What is Source and Sink Current

What is source and sink current? Source and sink currents are used by integrated circuits such as microcontrollers, digital signal controllers, digital signal processor and any application specific integrated circuits. Sometimes these are being interchanged by others that it should not be. In this article we will know the answer of the question what is source and sink current.

Source Current

This is the current capability of the device that can be provided to the external circuit. Most recent MCUs and DSPs have 4mA to 16mA source current capability in the output pins. In some integrated circuits these are configurable; you can set a 4mA source current, 9mA, and so on depending on the device.

Figure below demonstrates the direction of the source current. Source current has an outward direction (from the name source). This current flows through the internal circuit to the external circuit such as the resistor R1 and transistor Q1. The two transistors Q2 and Q3 are the internal representation of how the output pin is being driven internally. When the common base of the two transistors are applied by high level, the lower transistor which is a PNP will be cutoff and only the upper one will conduct which is an NPN. The upper transistor saturates and the current will flow outside.

Fig1

It is very important not to exceed the maximum source current the device can handle so that it will not damage. Some are confused and actually do not understand well the source current. They are thinking that the specified source current is the only current the device can provide and this level cannot be exceeded because the device itself will limit up to this level only. Supposing the specified source current is 4mA only; the device will clamp the current to this level only…This is a wrong understanding! Based from above figure, the upper transistor will saturate then the current will be limited by the value of the resistor R1 only. Therefore, the size of R1 must be carefully selected so that not to exceed the source current specified by the datasheet.

For the above figure the level of circuit source current Isource is

Isource=\frac{Vcc-VCEsat-VBE_Q1}{R1}

(Note: Isource is the actual source current of the circuit based from the given values. This value will be the one compared to the source current capability of the device.)

Since the upper transistor is saturating, the value of VCEsat is neglected and the equation will be deduced to

Isource=\frac{Vcc-VBE_Q1}{R1}

Since the value of VCEsat is omitted, the equation is then change to

Isource=\frac{Vpin-VBE_Q1}{R1}

To increase the reliability of the design, the value of Isource must be less than the specified source current such that the maximum stress is only 80%.

Isource\leq 80%Isource_rating

Sink Current

If the source current is an outgoing current, sink current on the other hand is an entering current. In device point of view, this is the maximum current before the pin will be damaged. Like with the source current, this can be configured as well (depends on the device if it has the feature).

Figure below demonstrates the direction of the sink current. When the applied base voltage on Q2 and Q3 is low, the upper transistor will cut-off and the lower one will saturate.

The PNP transistor will saturate and the current Isink will flow from Vcc to R2 then to the IC pin. The computed value of Isink must be less than the capability of the device so that to prevent damage.

Isink=\frac{Vcc-VCEsat}{R2}

Neglecting VCEsat

Isink=\frac{Vcc}{R2}

To increase the reliability of the design, maintain only an 80% stress.

Isink\leq80%Isink_rating

Fig2

In the above figure, the configuration of the IC pin is open drain that is why there is a need of the resistor R2. In case the pin configuration is push-pull, R2 will not be needed. With the above figure when push-pull is used, the sink current is not an issue because there is no path of the current coming from the outside and the current that flows through the PNP transistor is only the leakage current from the upper NPN transistor. To know what open drain and push-pull configurations are, read THIS.

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