MPS offers a scalable current sharing e-fuse solution that packs a built-in MOSFET, current and temperature sensing, soft-start ramp control, and protection features.

By Jason Bone, staff technical
marketing & apps engineer,
Monolithic Power Systems (MPS)

In typical
hot-swap or electronic fuse (e-fuse) applications, great care must be taken
when selecting the MOSFET to ensure that the safe operating area (SOA) of the device
is not exceeded during a soft start (SS) turn-on. Even if multiple MOSFETs are
paralleled, the soft-start condition causes large amounts of thermal stress.

Generally,
when using discrete MOSFETs in hot-swap and E-fuse applications, it is assumed
that only one MOSFET is conducting the entire soft-start current. This is due
to the variation in the gate threshold voltage of each MOSFET. Therefore, all
the power loss is in one device, even though there may be multiple MOSFETs in
parallel. This requires oversizing the MOSFET rating and package size, which
drives up the PCB area. With typical hot-swap/E-fuse applications, a precision
current sense resistor and a controller is needed to provide SS timing,
temperature protection, and over-current protection (OCP).

The MPS
MP5921 provides an innovative hot-swap/E-fuse application with a simple and
stackable building block. The MP5921 has a built-in MOSFET, current sensing,
temperature sensing, soft-start ramp control, and advanced protection features.
The use of advanced monolithic processes allows for user-friendly
implementation for hot-swap and E-fuse solutions (Fig. 1). 

Fig. 1: Hot-Swap/E-Fuse Application
Components

The MP5921
uses an advanced monolithic process to monitor and drive the internal MOSFET
actively during the soft-start process to ensure the SOA of the MOSFET. Using
this monolithic process, the MP5921 can measure the current flowing through the
internal MOSFET accurately.

With the
ability to monitor the current through the internal MOSFET, multiple MP5921
devices connected in parallel actively balance the current flowing through each
device during the soft-start condition. This ensures that each device carries
the soft-start current equally, and no one device carries the full soft-start
load current. With the soft-start current balanced among the devices in
parallel equally, the risk of violating the SOA of the MOSFET is reduced
greatly, and the thermal energy is more evenly distributed on the PCB.

Fig. 2 below
shows the current sharing of three MP5921 devices in parallel during a soft
start with a DC load. All three devices in parallel share the soft-start load
current evenly. 

MPS Figure 2 MP5921

Fig. 2: Three MP5921 Devices in
Parallel

If the
traces for the individual currents are set on the same origin point, it can be
seen that they overlap exactly (Fig. 3). 

MPS Figure 3 Currents with same origin point

Fig. 3: Currents with Same Origin
Point

The MP5921
can be scaled to support any current range needed for hot-swap and E-fuse
solutions. With a 60-A current rating in a 4-mm x 5-mm package, the MP5921
provides an extremely dense hot-swap/E-fuse solution. Each MP5921 has built-in
protection features that monitor damaged MOSFETs, over-temperature conditions of
the internal MOSFET, soft-start watchdog timer, and over-current protection
(OCP).

The MP5921
also has a built-in short-circuit protection (SCP) feature that can disable the
internal MOSFET within 200 ns of a short detection. This quick disabling function
prevents a large build-up of current on the PCB where output shorts occur.

The MP5921
provides a robust and user-friendly solution for space-critical designs that
can be scaled to meet the design requirements of all types of hot-swap/E-fuse
applications.