AN1596 datasheet(7/24 Pages) STMICROELECTRONICS

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AN1596 - APPLICATION NOTE

Under and over voltage lockout

Under and overvoltage protections occur when the supply voltage drops or raises to minimum and

maximum levels specified in the datasheet as VUSD and VOV. Under VUSD=5.5V value the PowerMOS

simply turns off, just because it would not work properly. The undervoltage condition may occur when

turning on a car headlamp for example, which is a near short circuit. The inductive effect of wires (typically

1µH/m) generates an opposing voltage across the wire and the apparent supply voltage drops. The

current increase rate for an HSD is about 1A/ms for a short-circuited load and using a 5m length wire, the

induced voltage will not be large enough to reduce the supply voltage below 5.5V and - therefore - the

HSD switches on. The overvoltage control circuit acts as a protection for the load against overvoltages

(VOV=36V and above that value the device switches off).

Reverse battery protection

Most auto manufacturers specify that any electronic device must be able to withstand a reverse battery

connection. The exact magnitude of the reverse voltage requirement varies per manufacturer, but the

worst case seems to be -24V for 10 min. The maximum allowed value of the ground current during reverse

battery is -IGND and it is specified in the device's datasheet. There are two possible solutions to this

problem.

Solution 1: Resistor in the ground line (RGND only). This can be used with any kind of load. The following

is an indication on how to dimension the RGND resistor.

Power dissipation in RGND during reverse battery situation is the following:

This resistor can be shared amongst several different HSDs. In this case in the formula (1) IS(on)max

becomes the sum of the maximum on-state currents of the different devices. When the microprocessor

ground is not common with the device ground then the RGND will produce a shift (IS(on)max * RGND) in the

input thresholds and the status output values. This shift will vary depending on how many devices are ON

in the case of several HSDs sharing the same RGND. This can lead to a very little value of RGND to comply

with formula (1) and formula (2) may not be fulfilled. To overcome this problem, ST suggests the following

solution.

Solution 2: a diode (DGND) in the ground line. A resistor (RGND=1kΩ) should be inserted in parallel to

DGND if the device drives an inductive load (see chapter about fast demagnetization). This small signal

diode can be safely shared amongst several different HSDs. Also in this case, the presence of the ground

network will produce a shift (~ 600mV) in the input threshold and in the status output values if the

microcontroller ground is not common to the device ground. This shift will not vary if more than one HSD

share the same diode/resistor network.

Micro-controller I/Os protection

If a ground protection network is used and negative transients are present on the VCC line, the HSD

control pins will be pulled negative due to parasitic internal structures. This may cause the microcontroller

I/O pins to latch up. The value of the resistors (Rprot) to be connected, is a compromise between the

voltage shift from the micro-controller output to the HSD control pins, and the latch-up limit current of

micro-controller I/Os. The following condition must be fulfilled:

Where:

-Vccpeak=negative peak voltage

max

)

(

600

GND

≤

(1)

(2)

GND

−

≥

()

GND

−

GND

out

prot

CCpeak

−

≤

−

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