Monthly Archives: May 2015

Accurate low-current measurements with a scope

Last week was incredibly busy so I wasn’t able to put the time in to complete the third part of the “Leakage currents & current leaks” post. This will be a short post with a link to a white paper on our website for more details.

Most engineers consider the oscilloscope their first tool of choice for hardware development work. Yet very few engineers ever consider how accurate their scope is. Most of the major oscilloscope manufacturers place great importance on the timing aspects of their products. Multi-gigahertz sample rates are fairly common today in mid-range digital scopes today yet most of those scopes only have 8-bit A/D converters. While timing accuracy is often spec’d in double-digit ppm, voltage measurement error on the same scope can be as much as the signal level you need to measure using a scope’s lowest volts/division setting.

Below are screen shots taken on the same mid-range MSO scope from one of the top scope companies of similar current waveforms using a “standard” scope probe and our CMicrotek µCP100 low current probe. Using a 10mV/division setting on the scope with the standard probe produced a waveform that was way too fuzzy to be useful. The measurements taken with the cursors were almost 2X too high for the peak and plateau portions of the waveform and well over 5X too high for the portions before the peak and after the plateau.

probe_SR_FW_task_shot“Standard” Scope Probe

uCP_FW_task_shotµCP100 Current Probe

For more information on these waveforms and an example of the calculations used to determine the measurement accuracy of a digital scope check out our “Accurate Current Measurements with Oscilloscopes” whitepaper.

My plan is to wrap up the “Leakage currents & current leaks” posts next week. If you find the Low Power Design blog interesting, please help spread the word about it so we can build up a large enough audience to make it worth the time it takes. We are on Facebook, Twitter, Google+, the Element14  Community and have a LinkedIn group.

Leakage current & current leaks – part 2

Virtually all semiconductor devices have some amount of leakage current. It is interesting to note as operating voltages and device power consumption keep dropping, leakage current is becoming a larger percentage of a device’s power consumption. In most cases there isn’t much you can do about leakage currents other than be aware of them and account for them in your power analysis. In some cases there may be a significant difference in leakage current levels from manufacturer to manufacturer for devices that perform the same function so it pays to take the time to include leakage current comparison in your component selection. For a CMOS device that isn’t actively being clocked, leakage current can make up a significant part of its power consumption that may be called out as “standby current” or “quiescent state current” in the datasheet specs.

Diodes (including LEDs) are one a few types of devices where the circuit design can play a part in determining the extent of the leakage current in that design.

Diode & LED leakage current

Diodes can present substantial leakage currents when in a reverse voltage condition, this is often referred to as reverse current. Similar to capacitors, there are a number of factors that come into play in determining the level of leakage current. Unlike capacitors, there aren’t any simple formulas to help you estimate what the reverse current is for a diode. Diode reverse current varies considerably from device to device and isn’t necessarily dependent on voltage rating, current rating or physical size.

The graph below shows a typical diode voltage/current curve. Notice that under forward voltage conditions (blue shaded area) diodes conduct very little current until the voltage starts approaching the diode’s forward voltage. Although not technically a leakage current, current will start flowing at a few hundred millivolts below the forward voltage level where the diode is expected to “turn on” and start conducting. Under reverse voltage conditions (pink shaded area), some amount of leakage current occurs as soon as the voltage is reversed and increases as the reverse voltage increases. Note that this graph is not to scale, the forward voltage of a diode is typically less than one volt while the reverse voltage and breakdown voltage are usually tens to hundreds of volts.

diode V-C curve

There are a few things to consider regarding diode reverse current:

  • Schottky diodes tend to have higher reverse currents than standard diodes.  In a recent search on DigiKey, for SMT Schottky diodes the reverse current specs ranged from 100nA to 15mA while for standard diodes the range was nearly an order of magnitude lower, from 500pA to 1.5mA. A Schottky diode may be appropriate for your design because of its low forward voltage but be aware that its leakage current can be considerable.
  • Similar to leakage current for caps, the applied voltage relative to the rated reverse voltage can have a significant impact on the reverse current of diodes. Reductions in reverse current as the applied voltage is reduced relative to the rated reverse voltage aren’t quite linear but can reach 90% or more.  This is often shown in a graph in the diode data sheet with reverse current plotted against percent of rated reverse voltage.
  • Temperature will also have a significant impact on a diode’s reverse current.  It is not uncommon for a 20°C temperature increase to cause a 10X or greater increase in reverse current. One option to improve this situation in power application where a diode can heat up while operating is to utilize a physically larger device and large copper areas on the circuit board to help transfer heat out of the diode and into the circuit board.
  • As shown above, the reverse current can become significant as the breakdown voltage is approached and increase to many times the rated current of the device if the breakdown voltage is exceeded. This is referred to as “avalanche current” because of the sudden increase and is the point where the part is likely to be destroyed.

LEDs will exhibit similar reverse and forward currents as other diodes. A few things to consider specific to how LEDs are typically used:

  • Reverse voltage with an LED typically is not a problem unless there are multiple power rails in a design and the cathode is driven to a higher voltage than the anode when the LED is off.
  • It can be tempting to use a high-drive GPIO to directly control an LED as shown in the diagram below. Because of the typical logic “high” and “low” levels of a CMOS micro, this can still result in hundreds of millivolts across the LED when it is off. This will create the condition just to the left of the “Forward Voltage” point on the voltage/current curve where there may be from tens of microamps to a few milliamps of current flow through the LED. If you choose to use a GPIO for cost/space reasons, it is better to connect the GPIO to the anode side and drive it high to turn on the LED. A CMOS micro will usually have a low level output below 0.4V while the high level can be as low as 70% of the Vcc rail. If you connect the GPIO to the cathode, at 70% of 3.3V that is only 2.3V which may not even be high enough to turn the LED completely off.
  • To virtually eliminate the current flow through an LED in the off state, use an N-channel MOSFET to control the cathode of the LED (see diagram below). This allows the cathode to float so the only current paths available are the circuit board itself and the solder mask (typically 100M ohm or higher) and the leakage current path through the MOSFET (typically in the low nanoamp range for a small N-channel FET but it can vary).

LED hookups

Unrelated to leakage current, LEDs can waste a lot of power if used without careful consideration.  Users love LED indicators but generally don’t have a clue about their impact on a device’s battery life. When LEDs are required, here are a few things to consider to reduce their power consumption:

  • Keep in mind that with current requirements of minimally several milliamps, an LED can draw much more current than a sleeping or slow running micro and high brightness LEDs can draw more current than a Bluetooth or ZigBee radio uses when transmitting.
  • Think about how bright your LEDs really need to be. Really bright LEDs generally aren’t needed unless a device is used outdoors or needs to be visible from across a large room. Light pipes and similar low cost plastic optics can be very useful for making an LED appear to be brighter or larger and may allow you to decrease the LED current by several milliamps. Particularly in red and green, high efficiency LEDS are available today that provide much better brightness/power performance than older LEDs.
  • When using an LED as an on/off indicator, consider a slow flash of the LED instead of having it on constantly.  Turning the LED on for ½ second every 3 seconds provides an almost 84% reduction in the power used for this indicator.
  • On a much smaller time scale, use a PWM to control the on/off duty cycle of the LED. LEDs tend to stay lit for a relatively long time after they are turned off. Switching the LED on/off with a 50/50 duty cycle at a rate faster than 1Khz will cut the power by half with an imperceptible reduction in brightness. The timers on many modern micros have PWM outputs or other output modes that can be used for this with little to no involvement by the firmware other than starting or stopping the timer.
  • If your device has more than a few LEDs that can be on simultaneously, consider adding an ambient light sensor to your product and controlling a PWM to adjust the brightness based on ambient light conditions.

Up to this point I have covered leakage currents, currents that may not be obvious but are usually specified in part data sheets. This part will deal with “current leaks”, non-obvious current flows and power losses that are caused by the circuit design. In some cases these “current leaks” may be reduced or managed somehow, in other cases you just need to be aware of them so they can be accounted for in your power budget.

Before leaving semiconductor devices, one of the biggest power wasters if not used carefully are MOSFETs. While MOSFETs usually have a leakage current spec, it is usually on the order of tens to a few hundred nanoamps. The bigger issue with MOSFETs is inefficient operation from not operating them under the right conditions to allow them to meet their Rds(on) spec. Borrowed from the “Low Power Design” e-book, here are a few things to consider when using MOSFETs:

  • The efficiency of a MOSFET is a function of gate voltage and load current. Most N-channel FETs usually need a gate voltage in the 8-10V range to fully turn on so simply driving the gate with a GPIO won’t put the FET in its lowest RDS(on) range. Logic level gate FETs may be better in this regard but typically they just have a lower minimum turn-on threshold and may still need over 5V to achieve their RDS(on) spec. In these cases you should consider using a P-channel FET or even a gate driver IC to drive the gate of the N-channel FET with the voltage it is switching (or use a step-up regulator or voltage doubler circuit to provide a higher voltage if the input voltage exceeds the maximum gate voltage). The graph below shows the impact of gate voltage on Rds(on) for the Fairchild FDS8449 N-channel FET. The FDS8449 has a max gate turn-on threshold of 3V but as you can see at 3V the Rds(on) is about 2.4X higher than at 10V at no load, much higher as the load increases.

Rds(on) vs gate voltage

  • When using a P-channel FET to drive a load, a GPIO may not drive the gate high enough to completely turn off the FET so you may be leaking power through the FET. This can often go un-noticed since the amount of power is too low to activate the load.
  • A P-channel FET of similar rated voltage and current as an N-channel FET will typically have 50-100% higher Rds(on) than the N-channel FET. With Rds(on) specs on modern FETs in the double-digit milliohm range even doubling the Rds(on) produces a fairly low value. However, that is simply wasted power that can easily be eliminated if low-side switching is an option for your application. This is also important to keep in mind if for some reason you can’t address one of the issues discussed here that prevents the FET from operating close to its lowest Rds(on), changing the type of FET may alleviate the problem. If you have a P-channel FET operating at 2X its lowest Rds(on) then you could possibly reduce the Rds(on) by a factor of 4X by using an N-channel FET.
  • Just like a resistor, the Rds(on) of a FET increases with temperature. The graph below shows the impact of temperature on Rds(on) for the Fairchild FDS8449 N-channel FET. As you can see, a 50°C increase in temperature results in a nearly 20% increase in Rds(on). Even if your product is normally used in a room temperature environment, a 20-50°C temperature rise at the FET’s die isn’t uncommon (another reason to operate the FET in its lowest Rds(on) range). This is another situation where keeping a part cooler helps prevent wasting power, as the Rds(on) increases the part will get hotter, increasing the Rds(on) and so on. Thermal runaway isn’t likely to happen but a hot FET and a non-optimal gate voltage can combine to generate a lot of excess heat and waste a lot of power.

Rds(on) vs junct temp

This started out to be a 2 part article, next week I’ll cover more sources of power loss commonly found in circuit designs to wrap-up the 3rd and final part.

 

Leakage current & current leaks – part 1

Accounting for all of the obvious points of power consumption and their current levels for active and sleep states can be a difficult task. In every design there are also points of power consumption that are often overlooked. Every type of semiconductor device has some amount of “leakage current” that may or may not be called out in its datasheet.  Leakage current can make up the majority of the deep-sleep current draw for a modern micro when it shuts off power to most of its internal circuits. There may also be aspects of your design that can “leak” microamps to milliamps of current that you don’t take into account.

This is the first of a two-part posting on leakage currents and current leaks. The first part will cover leakage current in capacitors. The second part will cover leakage current in other types of devices along with common types of current leaks in designs.

Capacitor leakage current

Capacitors can be a major cause of unexpected current draw due to their leakage current. Some amount of leakage is unavoidable for any type of cap but to some degree leakage current can often be reduced through part selection.  This is what you should know about leakage current for the three most common types of capacitors.

Aluminum electrolytic – While hard to beat for capacitance versus cost, these are the leakiest commonly used caps with the highest number of factors that determine their leakage current. The formula typically used to calculate their leakage current is shown below.

AE leakageAs you can see, capacitance and the rated voltage are the primary factors in the leakage current. A couple of things not shown in this equation:

  1. With aluminum electrolytic caps the leakage current changes significantly as the difference between applied voltage and rated voltage increases. This works both ways, the leakage current decreases as the voltage is decreased below the rated voltage. The leakage current also increases significantly when the applied voltage exceeds the rated voltage – in this situation the cap can be physically damaged, causing a permanent increase in leakage current.
  2. The leakage current of aluminum electrolytic caps is also greatly influenced by temperature. The leakage current can increase 2X-3X with a temperature increase from 20°C to 60°C.
  3. There a number of other factors that influence the leakage current of electrolytic capacitors. These include age, storage conditions, type of electrolyte and the physical construction of the capacitor. Electrolytic caps that have been abused (over voltage, excessive current, etc.) may exhibit significantly higher leakage currents due to internal damage.

Tantalum – Often the primary choice when high capacitance is needed in a small space, tantalum caps also offer potentially much lower leakage currents than aluminum electrolytic caps. The formula typically used to calculate their leakage current is shown below.

tant leakage As you can see, capacitance and both the rated voltage and applied voltage are the primary factors in the leakage current. The applied voltage also comes into play and the usual voltage derating used for safety reasons with tantalum caps can provide a significant reduction in leakage current.

Ceramic – While not capable of the high capacitance density of aluminum electric and tantalum caps, ceramic caps by far suffer the least from leakage currents. The formula typically used to calculate their leakage current is shown below.

 ceramic leakage

 It appears that capacitance is not involved at all and this just the Ohm’s Law formula for current based on voltage and resistance. In ceramic caps, the leakage current is primarily due to insulation resistance between the part terminals. However the insulation resistance is typically based on an “Ohms Farads” value even if it is listed as a resistance in the cap datasheet. While rated voltage is also not included in the formula directly, insulation resistance is typically a function of the rated voltage. Insulation resistance typically increases with rated voltage so leakage current can be decreased by using parts with a higher rated voltage. The insulation resistance of ceramic caps is also dependent on temperature but since their leakage currents are so low the impact of temperature can often be ignored unless you are concerned with sub-microamp current draw or your device will be exposed to extreme heat.

 

The chart below shows examples of the calculated leakage current for these three types of caps and summarizes the discussion above. Two examples are presented for each type of cap. For example “A” the cap is 10uF, rated voltage is 10V and the applied voltage is 3.3V; for example “B” the cap is 47uF, rated voltage is 16 volts and the applied voltage is 5V.

Cap Type

Leakage

Current

Capacitance Impact

Voltage Derating impact

Temperature

Impact

Other considerations

Aluminum electrolytic

“A” = 3uA

“B” = 8.2uA

High

Medium

High

Storage conditions, electrolyte type,

 age, part abuse

Tantalum

“A” = 330nA

“B” = 2.3uA

High

High

Medium

N/A

Ceramic

“A” = 66nA

“B” = 100nA

Low

Medium

Low

N/A

The differences in leakage currents between the types of caps are obviously very significant for ultra low power design. Multiply these numbers by the number of caps in your design and the leakage current can become a significant part of the sleep state current of your device. This is particularly true for a device with a wireless radio where there may be several hundred microfarads of capacitance to prevent a voltage droop when the radio starts to transmit.

Several more thoughts on capacitor leakage currents:

  • Since one of the main factors in leakage current is the amount of capacitance, for a low power design is important to determine how little capacitance is actually required for the circuits to operate reliably. This is particularly important for electrolytic and tantalum caps.
  • For all three types of capacitors, the rated voltage is a determining factor in leakage current. For electrolytic and tantalum caps, selecting caps with rated voltages at least 2X to 3X the applied voltage will greatly reduce leakage current compared to the same value cap with a rated voltage only slightly higher than the applied voltage. For ceramic caps, caps rated at 50V or higher typically reduce leakage current by a factor of 10 compared to the same value cap with a 25V or lower rated voltage.
  • There is also a significant time aspect to leakage current. When voltage is applied to a discharged cap, the leakage current can spike to 50X to 100X the calculated value, remain there for a minute or two then gradually fall to the calculated level over several minutes. This is sometimes called “absorption current”. This spike is so high and lasts so long that leakage current tests on aluminum electrolytic are typically performed at least two to five minutes after voltage is applied to the cap. This really needs to be taken into consideration when selecting caps for circuits that are frequently turned on and off as part of the power management scheme and for caps used in timing circuits.
  • This may seem like a fine point but leakage current on capacitors down-stream from a voltage regulator carries an extra expense in terms of power since the energy stored in those caps was subject to the inefficiency of the voltage regulator. Be careful about throwing extra capacitance on the output of regulators just to be safe. Many low power LDO regulators can get by with just a few microfarads of input and output capacitance. The bulk capacitance on the output of switching regulators can often be reduced if your design can tolerate more ripple on the voltage.

 

The second part of this post will discuss leakage current in other types of semiconductor devices along with sources of current leaks commonly found in many designs.