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Very simple project ran smooth virtually, but real prototype does not.

Category: Hardware
Product Number: LT1819
Software Version: LTspice XVII(x64) (17.0.35.0)

I have a task to get simple lever shifter from 3.3V to 10V, exactly, for feeding the MOSFET driver from ESP32 microcontroller. It is double op-amp LT1819 used as comparator between fixed 2.7V and incoming signal from MC and logic inverter based on next op-amp. The SPICE netlist is as follows:

V1 N001 0 10 Rser=0.1
V2 N006 0 3.3 Rser=1
V3 N002 0 PULSE(0 3.3 0 5ns 5ns 1ms 2ms 10) Rser=1000
R1 N004 0 5000 tol=1 pwr=0.1
XU1 N005 N002 N003 N001 0 LT1818
XU3 N005 N003 N004 N001 0 LT1818
R2 N005 N006 1k
R3 N005 0 9.1k
.tran 5
.lib LTC2.LIB
.backanno
.end

1000 Ohm parasitic serial resistance in V3 (the virtual port of MC) eliminates the usual serial resistor placing. The simulation gives perfect result. But op-amps, bought from an reliable supplier (digikey.com) warm up quickly and does not act as expected.

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  • Hi Vahe:  The LT1819 should not be used as a comparator.  Here is text from the data sheet;  "Sustained differential inputs, however, will result in excessive power dissipation and therefore this device should not be used as a comparator."

    Depending on gate capacitance and speed required, I would suggest using a comparator for this task.  An LT1713 would do well, as it can be supplied with 10V.  Or if speed is not required, an LT1716 might do it (very weak pullup).

  • Dear Glen, thank you, thank you and once again, thank you.

    You have solved a head destroying puzzle for me.

  • Hello Glen. I did another mistake and have immediately ordered (non-cancelable) at arrow.com which was the only Co. that has stock. As it seems, the LT1713 being good choice at first instance, requires 2 supply rails. Can I leave the ground floating and connect the "Minus" rail to ground, making single supply +10V? If floating GND is not a way, could be the synthetic ground, build up from serially connected capacitors, from rail-to-rail play role of ground just for comparator chip? Next to this chip will be connected a mighty driver-DEIC420. It is out of production since IXYS acquisition by LittleFuse, but has not an equivalent alternative for given project. The minimal threshold of high level is 7V for DEIC420 input port.

  • Vahe, the LT1713 is rated for single-supply operation.

    • Rail-to-Rail Complementary Outputs (TTL/CMOS Compatible)
    • Specified at 2.7V, 5V and ±5V Supplies

    A quick look at the datasheet shows an example on the first page.