Q
We are designing a benchmark circuit for testing diagnosis algorithms. We would
like to use the analog switches array AD75019 to build several circuit
estructures. But we need to know the maximum current allowed through the
switch, before buying the AD75019.
A
The THD and frequency response specifications on the AD75019 assume a load
resistance of 10kohms and a source resistance of 600ohms. Assuming +/-12V
supplies, input signal swing of +/-12V and a typical on resistance of 150ohms,
the typical input current is 1.12mA. If you reduce the load resistance, then
the insertion loss of the switch increases the -3dB bandwidth will reduce, and
the THD will degrade.
The absolute maximum input current is limited by the power dissipation of the
package. The 44pin PLCC package can dissipate 1W absolute Maximum. I would
recommend that you operate the device at 75% of it's absolute maximum power
rating 750mW.
To calculate the maximum current for each switch, we must consider all power
dissipated within the AD75019 package. I will assume VDD = +12V, VSS = -12V,
VCC = +5V. We must calculate the power dissipated by the analog and digital
circuitry, subtract this from the maximum power rating to find how much power
can be safely dissipated by all switches.
Analog Supplies:
+/-12V supplies, with a serial clock of 5MHz, analog supply current is 1mA.
Power dissipated by the analog circuitry is then 24V x 1mA = 24mW
Digital Supplies:
+5V supply, with a serial clock of 5Mhz, digital supply current is 800uA.
Power dissipated by digital circuitry is 5V x 800uA = 4mW
CMOS switches:
The CMOS switches can safely dissipate 750mW - 24mW - 4mW = 722mW
Each CMOS switch will dissipate power equal to:
P_one_switch = Iswitch x Iswitch x Ron
Assuming a maximum of 16 CMOS switches will be on at any time, the power
dissipated by the switches is:
P_all_switches = 16 x Iswitch x Iswitch x Ron
For this calculation we must use the maximum on resistance of 300ohms (assuming
+/-12V supplies).
P_all_switches = 16 x Iswitch x Iswitch x 300
Re-arranging for Iswitch.
Iswitch = sqrt ( 0.722 / [16 x 300] )
Iswitch = 12.3mA
The maximum current for each switch should therefore be limited to 12.3mA. From
the calculations above, you should be able to calculate the max current for
different supplies, and any number of on switches. For example if you could
guarantee that only one switch would be on at any given time, a single switch
could dissipate 722mW equivalent to 49mA and still be inside the max power
rating. Note that the thermal resistance of the 44pin PLCC is approx
40degC/Watt. You should expect the case temperature to rise by 30degC above
ambient when dissipating 750mW.
One word of warning however. All power dissipation calculations have assumed
steady state conditions. Die temperature is a slow moving quantity with time
constants of the order of seconds. Do not assume that you can allow a single
switch to carry 49mA and then quickly switch to 16 switches carrying 12.3mA.
This would likely cause the die to heat up beyond it's max temperature of
115degC.
Note on general AD75019 operation:
The power up configuration of the AD75019 is indeterminate (not guaranteed).
You have to shift configuration data into the part first.
Also pin #39 on the AD75019 which is marked as a NC (no connect) on the
datasheet should be connected to an analog ground.
This pin is actually connected to a shield that should reduce crosstalk.
