DC/DC converter and LDO evaluation

The single most expensive components in our boards are the RECOM DC/DC converters. 55 kr/pcs.

Our requirements:

• Input voltage up to 60V
• Output current maximum 50 mA
• Very low minimum current consumption - ideally below 50 uA
• Output voltage 3.3V or 5V or 12V, depending on the board

It is a rare combination that we want such large voltage drop, but so little current. The smallest RECOM model is 500 mA output, 10x what we need. It has only 12 uA no-load consumption!!

Cheaper models typically need an external inductor. These only cost 1-2 kr. Other than that, it is the same as the RECOM we use now!

Maxim MAX15062A

• 300 mA max current
• Max 60V input
• Fixed 3.3V
• 95 uA no-load consumption
• 20 kr/piece
• Internal soft start
• Good availability

ST L7983

• 300 mA max current
• Max 60V input
• Fixed 3.3V
• 10 uA no-load consumption
• 22 kr/piece
• Internal soft start

TI LMR36503MSC3RPERQ1

• 300 mA max current
• Max 65V input
• Fixed 3.3V
• 4 uA no-load consumption
• 23 kr/piece

Conclusion

Moving to one of these, we could save about 30 kr per board, the component count would increase by one, the footprint would be about the same.

New DC-DC converters selection (I)

The aid is to substitute the Recom DC-DC converters and use some switching regulators so it doesn't take that much space in the board and also makes the design way cheaper. Thus, we have 2 voltage levels: 12 V and 3.3 V, then two different regulators are needed.

Among the ones that we've found on the internet, the most suitable ones considering prize and availability are:

60 V to 12 V

Part number	Datasheet	Iq (uA)	Adjustable / Fixed	Price
LMR38020	https://www.ti.com/lit/ds/symlink/lmr38020.pdf?ts=1632127416736&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLMR38020	40	Adj with proposed circuit.	22.11 kr - TI Mouser
MAX5033CUSA	https://no.mouser.com/datasheet/2/256/maxim%20integrated%20products_max5033-1179310.pdf	270	Fixed. Needs feedback resistors anyway.	29.15 kr - Mouser

12 V to 3.3 V

Part number	Datasheet	Iq (uA)	Adjustable / Fixed	Price (unitary)
TPS562211	https://www.ti.com/lit/ds/symlink/tps562211.pdf?ts=1632148536733&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTPS562211	120	Adj with proposed circuit.	4.37 kr - Mouser 2.19 kr - TI
TPS629210	https://www.ti.com/lit/ds/symlink/tps629210-q1.pdf?ts=1632383575403&ref_url=https%253A%252F%252Fwww.google.com%252F	4	Adj with proposed circuit. Only one resistor for setting Vout	5.5 kr - TI Mouser
L7983PU33R	https://no.mouser.com/datasheet/2/389/dm00709056-2042185.pdf	10	Fixed	22.87 kr - Mouser
ADP2360	https://no.mouser.com/datasheet/2/609/ADP2360-1503291.pdf	12	Fixed	28.22 kr - Mouser

Despite all options exposer fit nice for our design, the two selected have been LMR38020 for 12V level, and TPS629203 for 3.3V level.

Design considerations

Let's summarize the layout for these two converters to be used.

LMR38020

According to the manufacturer recommendations:

Then, the layout would remain as:

Moreover, the rest of the non-commented components are

• C1: The device requires a bootstrap capacitor connected between the BOOT pin and the SW pin. This capacitor stores energy that is used to supply the gate drivers for the power MOSFETs.
• R1: Input resistor for avoiding sparking.
• R4: Resistor (RT) for setting the device frequency. Value according to manufacturer recommendation.

TPS629203

According to the manufacturer recommendations:

About the output voltage selection, we can fix it to the desired value through the FB pin. Here we have two options:

• Adjustable operation (external voltage divider)
• Settable operation (internal voltage divider)

Actually, this last one is the recommended so it will save component count and improve output accuracy.

As can be seen, for a voltage of 3.3V we can decide not to populate with any resistor. This makes the device kind of a fixed version.

The schematic is depicted below considering all these specifications:

Finally, R6 sets the operation mode of the device. Refer to the following table for this resistor selection:

For the 2.2uH coil, the manufacturer recommends a 2.5MHz switching frequency so we minimize output ripple at all. Also, this value sets the discharge feature to ensure a defined down-ramp of output voltage when the device is being disabled and also to keep the output voltage close to 0V when the device is off. Finally, it is recommended Auto PFM/PWM mode. In this mode, the switching frequency is adjusted automatically based on input/output and load to maintain the highest efficiency.

New DC-DC converters selection (II)

After realizing that the power consumption on the previous two power converters was not good at all, we should consider to use other different ones.

Thus, considering the existing global lack of chips it is being hard to find the proper one available.

Looks like the MAX5033 series would be nice for our application. With only 270 uA of open load consumption, it is available in both 3.3V and 12 V fixed version which simplifies a lot the schematics. Also, this time we do not depend on two different components since these two belong to the same family.

From its datasheet:
The MAX5033 delivers up to 500mA output current. The
output current may be limited by the maximum power
dissipation capability of the package. External shutdown
is included, featuring 10μA (typ) shutdown current. The
MAX5033A/B/C versions have fixed output voltages of
3.3V, 5V, and 12V, respectively, while the MAX5033D features
an adjustable output voltage, from 1.25V to 13.2V

Turning back to Linear Voltage Regulators (LDO)

Finally, we have decided to use LDO's instead of switching regulators once we realized about the consumption of each subsystem in the board separately. The methodology that we have followed for that consisted on supplying each of the loads directly at is own voltage level by obviating the power converters. This way we had:

• Resistor divider: 60 V
• Active low pass filter (Op Amp): 12 V
• Microcontroller unit: 3.3 V

The picture below relates the result:

The consumption at each voltage rail is detailed in the table:

Vbat Rail - Resistor divider	51.8 uA
12 V Rail - Op Amp	73.3 uA
3.3 V Rail - Uc	2.01 mA

Then it is clear how the microcontroller would be drawing almost all the current from the battery. For this reason, by choosing the LDO's we are not having big difference on the consumption, we will reduce the amount of components and also the prototype will get cheaper.

• 60 V - 15 V: https://no.mouser.com/ProductDetail/onsemi/NCP785AH150T1G?qs=k6dkFvHqcvCmANHAXaPZDg%3D%3D
• 12 V - 3.3 V: https://no.mouser.com/ProductDetail/NJR/NJW4181U3-33B-TE2?qs=rkhjVJ6%2F3EL3oi%252BYBpdDyg%3D%3D