Voltage droop is the intentional loss in output
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
from a device as it drives a load. Adding droop in a
voltage regulation circuit increases the headroom for load
transients
Transience or transient may refer to:
Music
* ''Transient'' (album), a 2004 album by Gaelle
* ''Transience'' (Steven Wilson album), 2015
* Transience (Wreckless Eric album)
Science and engineering
* Transient state, when a process variable or ...
.
All electrical systems have some amount of resistance between the regulator output and the load. At high currents, even a small resistance results in substantial
voltage drop
Voltage drop is the decrease of electrical potential along the path of a current flowing in an electrical circuit. Voltage drops in the internal resistance of the source, across conductors, across contacts, and across connectors are undesirable ...
between the regulator and the load. Conversely, when the output current is (near) zero, the voltage at the load is higher. This follows from
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equat ...
.
Rather than increasing output voltage at high current to try to maintain the same load voltage, droop instead simply allows this drop to take place and designs around it. The behaviour of the system with and without droop is as follows:
In a regulator not employing droop, when the load is suddenly increased very rapidly (i.e. a transient), the output voltage will momentarily sag. Conversely, when a heavy load is suddenly disconnected, the voltage will show a peak. The output decoupling capacitors have to "absorb" these transients before the
control loop
A control loop is the fundamental building block of industrial control systems. It consists of all the physical components and control functions necessary to automatically adjust the value of a measured process variable (PV) to equal the value of ...
has a chance to compensate. A diagram of such transients is shown below. The maximum allowed voltage swing in such a transient is
.
Comparing this to a regulator with droop, we find that the maximum allowed swing has doubled: it is now
. This increased tolerance to transients allows us to decrease the number of output capacitors, or get better regulation with the same number of capacitors.
References
Maxim-IC application note*Speed Droop and Power Generation. Application Note 01302. Woodward Governor Company (2004).
*Intersil Application Note 1021 (June 2002)
{{refend
Electrical parameters
Droop