New-Tech Europe Magazine | Feb 2017

will be lower figures in practice than this optimum value, but they should still be above 40%. The devices also deliver up to 0.5A and have a single pin programmable output with a fixed resistor. These figures are very impressive compared to older regulators, which would take a few milliamps with no load. If you are using a switching regulator with an external MOSFET, bear in mind that the MOSFET switching time can result in significant losses. The transition from non-conducting to conducting is the time when a switching MOSFET dissipates the most power. When it is turned fully on, the voltage drop will usually be very small and hence power dissipation will be low. However, partly turned on there will be a significant voltage drop across the MOSFET accompanied by significant current. You therefore want to minimize the time that the transistor spends in that state by choosing a fast switching device and low gate capacitance. Low ON resistance is a must. Power Supply Shutdown You can keep power supply capacitors small if power supplies are shut down in sleep mode. It takes energy to charge them and if the power supply is shut down when in a sleep mode then the energy in the capacitors is normally wasted. For example, a 1μF capacitor on the power supply of circuitry which is shut down 100 times per second will consume 165μA at 3.3V (same calculation as before). Many ICs will take less than that in shutdown or sleep mode, so it is often better to keep circuitry powered but in a sleep state than to actually do power switching to save power. The exception to this advice would be if the device used didn't have a sleep mode or if its sleep mode was not

Figure 1: ADP5301 functional block diagram

data to be transferred as much as possible. To demonstrate how much energy can be lost, if four pins have a capacitance of 5pF in a system running at 20MHz from a 3.3V supply, 660μA will be drawn from just pin capacitance. This figure can be determined from the equation I = 0.5CVf. The current drawn will be the total of both the data sent and received, which can mount up. This figure can be cut by using a highly integrated chip. Internal communications don’t suffer from pin capacitance, therefore having more peripherals on-board is better from a power consumption point-of- view. On-chip RAM and flash memory offer the same power savings. An Efficient Power Supply Choosing a switching regulator for a switched mode power supply is a key factor in maximizing efficiency. This is particularly true for synchronous regulators where efficiencies of

over 95% are possible. However, it is not just headline efficiency, or even standby efficiency, that is necessarily the most critical factor. It is necessary to look at the current in different modes for the device and determine the contribution to overall power consumption from each mode after taking into account the switching regulator efficiency at each current level. There are some quite impressive regulators around though, such as the new Analog Devices ADP5301 Step-down Regulators. The quiescent current of these devices is down as low as 180nA when not-switching, but still operating in hysteresis mode. It will switch for a short burst to add charge to the output capacitor using the inductor at very light loads, then return to just the quiescent current. The low quiescent current can give efficiencies as high as 80% at 1μA depending on the input and output voltages. It is more likely that there

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