Designers of portable electronics such as cell phones, portable media players, and GPS devices are always pushing to squeeze every ounce of battery life out of the application. In particular, in the realm of power conversion, engineers aspire to ensure that every coulomb that leaves the battery finds its way to a point-of-load (POL) without being lost or dissipated as heat.
The solution is not always straightforward, though, because as in most analog disciplines, multi-dimensional tradeoffs must be considered. Specifically, efficiency and footprint often seem to pull in opposite directions. However, a split-supply lowdropout regulator (LDO), such as the FAN2560, may let you have your cake and eat it, too.
Switchers have become popular for extending battery life in applications where the POL is at a voltage substantially lower than the battery supply. A typical “energy conscious” application may use two switchers to supply power to 1.8-V and 1.5-V loads (Fig. 1).
The efficiency in such a system is quite high. Assuming a battery supply (VBAT) of 3.6 V and 150 mA delivered to each load, the total power-conversion efficiency of two 3-MHz switchers with multilayer chip inductors is typically 90%. However, the tradeoff is that the application requires two inductors covering about 10 mm2.
Another solution is to use one switcher and one LDO (Fig. 2). The obvious benefit is the need for only one inductor, which consumes about 5 mm2. However, the tradeoff is a lower efficiency due to linear regulation of the second channel.
With VBAT = 3.6 V and 150 mA delivered to each load, total power-conversion efficiency is typically 60% (90% for the switcher and 42% for the LDO). Not only does this hurt functions like talk time, but the system designer also now faces thermal-dissipation issues across the LDO.
A third approach is to use an LDO with a split-supply architecture (Fig. 3). In this case, the switcher is post-regulated by an FAN2560 split-supply LDO. This device uses one supply as the input to the drain of the pass FET and the other supply to drive the gate.
In this circuit, most of the voltage drop is converted at the higher efficiency provided by the switcher, while the remaining 300 mV is converted linearly from the output of the switching regulator (V1). VBAT is used only to bias the gate and, as a result, consumes only on the order of tens of microamps. This configuration needs only one 5-mm2 inductor and achieves an overall efficiency of 83%, assuming 150 mA for each load and VBAT = 3.6 V. In addition, by having access to the VBAT line, the architecture can use an NMOS as the pass device, which typically allows for better dynamic performance in areas such as transient response.
To summarize the three scenarios:
1: Efficiency = 90%, inductor footprint = 10 mm2
2: Efficiency = 60%, inductor footprint = 5 mm2
3: Efficiency = 83%, inductor footprint = 5 mm2
The above scenarios may have been somewhat “hand waved.” However, the margin of difference between the scenarios in terms of efficiency and footprint is large enough to allow such liberties.
In response to the first comment... For applications(such as digital loads) that don't require the filtering of an LDO, ideally we could do away with LDO's. This would mean having switchers for the majority of rails in the system. This is what you suggested should be done if efficiency were paramount and that my article is contrived if my solution contradicts this suggestion. I stand by my belief however that system engineers do strive to maximize efficiency. However nothing is ever so simplistic meaning that the two other main vectors of cost and size must be taken in account. It would be foolish to assume these to be irrelevant. My point in stating that energy is so critical was to point out the creative ends engineers will go to in order to optimize the tradeoffs. With regards to this not being worthy of an article because post-regulation has been done in the past, you seem to neglect the fact that post regulation using high performance low input supply LDO's is a relatively new concept. The numerous devices which "regulate with a linear after a switcher" have historically been for higher voltage noise sensitive analog point of loads that required the filtering characteristics of an LDO. This article is targeted towards lower voltage point of loads such as digital processors which can not effectively use a traditional LDO connected to the battery due to thermal issues and gross energy waste. Finally 5mm^2 does matter. In fact, it matters a lot. If you take it a step further and calculate the volumetric "footprint" of a state of the art 0805 inductor then you would be looking at 2.5x2x1mm = 5mm^3. 5mm^3 starts to add up quickly in high density consumer products which is why there is so much investment in the areas of power management IC's, passives, mechanics, display technologies, and battery chemistries aiming to shave off every 1mm^3 possible. These incremental improvements are how we have gotten to where we are today.
Peter Khairolomour -July 22, 2008
The article seems contrived, leading with a statement about pushing to squeeze battery life then ultimately suggesting a solution that reduces battery life for a small difference in the inductor footprint, and yet that at a cost of higher heat in a device implied to be designed as small as possible (else what was the point?).
This doesn't seem to be worthy of an article IMO, it's not so different than so many other devices out there which regulate with a linear after a switcher, except to find some use for a dual supplied FAN2560 - backwards engineering a suggested "need" for that slightly more integrated IC .
That in itself is a fair option, but if this is put towards such a small device that 5mm^2 matters much, it'll tend to have such a small battery that battery life mattered even more. Bring back devices big enough that adult hands can use them, and let's not throw away the gains made by Li-Ion, let's finally have what we secretly wanted all along, being able to use a device all day before recharging it.
Anyway, just one subjective opinion from someone who is a consumer also.
Anonymous -July 08, 2008
GOOD
Anonymous -June 22, 2008
INVETER
AJEET -June 22, 2008
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