STEF12 Datasheet, PDF (9/20 Pages) STMicroelectronics – Electronic fuse for 12 V line

STEF12
Typical application
5.1.2
5.1.3
5.1.4
5.1.5
Normal operating condition
The STEF12 E-fuse behaves like a mechanical fuse, buffering the circuitry on its output with
the same voltage shown at its input, with a small voltage fall due to the N-channel MOSFET
R_DSOn.
Output voltage clamp
This internal protection circuit clamps the output voltage to a maximum safe value, typically
15 V, if the input voltage exceeds this threshold.
Current limiting
When an overload event occurs, the current limiting circuit reduces the conductivity of the
power MOSFET, in order to clamp the output current at the value selected externally by
means of the limiting resistor R_Limit (Figure 3).
Thermal shutdown
If the device temperature exceeds the thermal latch threshold, typically 165 °C, the thermal
shutdown circuitry turns the power MOSFET off, thus disconnecting the load. The EN/Fault
pin of the device is automatically set at an intermediate voltage, in order to signal the
overtemperature event. In this condition the E-fuse can be reset either by cycling the supply
voltage or by pulling down the EN pin below the V_il threshold and then releasing it.
5.2
R limit calculation
As shown in Figure 3, the device uses an internal N-channel sense FET with a fixed ratio, to
monitor the output current and limit it at the level set by the user.
The R_Limit value for achieving the requested current limitation can be estimated by using the
following theoretical formula, together with the graph in Figure 13: Current limit vs. RLimit.
Equation 1
RLimit = ----⁹----⁵-----
IShort
5.3
C_dv/dt calculation
Connecting a capacitor between the C_dv/dt pin and GND allows the modification of the
output voltage ramp-up time.
Given the desired time interval Δt during which the output voltage goes from zero to its
maximum value, the capacitance to be added on the C_dv/dt pin can be calculated using the
following theoretical formula:
Equation 2
C_dvdt = 24 × 10^–9Δt – 30x10^–12
Where C_dv/dt is expressed in Farads and the time in seconds.
Doc ID 019056 Rev 3
9/20