[SOLVED] Looking for equivalent ML4790 Adjustable Output Regulator

Hello,

I have a blown ML4790 Adjustable Output Regulator in my instrument , it takes battery voltage 3V as input and produces a stable 3V output with its own GND and has on/shutdown signal. I tried to loacte a source for the ML4790 but did not find any.
Attached is the datasheet of the ML4790 , any experience with an equivalent regulator will be appreciated.
Many thanks in afvance!

Hi,

is your input voltage really 3 V and your output voltage also 3 V? If so, it makes no sense to me to use the ML4790. If your input is provided by a 3 V battery, it should be stable/constant.
If your input voltage is below 3 V, the use of the LM4790 makes sense, as it boosts your input voltage and ragulates the output voltage by means of a LDO.

According to the schematic on the first side, the LM4790 does not provide a galvanic isolation, thus input and output are referenced to the same ground. This is also indicated by the used GND symbol.

Please provide some information about your input voltage source. Is it really 3 V, maybe two AA batteries in series?
I would expect an input voltage of 1.2 V to 1.5 V (single AA-accumulator or AA-battery).

Greets

the data sheet indicates end of life July 1, 2000

try this:
https://www.digikey.com/products/en?keywords=ML4790

stenzer said:

Hi,

is your input voltage really 3 V and your output voltage also 3 V? If so, it makes no sense to me to use the ML4790. If your input is provided by a 3 V battery, it should be stable/constant.
If your input voltage is below 3 V, the use of the LM4790 makes sense, as it boosts your input voltage and ragulates the output voltage by means of a LDO.

According to the schematic on the first side, the LM4790 does not provide a galvanic isolation, thus input and output are referenced to the same ground. This is also indicated by the used GND symbol.

Please provide some information about your input voltage source. Is it really 3 V, maybe two AA batteries in series?
I would expect an input voltage of 1.2 V to 1.5 V (single AA-accumulator or AA-battery).

Greets

Click to expand...

Hello,

Many thanks for your input.

Yes, the input voltage is 3V from 2x1.5v batteries, the instrument is a laser source unit from agilent/hp N3974A. It has 2 DC-DC regulators, one is step up type for the laser source frontend electronics unit which is functioning ok and has a stanby function
for the "soft" power on button. When power-on is pressed, it wakes up and sends "ON" signal to the damaged regulator ML4790 (at pin 8) which starts the 3v-3v regulator and feeds the computer part together with the LCD display. No schematics are available.

Regarding the GND:
Here is the trycky part, The input ground of the ML4790 is the same as the battery ground GNP pin 2 but the output GND is at pin 8 (PWR GND) and are not the same! According to the datasheet , boost control block delivers seperate GND for the output.
The same scenario is fo rthe other regulator that controls the laser frontend.

The best way was to find a pin compatable equivilent regulator for the N3974A, but I could may be use another 3v-3v regrulator that delivers different GND and has a shutdown function similar to the N3974A (pin 7 SHDN Pulling this pin high shuts down the regulator, isolating the load from the input ).
Any input on which regultor can do the job?
Many thanks

Hi,

have you measured the isolation between GND and Power-GND by means of a multimeter? I'm just curious, because I'm aware of this kind of dedicated GND labeling from other kind of power ICs although they are not isolated. Maybe the LDO output is referenced to the Power-GND to ensure proper regulation.

Unfortunatly, you don't get a single ML4790 at digikey, linked by wwfeldman.

By using an other IC, you also have to modify your external components e.g. due to a different switching frequency and resistive voltage divider.

Two possible solutions would be:

(i) Use a step up converter, maybe a low ripple one with 6 or 8 pins. You may have to desolder almost all your external parts. Use an additional 3-pin LDO e.g. TO-92. Keep the TO-92 thermal resistance of 160 °C/W in mind.
(ii) If there is some space left, design a simple PCB housing an appropriate DCDC & LDO combination, or a commbined IC like the MAX1765 [1]. Have a look at digikey at "PMIC - Voltage Regulators - Linear + Switching".

[1] https://datasheets.maximintegrated.com/en/ds/MAX1765.pdf

greets

stenzer said:

Hi,

have you measured the isolation between GND and Power-GND by means of a multimeter? I'm just curious, because I'm aware of this kind of dedicated GND labeling from other kind of power ICs although they are not isolated. Maybe the LDO output is referenced to the Power-GND to ensure proper regulation.

Unfortunatly, you don't get a single ML4790 at digikey, linked by wwfeldman.

By using an other IC, you also have to modify your external components e.g. due to a different switching frequency and resistive voltage divider.

Two possible solutions would be:

(i) Use a step up converter, maybe a low ripple one with 6 or 8 pins. You may have to desolder almost all your external parts. Use an additional 3-pin LDO e.g. TO-92. Keep the TO-92 thermal resistance of 160 °C/W in mind.
(ii) If there is some space left, design a simple PCB housing an appropriate DCDC & LDO combination, or a commbined IC like the MAX1765 [1]. Have a look at digikey at "PMIC - Voltage Regulators - Linear + Switching".

[1] https://datasheets.maximintegrated.com/en/ds/MAX1765.pdf

greets

Click to expand...

Hello,
Thanks for the information.
I measured the battery ground and the ground of the digital board (PWR GND) with an Ohm meter, it gives different resistences on different polarity positions of the Ohm meter connections , so some active devices involved, I can see one 8 pin IC or mosfet is connected to battery ground and som resistors caps, etc.. It should be some kind of isolation and some battery standby circuit involved.
What I need is a step up converter with isolated GND and shutdown function.
Best regards

- - - Updated - - -

stenzer said:

Hi,

have you measured the isolation between GND and Power-GND by means of a multimeter? I'm just curious, because I'm aware of this kind of dedicated GND labeling from other kind of power ICs although they are not isolated. Maybe the LDO output is referenced to the Power-GND to ensure proper regulation.

Unfortunatly, you don't get a single ML4790 at digikey, linked by wwfeldman.

By using an other IC, you also have to modify your external components e.g. due to a different switching frequency and resistive voltage divider.

Two possible solutions would be:

(i) Use a step up converter, maybe a low ripple one with 6 or 8 pins. You may have to desolder almost all your external parts. Use an additional 3-pin LDO e.g. TO-92. Keep the TO-92 thermal resistance of 160 °C/W in mind.
(ii) If there is some space left, design a simple PCB housing an appropriate DCDC & LDO combination, or a commbined IC like the MAX1765 [1]. Have a look at digikey at "PMIC - Voltage Regulators - Linear + Switching".

[1] https://datasheets.maximintegrated.com/en/ds/MAX1765.pdf

greets

Click to expand...

Hi ,

WHat I have done now is that I removed the IML4790 and shorted pad Vin and Vout and the instrument worked fine when I inserted the batteries (2x1.5V) but I lost the standby functionality, I need to remove the betteries to shutdown.
I think the ML4790 is setup to convert 3v to 3.3v but 3v seems to work fine.
it seems that the battery GND is handled some where else , I will try to track the battery ground

Ok, that's how i expected the circuit works, because I can not see any galvanic isolation between input and output by having a look on Fig. 2 in the datasheet.

As the shut down logic level is active HIGH, a PMOST should do the job to disconnect your battery. What's the shut down voltage level?

greets

hafrse said:

Hello,
Thanks for the information.
I measured the battery ground and the ground of the digital board (PWR GND) with an Ohm meter, it gives different resistences on different polarity positions of the Ohm meter connections , so some active devices involved, I can see one 8 pin IC or mosfet is connected to battery ground and som resistors caps, etc.. It should be some kind of isolation and some battery standby circuit involved.
What I need is a step up converter with isolated GND and shutdown function.
Best regards

- - - Updated - - -



Hi ,

WHat I have done now is that I removed the IML4790 and shorted pad Vin and Vout and the instrument worked fine when I inserted the batteries (2x1.5V) but I lost the standby functionality, I need to remove the betteries to shutdown.
I think the ML4790 is setup to convert 3v to 3.3v but 3v seems to work fine.
it seems that the battery GND is handled some where else , I will try to track the battery ground

Click to expand...

Found it! You wwre correct!
I was wrong about GND was isolated: The battery ground goes to a current sense amplifier MAX9928 https://datasheets.maximintegrated.com/en/ds/MAX9928-MAX9929.pdf the output from MAX9928 goes to both battery GND input and PWR GND so NO isolation of ground!
so only a simple step up regulator 3v->3.3v with shutdown function will work .

Many thanks for your valuable comments

- - - Updated - - -

stenzer said:

Hi,

have you measured the isolation between GND and Power-GND by means of a multimeter? I'm just curious, because I'm aware of this kind of dedicated GND labeling from other kind of power ICs although they are not isolated. Maybe the LDO output is referenced to the Power-GND to ensure proper regulation.

Unfortunatly, you don't get a single ML4790 at digikey, linked by wwfeldman.

By using an other IC, you also have to modify your external components e.g. due to a different switching frequency and resistive voltage divider.

Two possible solutions would be:

(i) Use a step up converter, maybe a low ripple one with 6 or 8 pins. You may have to desolder almost all your external parts. Use an additional 3-pin LDO e.g. TO-92. Keep the TO-92 thermal resistance of 160 °C/W in mind.
(ii) If there is some space left, design a simple PCB housing an appropriate DCDC & LDO combination, or a commbined IC like the MAX1765 [1]. Have a look at digikey at "PMIC - Voltage Regulators - Linear + Switching".

[1] https://datasheets.maximintegrated.com/en/ds/MAX1765.pdf

greets

Click to expand...

Found it! You wwre correct!
I was wrong about GND was isolated: The battery ground goes to a current sense amplifier MAX9928 **broken link removed** the output from MAX9928 goes to both battery GND input and PWR GND so NO isolation of ground!
so only a simple step up regulator 3v->3.3v with shutdown function will work .

Many thanks for your valuable comments

- - - Updated - - -

stenzer said:

Ok, that's how i expected the circuit works, because I can not see any galvanic isolation between input and output by having a look on Fig. 2 in the datasheet.

As the shut down logic level is active HIGH, a PMOST should do the job to disconnect your battery. What's the shut down voltage level?

greets

Click to expand...

Found it! You wwre correct!
I was wrong about GND was isolated: The battery ground goes to a current sense amplifier MAX9928 **broken link removed** the output from MAX9928 goes to both battery GND input and PWR GND so NO isolation of ground!
so only a simple step up regulator 3v->3.3v with shutdown function will work .

Many thanks for your valuable comments

Keep the output ripple of your DCDC converter in mind. I don't know what's connected to its output e.g. an analog signal conditioning circuitry. The ML4790 has a typical output ripple of 5 mV. Also have a look on the quiescent current, to ensure a long operating time whitout replacing the batteries.

greets

stenzer said:

Keep the output ripple of your DCDC converter in mind. I don't know what's connected to its output e.g. an analog signal conditioning circuitry. The ML4790 has a typical output ripple of 5 mV. Also have a look on the quiescent current, to ensure a long operating time whitout replacing the batteries.

greets

Click to expand...

The output goes to a digital circuit with cpu, display, keyboard, etc...
I orderd a complete regulator board for about 5USD from my local dealer https://www.tme.eu/Document/acd74734ca04395204f859807ba64ba5/POLOLU-2561.pdf hope it work fine