LTC2662 FAQ

Q1: Can the outputs be connected together to sum the currents?

A1: Yes, any or all of the outputs can be connected together to increase the current output.  Since the outputs are current sources no additional circuitry, like a resistor, is require to sum two channels of the LTC2662-16/12.  This is useful if more than 300mA is required for a specific load, or if fine control is required at some higher output current.

Q2: What external reference do you recommend for the LTC2662-16/12

A2: For most applications the internal voltage reference is adequate.  For applications that require an external reference we recommend the LTC6655-1.25.  All of the datasheet specifications that used an external reference were taken with the LTC6655-1.25.

Q3: Can you sink current with the LTC2662-16/12

A3:  The LTC2662-16/12 does not allow for controlled current sink.  However, there is a mode that allows current to be shunted to the negative supply which will sink current off of the load.  This is a useful mode if the current on a load needs to be taken away quickly and completely.

Q4: Can I use a switching regulator to power the VDD0/1/2/3/4  pins?

A4:  Switching regulators can be used to power the VDD0/1/2/3/4 to improve power efficiency in the system.  The typical power supply rejection ratio of the LTC2662-16/12 is -80dB at 10kHz, but increases over frequency to -20 at 10MHz

Q5:  Can I use the LTC2662-16/12 with a single supply? 

A5:  Yes, but because Vcc and IOVcc have an upper limit of 5V the maximum single supply voltage that is possible is 5V.  If higher voltage are required 5V will need to be supplied to Vcc and IOVcc and the high supply can be used on V+ and VDD0/1/2/3/4.

Q6: How many cycles does it take to clear a fault condition from the fault register?

A1:  It will take a minimum of three SPI transfers to clear a fault condition from the fault register.  After the first SPI transfer the fault will be detected.  The second transfer will likely change a condition that will eliminate the fault condition.  After the fault condition is mitigated the next transfer will show that the fault is cleared.  

Q7: Can multiple parts be programmed by connecting the SDO of one part to the SDI of the next in a daisy chain configuration?

A7:  Yes, using a daisy chain configuration is a convenient way to program multiple parts simultaneously.  This is done by connecting the SDO of the first part to the SDI of the second, and so forth.  This technique requires 32-bit serialization to be used.

Q8: Does using the “daisy chain” technique reduce throughput?

A8: Yes, throughput will not be as high  when daisy chaining multiple parts. First the SDO will be shifted out on the falling edge of /CS and an additional delay to be shifted out of the first part.  There will also be a setup time associated with the second part in the daisy chain.  These delays require a throughput that is lower than the 50MHz that is achievable with a single SPI transfer.

Q9. What are the layout guidelines for this part?

A9.  The thermal design is critical to the performance of the LTC2662-16/12.  The exposed pad should be tied to a solid ground plane one layer down from the IC.  There should be an array of vias connecting the paddle to this ground plane.  If the part were to overheat there is a thermal shutdown in the part, but using a good thermal design for the application should reduce the need for this thermal shutdown.

Q10. How fast does the LTC2662-16/12 settle after a change in the output?

A10. For large current output steps, internal thermal effects result in a final settling tail. In most cases the tail is too small to affect settling to ±0.024%, but several milliseconds may be needed for full settling to the ±0.0015% level. For best results, always solder the exposed pad to a solid GND plane, and set VDD0/1/2/3/4 as low as practicable for each channel to reduce power dissipation in the part.