Hybrid DC-DC/LDO chip, do you think it's useful? - Page 1
Forum section: electronics / projects

author: ivan747
author_rank: Super Contributor
timestamp: February 18, 2014, 02:42:38 pm
content: Does such thing exists. I was thinking of a project where I have to step down 9V to 5V (maybe 3.3V but that doesn't come into play). The project will spend most of its time in sleep mode, but during briefs periods of 2 to 5 minutes, it will draw 100 to 200mA.As far as I know, DC-DC converters tend to have high quiescent current, even if they implement some method of skipping PWM pulses. LDOs can be made with extremely low quiescent currents. The problem with LDOs is when current gets high, they waste a lot of power. DC-DC converters shine in this application.And what if the product is battery powered and I want to squeze the last bit of that discharge curve of my batteries. LDOs are the best for this. DC-DC converters rarely have millivolt dropout voltages.That makes me wonder if it is feasible to design a hybrid DC-DC converter mixed with an LDO. The same switching MOSFET could be made to operate in its linear region during low current consumption situations and then start to switch the series inductor when the current gets higher. When the drop-out voltage is too low, it could operate in the linear region again.Another method would be to make the DC-DC converter and the LDO separate feedback loops with an extra circuit that decides which one should be enabled. We can do this with existing chips. DC-DC regulators and LDOs come with enable pins so you could technically turn one or the other off with a TTL signal. All the logic can be made with an 8 pin very low power microcontroller that has decent comparators or ADCs built in.A better solution would be to have a chip substitute the microcontroller, something that doesn't require a clock and has the precision needed. You could set the critical values with (like at what voltage or current should I switch between regulators) resistors in the 500k range.The ideal solution is to have an LDO with the chip I described in the previous paragraph built in.I guess this is useful only in some applications, where there can be relatively long bursts of high power (maybe a transmission) and lots of standby time, maybe waiting to receive packets of wireless data. Some application where preserving power is critical. I am thinking of cellphones and devices like weather stations or some sort of device that sits all by itself out there and harvests its own energy to store it in a battery or ultra-capacitor.

author: dannyf
author_rank: Super Contributor
timestamp: February 18, 2014, 03:43:15 pm
content: Buck converters are natural LDO in the sense that if the input current is too low it goes fully on (100 percent duty cycle).Burst mode dc converters try to solve your issue: they would skip beats when the load is light (so to minimize switching losses) but goes full out when the load is heavy. The disadvantage there is emf and high ripple under light load.

author: tszaboo
author_rank: Super Contributor
timestamp: February 19, 2014, 10:18:10 pm
content: I would place the LDO and the switching in paralel, and connect the outputs together with an ORing controller. There are even prioritized ORing controllers, you turn on every now and then the switcher. If it drains too much standby, then put maybe a power switch before it.Hybrid chips exist, analog devices has usually some 5V input, and linear technology some which is way to expensive for what it does, but good for the lazy.

author: Neverther
author_rank: Regular Contributor
timestamp: February 19, 2014, 11:23:21 pm
content: I assume that is a regular 9V battery.I may be derailing this, but have you considered using AA/AAA as they have a bit better energy density.I got battery powered stuff and I throw boost regulators at them. You are considering buck, so the ripple shouldn't be an issue.Currently I got SP6641 (AEK low current version as It can start at pretty low battery voltage, BEK can give half an amp) there and it doesn't cause too much excess current draw (quoted 10uA typical at output, never bothered to check it as it wasn't too bad) when the board is on listen mode (0.5mA to 2.5mA) but works fine when the board is on full load (40-60mA, usually less as that is worst case).Currently looking at NCP1402 as it seems similar but has a lot more in stock for future proofing.Two AAs in series and they go pretty far as when they are started, they operate down to 0.5V. Might not work with high currents as 200mA, but 200mA is pretty high for battery powered stuff. But same goes for 9V battery (I assume?) when it starts going empty.The 200mA current is going to sag normal 9V battery pretty bad when it starts going.Hybrid setup makes more sense when you want smooth output and better than linear efficency from wide voltagerange.Single lithium 18650 for better energy density with good current output across the whole discharge, the size is still similar to 9V.Choochoo into the ravine.