Datasheet LT3092 (Analog Devices) - 13

LT3092
applicaTions inForMaTion Quieting the Noise Paralleling Devices
When a reduction in the noise of the current source is Obtain higher output current by paralleling multiple desired, a small capacitor can be placed across RSET (CSET LT3092’s together. The simplest application is to run in Figure 7). Normally, the 10µA reference current source two current sources side by side and tie their inputs generates noise current levels of 2.7pA/√Hz (0.7nARMS together and their outputs together, as shown in Figure over the 10Hz to 100kHz bandwidth). The SET pin resistor 8. This allows the sum of the current sources to deliver generates a spot noise equal to in = √4kT/R (k = Boltzmann’s more output current than a single device is capable of constant, 1.38 • 10–23J/°K, and T is absolute temperature) delivering. which is RMS-summed with the noise generated by the 10µA reference current source. Placing a C Another method of paralleling devices requires fewer SET capacitor across R components and helps to share power between devices. SET (as shown in Figure 7) bypasses this noise current. Note that this noise reduction capacitor increases Tie the individual SET pins together and tie the individual start-up time as a factor of the time constant formed by IN pins together. Connect the outputs in common using R small pieces of PC trace as ballast resistors to promote SET • CSET. When using a capacitor across the SET pin resistor, the external pole introduced usually requires equal current sharing. PC trace resistance in milliohms/ compensation to maintain stability. See the Stability and inch is shown in Table 1. Ballasting requires only a tiny Frequency Compensation section for detailed descriptions area on the PCB. on compensating LT3092 circuits.
Table 1. PC Board Trace Resistance
A curve in the Typical Performance Characteristics section
WEIGHT (oz) 10mil WIDTH 20mil WIDTH
depicts noise spectral density for the reference current 1 54.3 27.1 over a 10Hz to 100kHz bandwidth. 2 27.1 13.6 Trace resistance is measured in mΩ/in The worst-case room temperature offset, only ±2mV be- tween the SET pin and the OUT pin, allows the use of very LT3092 IN small ballast resistors. RCOMP 10µA As shown in Figure 9, each LT3092 has a small 40mΩ CCOMP OR ballast resistor, which at full output current gives better + CCOMP than 80% equalized sharing of the current. The external – resistance of 40mΩ (20mΩ for the two devices in paral- SET OUT lel) only adds about 8mV of output voltage compliance at an output of 0.4A. Of course, paralleling more than two CSET LT3092’s yields even higher output current. Spreading the RSET ROUT device on the PC board also spreads the heat. Series input 3092 F07 resistors can further spread the heat if the input-to-output difference is high.
Figure 7. Adding CSET Lowers Current Noise Thermal Considerations
The LT3092’s internal power and thermal limiting circuitry protects itself under overload conditions. For continuous normal load conditions, do not exceed the 125°C maximum junction temperature. Carefully consider all sources of thermal resistance from junction-to-ambient. This includes (but is not limited to) junction-to-case, case-to-heat sink 3092fc For more information www.linear.com/LT3092 13 Document Outline Description Typical Application Absolute Maximum Ratings Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Applications Information Package Description Related Parts