows, you can again include current-clamping and open-laser-protection options. Additionally, higher output current is possible at increased supply voltages, although you must respect Q1's dissipation limits.
Figure 13b details operation during supply turn-on. At supply ramp-up (Trace A), output current (Trace D) is disabled. When the supply reaches 2V, IC1B (Trace B) goes low, permitting IC1A's output (Trace C) to rise. This action biases Q1, and laser current flows (Trace D). The LT1635 operates on supply voltages as low as 1.2V. Below this level, the circuit prevents spurious outputs using junction stacking and bandlimiting at Q1's base. Q1's base components also prevent unwanted outputs when the supply rises rapidly. Such rapid rise could cause uncontrolled IC1A outputs before the amplifier and its feedback loop are established.
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