 |
|
|
 |
||
 |
|
|
| ST Home | Analog & Mixed Signal ICs | Intelligent Power Switches for Industrial | FAQ | ||
Intelligent Power Switches for Industrial
|
|
|
|
|
|
|||||||||||||||||||||||||||||||||||||||||||||||
The dotted rectangle is a simplified scheme of the current sense block shown in fig.2.
The principle of the operation is to compare the currents flowing through two paths: the sense path made up of the series of n-cells MOS plus the sense resistor (Isense) and the power path made up of the series of N-cells MOS plus the connected load (Iload). In normal operation: Iload=K*Isense Where K is the ratio between the N-power MOS cells and the n-sense MOS cells.
To better clarify the current sense feature, we can consider, in the on-state condition (high input), the following steps: 1)the load current creates a voltage drop Vds(on) on the output pin (see fig.3); 2)the operational amplifier compares Vds(on) to Vs voltage generated by an internal reference; 3)if Vds(on) < Vs, the voltage on the sense resistor Vsense, is about zero volt. As a consequence the sense current loses the proportionality with the load current. 4)if Vds(on) > Vs, the sense circuit controls the sense current in such a way that Vsense=(K/Rsense)*Iload. 5)if a short circuit fault occurs: Vds(on) >>Vs. In this condition Vsense is pull-upped to a voltage VsenseH ( 5.5Volts typical). It is necessary to take into account that the K ratio must be influenced by some external and physical parameters. For this note we only consider one K-variation factor: the bonding position versus sense location. Following our data, it impacts for about the 50% of the K ratio spread.
K spreads from 4400 to 5250. A standard application with a current sensing HSD (fig.4) uses an A/D converter to read the Vsense and a microcontroller to manage the data. This application requires a good sense accuracy, it is then necessary to decrease the K spread. Goal of the calculations here below is to give a method with which the K value is calibrated.
For the VN920 @T=25ºC; Vcc=13Volts, Rsense=3.9kW, the data table of the Vsense values versus Iout and the related plot are shown below. When Iout is in the range 1.5-6.5Amps, Vsense is proportional to Iout (linear zone).
The key method consists in measuring Vsense1 and Vsense2 in the linear zone by using two precise load currents Iref1 and Iref2. During the measurement : T=25ºC (case temperature); Rsense=fixed; Vcc=13Volts. This way the K ratio spread will be reduced of about 50%, even if other drift causes ( temperature for example) will still be present. As explained before:
The formula (2) will give the sense voltage Vsense1 and Vsense2 with the output currents Iref1 and Iref2:
The pairs (Iref1 ; Vsense1) and (Iref2 ; Vsense2) fix two points on the Vsense plot versus Iout (see fig.4).
The K ratio is the angular coefficient of the dotted red line (fig.4). In the linear zone of the plot we can suppose that Vsense varies linearly with Iout :
In the formula (5) "b" is the Iload value which corrisponds to a zero sense voltage: pair (b ; 0) in fig.4. In order to calculate "b" and "K" we solve the system of two equations (6) and (7). They are obtained from the formula (5) with the fixed values Iref1 ; Vsense1 and Iref2 ; Vsense2 .
Now an easy algorithm can give us the "K" (8) and "b" (9) values.
We now substitute in the (5) the obtained values for "K" (8) and "b" (9);
During module final test the values (Iref1 ; Vsense1) and (Iref2 ; Vsense2) are stored in the microcontroller EEPROM using the flow-chart in fig.5. 1st ): Set Vcc=13V; T=25ºC; IN=HIGH. 2nd): Set Iout=Iref1; read Vsense1 and store this data. 3rd): Set Iout=Iref2; read Vsense2 and store this data. 4th): Read Vsense. 5th): Evaluate the formula (10). 6th): Read the value. 7th): End. This way the K ratio spreads is reduced by about 50% leaving only Vcc and temperature drifts.
|
|
||||||||||||||||||||||||||||||||||||||||||||||||
|
|
 |
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
|
|
|
|
||