STEL12H24 datasheet(10/26 Pages) STMICROELECTRONICS

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Application information

6.1

Enable pin

If the VCC is inside its operating range, the STEL12H24 is enabled by driving the EN pin above the VIH threshold.

When the VIH threshold is crossed, the internal blocks are turned on and, after TON time, the soft-start procedure

to activate the MOSFET starts.

6.2

Undervoltage lockout

The STEL12H24 has two UVLO circuits: one on the VCC pin and another on VIN pin.

When the undervoltage lockout threshold VCC-UVLO is crossed (VCC rising), the STEL12H24 enables a timer

of 1.2 ms, which activates a discharge circuit for the soft-start cap. This avoids inrush current if the soft-start

capacitor remained charged before the VCC-UVLO threshold was crossed.

The VIN-UVLO acts on the VIN pin. If the voltage on the VIN pin is below VIN-UVLO threshold, the MOSFET is OFF

and the discharge circuit is activated.

Both the UVLO circuits are disabled when EN pin is low.

6.3

Quick discharge

The STEL12H24 has an internal discharge switch and resistor connected between BLEED pin and GND which

allows discharging the load capacitor when the MOSFET is turned off (please refer to Figure 1). The discharge

switch and the MOSFET work always in an opposite manner, when one is ON the other is OFF and vice versa.

The BLEED pin must be connected to VOUT through an external series resistor RBLEED. The series resistor

minimum value must be calculated in order to limit the power dissipation on the internal Rb below 160 mW and to

limit its current to 20 mA maximum.

The minimum RBLEED can be calculated using the following formula:

RBLEED [kΩ] = VIN / 20 – Rb (min).

6.4

Power Good

The Power Good (PG) pin provides the information about the gate of the MOSFET. It is an open-drain output

that goes high when the gate of the MOSFET is fully charged thus giving indication about the operation of

the STEL12H24. For whatever reason the MOSFET is turned off (Enable, Thermal protection, UVLO, OVP or

short-circuit) the PG pin is pulled low. It is recommended to connect a pull-up resistor greater than 1 kΩ between

PG pin and any supply voltage in the system lower than PG pin AMR.

6.5

Short-circuit protection

The STEL12H24 is protected against short-circuit condition at the output. An internal comparator monitors the

voltage difference between VIN and BLEED pins. When this voltage exceeds the VSC threshold, the MOSFET is

turned off and the discharge switch is activated. The device restarts automatically with a controlled soft-start.

The short-circuit trigger point can be reduced by connecting an external resistor between BLEED and VOUT. The

leakage current flowing into the BLEED pin increases the voltage drop monitored by the internal comparator thus

triggering a lower short-circuit current protection.

6.6

Slew rate control

One of the key features of the STEL12H24 is the controlled ramp-up of the MOSFET current during turn-on. This

feature allows to limit the inrush current in hot swap applications.

The gate voltage of the MOSFET is increased linearly by mirroring the voltage ramp on the internal soft-start

capacitor which provides a default value for the ramp-up time. This time can be increased by connecting a

capacitor to CSR pin. The CSR pin is charged with a constant current of KSR so the slew rate is given by the

formula:

SREXT=8 × KSRCSR Vs

(1)

STEL12H24

Application information

DS13821 - Rev 1

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