| How to protect an HIGH SIDE DRIVER in the event of reverse polarity when driving resistive or inductive loads? |
In the event of polarity inversion the HIGH SIDE DRIVER must be protected by connecting a protection network diode plus resistor between the GROUND pin and the system ground and by using protection resistor in the INPUT and STATUS/SENSE pin.
Explanation:
All M0 HSDs need an external circuitry to protect them against reverse battery polarity. For a complete reverse polarity protection, any external circuitry has to:
- avoid any current through the pins IN, STATUS and GND exceeding the maximum ratings fixed in the datasheet;
- avoid that the chip power dissipation exceeds the maximum permissible. It means that particular care has to be taken in account with respect to the cooling.
As shown in the AN514/1092 about HSDs, to protect the INPUT and STATUS pin against extra currents in reverse polarity condition, it is necessary to provide external series resistors.
The power stage is, usually, protected against reverse battery by the load itself even if the power dissipation is increased compared to normal operating conditions. This extra power dissipation, during reverse polarity condition, has to be considered when calculating the cooling. Sometimes, depending on the kind of load, an extra circuitry is necessary to protect the device against an uncontrolled thermal runaway as the load characteristics do not guarantee acceptable working conditions.
As told before, it is also necessary to limit the current through the GND pin. Many different solutions can be adopted. Their choice depends on the characteristics of the load. In all our datasheets of M0 HSDs we have proposed a typical application circuit with a Schottky diode (or also a zener diode or a simple diode) for reverse supply protection. The same application circuit is proposed in the AN before mentioned (please look at n.6 applicative circuit). We want to explain with this technical note that this solution is not suitable for inductive load and, consequently, a modification is needed. Refer to Fig.1 to have an explanation of the phenomenon.
With an inductive load, immediately after the turn-off phase we have a demagnetization phase in which the Output voltage goes negative. This value could be deeply negative if a fast demagnetization circuit is implemented inside the HSD. As long as the output voltage remains negative the current through the GND pin tries to change its normal direction as a result of the current through the pull-down resistance beetwen OUT and GND. If this reverse current flow is prevented by a diode, as in the proposed applicative circuit, the GND is pulled deeply negative. This negative GND voltage can be interpreted by the internal logic, even if at low logic level, like a positive input voltage. Consequently the M0 HSD attempts to turn on with the result of high frequency oscillations of the output voltage.
Solution:
A resistance paralleled to the diode prevents this effect as the GND voltage can not assume so negative voltages. The proposal value is 1KOhm (see Fig.2).
| How to detect an OPENLOAD fault ? |
An HIGH SIDE DRIVER with digital status and openload detection features allows to monitor the load circuit. If openload fault occurs STATUS pin will report the openload condition (STATUS=low). In the following note is explained how detect an openload fault in ON and OFF state.
| How to detect the OPENLOAD fault if the HIGH SIDE DRIVER is in ON STATE ? |
We can consider the case of a load disconnection. The scheme below illustrates the power MOS (consisting of N cells) connected gate to gate with the sense MOS (consisting of n cells) and the comparator connected on the power and sense MOS sources. It compares the source voltage of the power MOS with the source voltage of the sense MOS in which flows a reference current IREF. When the sense and power MOS work with the same current density (that is under nominal condition), the voltage difference between their sources is zero. Then a current: ILOAD=IOL=(N/n)IREF is flowing in the power MOS. When ILOAD<IOL the comparator reads a different voltage on the power and sense MOS sources setting the STATUS pin at LOW level (see fig. 3).
OPEN LOAD table of the datasheet reports IOL parameter: with ILOAD<IOL openload indication STATUS=low; see below an extract (first row) of the openload detection table of the VNQ810 datasheet.
|
Symbol
|
Parameter
|
Test Conditions
|
Min
|
Typ
|
Max
|
Unit
|
|
IOL
|
Openload ON State detection threshold
|
VIN=5V
|
20
|
40
|
80
|
mA
|
The truth table below contains the status specification in normal and openload on state condition.
| INPUT |
OUTPUT |
STATUS |
Comments |
| HIGH |
HIGH |
LOW |
OPEN LOAD CONDITION |
| How to detect the OPENLOAD fault if the HIGH SIDE DRIVER is in OFF STATE ? |
In this case is necessary a pull-up resistor connected between the OUTPUT pin and a supply voltage line +V
CC. The note below drives you to an easy calculation of the pull-up resistor.
| How to choose the pull-up REXT resistor used in the OPENLOAD OFF state detection? |
Our goals are:
1) assure the indication of openload condition when load is disconnected;
2) assure no openload indication when load is connected.
To do this an external pull-up resistor is connected between the output pin and a positive voltage (we suggest to connect it to the +5V line used to supply the microcontroller). The resistor value must be choose accurately, keeping in mind the spread of the VOL parameter indicated in the openload table of the datasheet. A bad dimensioning of the external resistor could result in a false openload indication. For VNQ810 in the worst case we have to consider max and min VOL values: 3.5V and 1.5V (see the VOL values for VNQ810).
|
Symbol
|
Parameter
|
Test Conditions
|
Min
|
Typ
|
Max
|
Unit
|
|
VOL
|
Openload OFF State detection threshold
|
VIN =5V
|
1.5
|
2.5
|
3.5
|
V
|
Condition 1)
If load disconnection occurs, a current IL(off2) (specified in POWER OUTPUT section of the datasheet) flows through the integrated on-chip resistor R (see figure 4). For the openload detection, the external resistor (REXT) must set the output pin at a voltage higher than VOL max (VOL>3.5V); in this case the STATUS becomes low.
Connecting the pull-up resistor to the +V line; REXT value is:
REXT<(+V-VOL max)/IL(off2)
Using the following data: +V=5V; VOL max=3.5V; IL(off2)=75mA then:
REXT<(5-3.5)(V)/75mA; REXT<20kW
Condition 2)
In order to assure no openload indication if load is connected, VOL value must be lower than the VOL min (1.5V). R2 and REXT form a divider which set the output pin voltage at a value:
VOL=+V*R2/(REXT + R2)<1.5V
To assure VOL<VOLmin, the value of REXT must be higher than:
REXT>R2*(+V/VOL min)-R2;
In our example if R2=1kW and +V=5V;
REXT>103*(5/1.5)W-103W=2.3kW;
The external pull-up resistor is therefore in the range of:
R2*(+V/VOL min)-R2 <REXT<(+V-VOL max)/IL(off2); 2.3kW<REXT<20kW
As a note for you the connection of REXT to the +5V line avoids false open load indication caused by noises from the Vbatt. Line.
| Can I apply a 5V (TTL input) signal to INPUT pin to drive the HIGH SIDE DRIVER? |
Yes, surely; all our High SIDE DRIVERS can be driven by a logic signal.
Our HIGH SIDE DRIVERS are designed to drive DC motor or coil. An integrated charge pump provides the sufficient voltage to turn on the power MOSFET stage. During the on period the current in the load rises linearly to a maximum value. At turn off the load stored energy is removed through an internal circuit which has a typical V
demag indicated in the datasheets.
| What is an HIGH SIDE DRIVER with current sense feature? |
An HIGH SIDE DRIVER with PROPORTIONAL LOAD CURRENT SENSE has an analog sense output e signal proportional to the load current.
In the following article we would like to give you some information about the current sense feature of the High Side drivers family VN6XX.
An High Side Driver with analog output sense feature delivers to the sense pin a current proportional to the output load current. One of the most common way of using the current sense feature is to connect a resistor between the sense pin and ground (see figure5).
With reference to a datasheet of an High Side Drivers of VN6XX series (with current sense feature) in the electrical characteristics CURRENT SENSE you will find the table which gives the ratio "K" of output load current in respect to sense current. If sense pin is left open (see figure 6) it will rise to saturation voltage (respect to ground) for any output current.
A 10KW resistor will drive the sense to saturation voltage for a output load current greater than 2Amps (see Fig. 7). Obviously in the above case you will not be able to use the information from sense voltage because it won’t be proportional to the output load current.
In the datasheet, CURRENT SECTION section, the parameter VSENSE will be changed in VSENSE(SAT).
| How to choose the sense resistor? |
The sense signal is applied to an analog to digital (A/D) converter (see figure 8). A voltage of 1.5V is usually the minimum level required to get the full 8 bit accuracy of the A/D that is maintained for higher voltages. For example if you have to monitor a variable output load current in the range of 0-13Amps, to which corresponds a sense voltage in the range of 0-4V, with a "K" value of 5000, you will need a resistor of:
RSENSE(W)=VSENSE(V) xK ÷ILOAD(A)
So, in our case:
RSENSE(W)= 4(V) x5000 ÷13(A) (see figure 8)
In the above example a 1500W resistor allow to well monitor the output load current.
We remark that the sense may saturate at IOUT>13A.
In overtemperature condition the High Side Driver is in thermal shut-down; sense voltage in this condition is pulled-up at 5.5Volts which is a value that the micro can easy recognize like fault.
We are going to update the datasheet in such a way to insert a graph of the sense voltage versus output load current.
