Puzzling Problem of Using a LDO Regulator for the Power Supply of a Comparator

[Jiaqi]

Recently I am using a NEGATIVE output low-dropout (LDO) linear
regulator, TI's TPS72301, for the NEGATIVE power supply of a
comparator (designed as: -9V input, -8V output). By the way, the
positive power supply terminal of the comparator was connected to the
output of a positive LDO regulator keeping okay all the time.

Initially, TI's TLC3702 was chosen as the comparator (dual per
package). When turned on, as observed by an oscilloscope, the voltage
at both the output and the FB terminal of TPS72301 creeped from the
designed value, -8V, towards ground. The output of TPS72301 stops
creeping finally at -1.5V ca, while the FB voltage at -0.6V ca.

I'm sure my usage of TPS72301 is okay, since another TPS72301 with the
same circuitry (but used for power supply of some op amps) on my PCB
works well. Thus the comparator, TLC3702, was suspected. As mentioned
in its datasheet, TLC3702 was designed to operate from a SINGLE
supply. To implement dual supply operation, another type of
comparator, LM393, was substituted for TLC3702. However, its output
needs additional push-pull. Furthermore, TPS72301 still did not work
properly and this time its output turned to be -1.0V (I also replaced
TPS72301 with a new piece before the 2nd test). All external
components including the power supply security has been checked and no
fault was found. Now I have to doubt whether TPS72301 can be used with
general comparators.

I required TI's technical support by e-mail and the reply was that
TPS72301 had better not be used here and the positive supply must be
ensured during use of such a comparator. That called up an important
detail in my design: a deliberate turn-on time gap between the
positive supply and the negative (hundreds of microseconds). Is this
the cause of my problem? Or TPS72301 itself fooled me? As for me, the
latter is more likely.

Here list my next steps to study the problem:
1. Remove the comparator and see whether TPS72301 works properly;
2. If yes, replace LM393 with LT1018CN8 (a device of unique design,
see pg. 8 of its datasheet, 'NEGATIVE Voltage Regulator') and see
whether it is okay;
3. If it fails, remove TPS72301 and connect the negative supply of the
comparator directly to -9V input.

I would appreciate any instruction or information about the cause of
the problem or the possible solution.

 

[colin]

Recently I am using a NEGATIVE output low-dropout (LDO) linear
regulator, TI's TPS72301, for the NEGATIVE power supply of a
comparator (designed as: -9V input, -8V output). By the way, the
positive power supply terminal of the comparator was connected to the
output of a positive LDO regulator keeping okay all the time.

Initially, TI's TLC3702 was chosen as the comparator (dual per
package). When turned on, as observed by an oscilloscope, the voltage
at both the output and the FB terminal of TPS72301 creeped from the
designed value, -8V, towards ground. The output of TPS72301 stops
creeping finally at -1.5V ca, while the FB voltage at -0.6V ca.

tlc3702 is push pull but only pulls down to gnd not -v.
not sure if this is anything to do with this or not.

Colin =^.^=

 

[John Larkin]

Recently I am using a NEGATIVE output low-dropout (LDO) linear
regulator, TI's TPS72301, for the NEGATIVE power supply of a
comparator (designed as: -9V input, -8V output). By the way, the
positive power supply terminal of the comparator was connected to the
output of a positive LDO regulator keeping okay all the time.

Initially, TI's TLC3702 was chosen as the comparator (dual per
package). When turned on, as observed by an oscilloscope, the voltage
at both the output and the FB terminal of TPS72301 creeped from the
designed value, -8V, towards ground. The output of TPS72301 stops
creeping finally at -1.5V ca, while the FB voltage at -0.6V ca.

I'm sure my usage of TPS72301 is okay, since another TPS72301 with the
same circuitry (but used for power supply of some op amps) on my PCB
works well. Thus the comparator, TLC3702, was suspected. As mentioned
in its datasheet, TLC3702 was designed to operate from a SINGLE
supply. To implement dual supply operation, another type of
comparator, LM393, was substituted for TLC3702. However, its output
needs additional push-pull. Furthermore, TPS72301 still did not work
properly and this time its output turned to be -1.0V (I also replaced
TPS72301 with a new piece before the 2nd test). All external
components including the power supply security has been checked and no
fault was found. Now I have to doubt whether TPS72301 can be used with
general comparators.

I required TI's technical support by e-mail and the reply was that
TPS72301 had better not be used here and the positive supply must be
ensured during use of such a comparator. That called up an important
detail in my design: a deliberate turn-on time gap between the
positive supply and the negative (hundreds of microseconds). Is this
the cause of my problem? Or TPS72301 itself fooled me? As for me, the
latter is more likely.

Here list my next steps to study the problem:
1. Remove the comparator and see whether TPS72301 works properly;
2. If yes, replace LM393 with LT1018CN8 (a device of unique design,
see pg. 8 of its datasheet, 'NEGATIVE Voltage Regulator') and see
whether it is okay;
3. If it fails, remove TPS72301 and connect the negative supply of the
comparator directly to -9V input.

I would appreciate any instruction or information about the cause of
the problem or the possible solution.

What's the output of the TLC3702 connected to?

John

 

[crominator]

Recently I am using a NEGATIVE output low-dropout (LDO) linear
regulator, TI's TPS72301, for the NEGATIVE power supply of a
comparator (designed as: -9V input, -8V output). By the way, the
positive power supply terminal of the comparator was connected to the
output of a positive LDO regulator keeping okay all the time.

Initially, TI's TLC3702 was chosen as the comparator (dual per
package). When turned on, as observed by an oscilloscope, the voltage
at both the output and the FB terminal of TPS72301 creeped from the
designed value, -8V, towards ground. The output of TPS72301 stops
creeping finally at -1.5V ca, while the FB voltage at -0.6V ca.

I'm sure my usage of TPS72301 is okay, since another TPS72301 with the
same circuitry (but used for power supply of some op amps) on my PCB
works well. Thus the comparator, TLC3702, was suspected. As mentioned
in its datasheet, TLC3702 was designed to operate from a SINGLE
supply. To implement dual supply operation, another type of
comparator, LM393, was substituted for TLC3702. However, its output
needs additional push-pull. Furthermore, TPS72301 still did not work
properly and this time its output turned to be -1.0V (I also replaced
TPS72301 with a new piece before the 2nd test). All external
components including the power supply security has been checked and no
fault was found. Now I have to doubt whether TPS72301 can be used with
general comparators.

I required TI's technical support by e-mail and the reply was that
TPS72301 had better not be used here and the positive supply must be
ensured during use of such a comparator. That called up an important
detail in my design: a deliberate turn-on time gap between the
positive supply and the negative (hundreds of microseconds). Is this
the cause of my problem? Or TPS72301 itself fooled me? As for me, the
latter is more likely.

Here list my next steps to study the problem:
1. Remove the comparator and see whether TPS72301 works properly;
2. If yes, replace LM393 with LT1018CN8 (a device of unique design,
see pg. 8 of its datasheet, 'NEGATIVE Voltage Regulator') and see
whether it is okay;
3. If it fails, remove TPS72301 and connect the negative supply of the
comparator directly to -9V input.

I would appreciate any instruction or information about the cause of
the problem or the possible solution.

Perhaps I missed this in your text - what's going on with the -9V
during the collapse of the -8V? How much current can the negative
supply handle?

the problem is the solution
Crom

 

[Jiaqi]

The comparator output is driving a N-channel MOSFET.

 

[Jiaqi]

The external -9V supply keeps okay all the time. The rated current is
0.8A. As to TPS72301, the value is 0.2A.
I'll examine the output of TPS72301 after removing the comparator. But
that have to be done after a week's holiday. Now the laboratory is
closed.

 

[Robert Latest]

Jiaqi wrote:

[pos/neg regulator troubel snipped]

I don't know what causes your problem, but as a rule I always put
reverse-biased diodes between reg output and ground. That prevents any rail
from getting pulled into the wrong polarity by the "opposite" regulator
during transitional startup phases or some other fault (short between pos
and neg rail), which could wreak all sorts of havoc in the supplee.

robert

 

[Jiaqi]

At each input supply there put a proper TVS diode.

 

[crominator]

Recently I am using a NEGATIVE output low-dropout (LDO) linear
regulator, TI's TPS72301, for the NEGATIVE power supply of a
comparator (designed as: -9V input, -8V output). By the way, the
positive power supply terminal of the comparator was connected to the
output of a positive LDO regulator keeping okay all the time.

Initially, TI's TLC3702 was chosen as the comparator (dual per
package). When turned on, as observed by an oscilloscope, the voltage
at both the output and the FB terminal of TPS72301 creeped from the
designed value, -8V, towards ground. The output of TPS72301 stops
creeping finally at -1.5V ca, while the FB voltage at -0.6V ca.

I'm sure my usage of TPS72301 is okay, since another TPS72301 with the
same circuitry (but used for power supply of some op amps) on my PCB
works well. Thus the comparator, TLC3702, was suspected. As mentioned
in its datasheet, TLC3702 was designed to operate from a SINGLE
supply. To implement dual supply operation, another type of
comparator, LM393, was substituted for TLC3702. However, its output
needs additional push-pull. Furthermore, TPS72301 still did not work
properly and this time its output turned to be -1.0V (I also replaced
TPS72301 with a new piece before the 2nd test). All external
components including the power supply security has been checked and no
fault was found. Now I have to doubt whether TPS72301 can be used with
general comparators.

I required TI's technical support by e-mail and the reply was that
TPS72301 had better not be used here and the positive supply must be
ensured during use of such a comparator. That called up an important
detail in my design: a deliberate turn-on time gap between the
positive supply and the negative (hundreds of microseconds). Is this
the cause of my problem? Or TPS72301 itself fooled me? As for me, the
latter is more likely.

Here list my next steps to study the problem:
1. Remove the comparator and see whether TPS72301 works properly;
2. If yes, replace LM393 with LT1018CN8 (a device of unique design,
see pg. 8 of its datasheet, 'NEGATIVE Voltage Regulator') and see
whether it is okay;
3. If it fails, remove TPS72301 and connect the negative supply of the
comparator directly to -9V input.

I would appreciate any instruction or information about the cause of
the problem or the possible solution.

Jiaqi,

A couple of things come to mind after looking at the data sheet for
the LDO.

One is whether the output capacitor, if electrolytic, might be
reversed. The other is whether there is sufficient output
capacitance, with the output capacitor having a low enough ESR for the
regulator to be stable. The ESR is critical in some of these LDOs
regarding stability. Have seen output oscillation on LDOs with what
otherwise should have been good ceramic capacitors, but were a bit
high on ESR.

To test this if you don't have something to measure the ESR, add some
extra capacitors directly to the output to circuit ground.

If you have an oscilloscope available, look at the output to see if it
is oscillating.

Make sure there is no extra capacitance on the feedback resistors.
Depopulate the capacitor, if one is there, between the feedback pin
and ground.

Crom

 

[Jiaqi]

Thanks for your remark, Crom. I appreciate your carefulness.
Please read the facts listed in my first message:As soon as the lab reopens, the output of TPS72301 will be checked
with the comparator removed.

Jiaqi