Analysis of electrical performance Item investigated: High current LED driver SL-D1 while driving laser diode NDB7352 Overview of SL-D1 SL-D1 is a switching power supply that is intended to operate in “buck” mode – input voltage greater than output voltage. The SL-D1 power supply regulates current, not voltage; the current through the LED (laser) is held constant, regardless of the input voltage changes, as long as the input voltage remains within a defined region – see specification in Table 1. The switching frequency of the regulator is high which results in the design being able to use smaller components than a typical power supply of this type. The high switching frequency also results in high efficiency, which lengthens the life of a battery if used for input power. However, the unit still draws significant current when in operation and will drain a battery quickly if power is applied continuously – see table 1. Minimum Typical Maximum Input Voltage 3.6 23V VOLTS Input Current 0.37 1.21 1.32 AMPS Switching Frequency 330 Kilo-Hz Diode Turn On Delay 7.2 (see note 1) 18 19 Milli-Seconds (ms) Current Out 1.19 1.25 1.31 Amps TABLE 1; Key specification for SL-D1 when driving Laser Diode NDB7352 Note 1: The time between the application of power and the diode going into forward conduction (lasing) is a function of the time period it has been off and the application of input power. The longer it has been off, the longer it takes to charge up the input circuit of the regulator before it will function. The fastest turn-on time was observed to be 7.2 ms after being off for 10 ms. SL-D1 Circuit Operation The analysis of the SL-D1 LED driver was accomplished with the configuration shown in figure 1. Figure 1; Test Circuit Power Supply Tek Digital O’Scope CH1 CH2 CH3 SL-D1 NDB7352 10.02
Transcript
Analysis of electrical performance
Item investigated:
High current LED driver SL-D1 while driving laser diode NDB7352
Overview of SL-D1 SL-D1 is a switching power supply that is intended to operate in “buck” mode – input voltage greater than output voltage. The SL-D1 power supply regulates current, not voltage; the current through the LED (laser) is held constant, regardless of the input voltage changes, as long as the input voltage remains within a defined region – see specification in Table 1. The switching frequency of the regulator is high which results in the design being able to use smaller components than a typical power supply of this type. The high switching frequency also results in high efficiency, which lengthens the life of a battery if used for input power. However, the unit still draws significant current when in operation and will drain a battery quickly if power is applied continuously – see table 1. Minimum Typical Maximum Input Voltage 3.6 23V VOLTS Input Current 0.37 1.21 1.32 AMPS Switching Frequency 330 Kilo-Hz Diode Turn On Delay 7.2 (see note 1) 18 19 Milli-Seconds (ms) Current Out 1.19 1.25 1.31 Amps TABLE 1; Key specification for SL-D1 when driving Laser Diode NDB7352 Note 1: The time between the application of power and the diode going into forward conduction (lasing) is a function of the time period it has been off and the application of input power. The longer it has been off, the longer it takes to charge up the input circuit of the regulator before it will function. The fastest turn-on time was observed to be 7.2 ms after being off for 10 ms.
SL-D1 Circuit Operation The analysis of the SL-D1 LED driver was accomplished with the configuration shown in figure 1.
Figure 1; Test Circuit
Power Supply
Tek Digital O’Scope
CH1 CH2 CH3
SL-D1
NDB7352
10.02
The SL-D1 is based on the example design of the manufacturer of the regulator chip –AXElite. The first figure on page 4 of the AX2002 data sheet was used with the value of R2 changed to 0.2 ohms and the optional parts omitted. Figure 2 is the schematic of the SL-D1, D7 is the laser diode NDB7352.
The value of R2 sets the current that will flow thru the diode to a typical value of 1.2 amps, +/- 5%. This is the maximum current allowed by the data sheet for the NDB7352 laser diode. The delay noted in table 1 between the application of power and the laser diode starting to operate it due to the charging time of C2 and then the charging of C3 which is delayed by R1. The delay of R1 /C3 allow the input voltage to settle and the regulator internal circuitry to start to function before the power is applied to the laser diode. This delay results in a relatively smooth turn on voltage being seen by the laser diode. This is shown in figure 3, which is a screen capture of from the O’scope shown in figure 1. Note that there is neither voltage spikes nor overshoot that could be present when a switching power supply is used.
Figure 2; Schematic of SL-D1
Figure 3; O’Scope screen capture – Laser diode turn-on time Top trace: voltage into Laser Diode ~ 5v peak, Bottom trace: voltage into SL-D1 –7.5v peak
Figure 4 shows the start of the current flow through the diode in relationship to the voltage seen by the input to the laser diode. The turn-on of the diode is quick and with out transient voltage/current spikes. Connections are per figure 1. R5 and Ccomp are used to compensate (smooth-out) the regulation of the output drive voltage. This also slows the initial turn on voltage to the laser diode. The optional components shown in the manufacturer’s data sheet have been omitted, they would have provided output short-circuit protection.
Figure 3; O’Scope screen capture – Laser diode going into conduction (lasing) Top trace: voltage into Laser Diode ~ 5v peak, Bottom trace: current flow though laser diode, 250mV/0.2 ohm =1.25A peak
Figure 5 shows the shut-off characteristics of the driver circuit. There was no ringing of any type- this is due to the low resistance of the R2, which sets the current through the diode.
Figure 5; O’Scope screen capture – Laser diode turning off Top trace: voltage into the laser diode Bottom trace: current through the laser diode (measured as voltage across R2)
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