What is Headroom?
The headroom is the difference between the input and output voltages in a series pass regulator. The headroom voltage refers to the actual voltage drop across the regulator that must occur during operation. This is the difference between the amount of input voltage that must be supplied to the regulator, and the specified regulated output voltage.
The headroom voltage is one of the considerations when designing linear voltage regulators in power supplies. And the designer must calculate and decide on how low the input voltage to the regulator can go and still maintain the control of the output voltage within the rated specifications.
The linear voltage regulation involves taking in a high voltage, and then producing a low and steady output voltage. It is entirely powered from the input voltage, while the current through the regulator is controlled using a switching device such as a transistor. The series device limits the amount of current flowing through it, hence the voltage drop across the device, the headroom.
Headroom in a series pass transistor – Image Credit
The headroom ensures that the regulator to work properly. That is, the input voltage must be more than the required regulated output voltage by a few volts or hundreds of millivolts. It is also good to note that the dropout voltage of a regulator is usually the minimum allowable headroom.
The older voltage regulators had headroom of several volts. For example, the LM7805 had 2.5Volts; meaning that the 5V regulator required at least 7.5V input voltage so as to produce a regulated 5 Volts. Due to the need to have more efficient and low power dissipation in power supplies, manufacturers are now making the newer linear regulators with low headroom voltages in the order of a few 100 mV, typically between 0.2 Volts and 0.5 volts. This is especially one of the major parameters for the Low-Dropout Regulators (LDO).
The regulator or power supply should be able to handle the heat dissipated due to the headroom losses at the specified maximum ambient operating temperature; otherwise the design should be reconsidered. The headroom losses should be minimized to increase efficiency and reduce power dissipation and heat issues in the regulator. The headroom loss is given by
PHR = (V in(max) – V out) I load(rated)
If the headroom is higher, it means that more voltage must be dropped across the regulator, requiring a larger heat sink. A low headroom voltage has less heat dissipation problems and the design should be geared towards such.
The low headroom linear regulators as well as the low dropout regulators are usually used to clean up noisy power supply rails.