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BQ79616-Q1: Using DCDC instead of on-chip LDO?

Part Number: BQ79616-Q1
Other Parts Discussed in Thread: BQ79616, LM5168,

Team,

It seems that Vin is 6V and Vout 5V routed to the different internal blocks.
Do we have some information about the overall efficiency when using the on-chip LDO?

Does it make sense not to use the on-chip LDO to power the LDOIN pin? Is it at all supported not to use the on-chip LDO?
Do you have some recommendation (schematics, DCDC IC recommendation, PCB design) to use an external DCDC instead?
Are there some precaution to take with the on-chip LDO if not used to power LDOIN?

What would be the system level careabout in case a DCDC will be used?
Are there some specific design aspects (like power up sequencing, ripple, ..etc) that need to be carefully considered?
For example I have read in the datasheet that 8V is required on Vin for the OTP.

A.

• Hi Anthony,

The overall efficiency with NPN transistor LDO varies depending on the current consumption by the system and the system supply Voltage. From the Datasheet specification you can see our typical current consumption numbers in each state. Just for clarification here we can assume that the average current consumption of the BQ79616 device in active state is approx. 13mA. Further if we assume that the system is powered by 16 battery cells, 4V each cell then the system voltage is 64V. The voltage drop on the external NPN transistor and Resistor (if used) is 64V - 6V = 58V and with a current of 13mA we get 0.754W of dissipation on the external components and the used power here is 6V * 13mA = 0.078W so the efficiency in the system is approx. 9.375%. We sometimes add additional margin to the current draw (up to 25mA) so the efficiency here will further drop.

Yes it does make sense to use an external DCDC converter instead of the in-built LDO with external FET in order to increase the efficiency. In this case the on-chip LDO is bypassed and the DCDC output supplies directly to the LDOIN pin.  We don't have any official documentation on this system however we have done some preliminary testing with the TPS55160 buck-boost device and the preliminary measurements were satisfactory.

The key careabouts to this approach would be the output voltage regulation (6V vs. 8V if OTP programming is needed), the current capability of the DC-DC, low ripple and considerations for EMI/EMC performance.

Regards,

Viktor.

• Hi Viktor,

thanks for your answer. Our idea so far is to use an DCDC convert with enable Pin. We would like to be able to disable the converter. As enable signal we would like to use the NPNB signal (with voltage divider). Would that be possible? We don't need OTP (or programm the OTP during production on ICT).

You mentioned the TPS55160, but this one has a input voltage of maximal 36V. We would need around 64V. Do you have any proposal for a suitable DCDC converter?

Another idea would be to use this concept to supply two BQ:

But than we would need a DCDC with a max voltage of at least 128V. The shown LM5168 has a maximal input voltage of 115V. Does TI has DCDC converters with more than 120V. I could not find one.

BR

Christian

• Hi Christian,

I will answer the questions regarding the BQ79616-Q1 device here and then transfer this topic to the team that supports Buck DC-DC converters.

I don't believe you can use the NPNB pin, due to two reasons:

1. The NPNB pin is always active when the BQ device is power supplied, you cannot disable it, so this beats the purpose.

2. The NPNB pin can go as high as VBAT, so if your system is powered by 16 battery cells, this means that the NPNB pin can potentially go up to approx. 72V.

Regards,

Viktor.

• Hello Christian,

The largest input voltage range we have for flybuck is up to 120V, normal operating.