Overview
Engineers developing electronic projects may encounter the following design challenge: a PCB labeled A includes a 5V power rail that supplies internal circuits such as a microcontroller, a PM2.5 sensor, and a speaker, and it must also provide power to external circuitry. Supplying internal circuits at 5V is straightforward, but powering external circuits introduces additional concerns. For example, what happens if the external circuit is shorted? What if the external circuit forces a voltage above 5V back into the rail? How can internal circuitry detect an external fault?
A dedicated current-limiting protection IC, the NCP380, can limit the 5V output current to 1.0 A, 1.5 A, or 2.0 A, preventing excessive current and short-circuit damage to the internal 5V supply.
The internal 5V supply outputs 5V to external circuits.
Typical Application
First, review the IC's intended function and usage.
The application circuit shows VIN connected to a 5V input and VOUT providing the 5V output. Under normal conditions the IC allows a nominal working current of 500 mA.
If an external fault increases the output current to about 1.2 A, exceeding a 1.0 A limit, the IC's overcurrent protection will trigger and immediately shut down VOUT. This protects the internal 5V supply from damage. The key question is: how is that 1.0 A limit determined?
How the 1A Limit Is Set
The limit is set by an external resistor labeled R1. Different R1 values correspond to different current limits. For example:
- R1 = 23.7 kΩ → current limit ≈ 1.0 A
- R1 = 37.4 kΩ → current limit ≈ 0.7 A
- R1 = 14.3 kΩ → current limit ≈ 1.5 A
- R1 = 9.0 kΩ → current limit ≈ 2.0 A
Engineers select R1 according to project requirements to achieve the desired maximum output current.
Related Reading: A Comprehensive Guide to Selecting PCB Fuses for Overcurrent Protection
Chip Current-Limit Principle
Why does the NCP380 provide current-limiting protection and how does it work? The answer lies in the IC's internal circuitry.
Internally, a P-channel MOSFET sits between VIN and VOUT and acts as the power switch controlling the output. When the MOSFET is on, 5V is passed to VOUT; when it is off, VOUT is disconnected.
The MOSFET gate is controlled by the EN pin: a high level on EN turns the MOSFET on, and a low level turns it off.
Note that on the VIN side there is a small series resistor used to measure the output current; this functions as a sense resistor. The NCP380 uses this internal sensing to monitor output current in real time.
Do not confuse this internal sense resistor with the external R1 resistor. The internal sense resistor only detects the actual output current, while the external R1 sets the threshold for the maximum allowable current.
How to Detect an Overcurrent Event
The IC provides a FLAG pin to indicate fault conditions. When the chip detects overcurrent or overtemperature, the FLAG pin is pulled low. Under normal conditions the FLAG pin remains high via a 47 kΩ pull-up resistor labeled R2.
While the NCP380 is useful for designs requiring current-limited outputs, note that its VIN pin supports a maximum voltage of 5.5 V, so it is not suitable for higher-voltage applications such as 12 V systems.
