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LM1921 Datasheet(PDF) 5 Page - National Semiconductor (TI) |
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LM1921 Datasheet(HTML) 5 Page - National Semiconductor (TI) |
5 / 6 page Application Hints HIGH CURRENT OUTPUT The 1 Amp output is fault protected against overvoltage If the supply voltage rises above approximately 30 volts the output will automatically shut down This protects the inter- nal circuitry and enables the IC to survive higher voltage transients than would otherwise be expected The 1921 will survive transients and DC voltages up to 60 volts on the supply The output remains off during this time independent of the state of the input logic voltage This protects the load The high current output is also protected against short cir- cuits to either ground or supply voltage Standard thermal shutdown circuits are employed to protect the 1921 from over heating FLYBACK RESPONSE Since the 1921 is designed to drive inductive as well as any other type of load inductive kickback can be expected whenever the output changes state from on to off (see waveforms on Figure 1 ) The driver output was left un- clamped since it is often desirable in many systems to achieve a very rapid decay in the load current In applica- tions where this is not true such as in Figure 2 a simple external diode clamp will suffice In this application the inte- grated current in the inductive load is controlled by varying the duty cycle of the input to the driver IC This technique achieves response characteristics that are desirable for cer- tain automotive transmission solenoids for example For applications requiring a rapid controlled decay in the solenoid current such as fuel injector drivers an external zener and diode can be used as in Figure 3 The voltage rating of the zener should be such that it breaks down be- fore the output of the LM1921 The minimum output break- down voltage of the IC output is rated at b57 volts with respect to the supply voltage Thus on a 12 volt supply the combined zener and diode breakdown should be less than 45 volts The LM1921 can be used alone as a simple relay or sole- noid driver where a rapid decay of the load current is de- sired but the exact rate of decay is not critical to the sys- tem If the output is unclamped as in Figure 1 and the load is inductive enough the negative flyback transient will cause the output of the IC to breakdown and behave similarly to a zener clamp Relying upon the IC breakdown is practical and will not damage or degrade the IC in any way There are two considerations that must be accounted for when the driver is operated in this mode The IC breakdown voltage is process and lot dependent Clamp voltages ranging from b 60 to b120 volts (with respect to the supply voltage) will be encountered over time on different devices This is not at all critical in most applications An important consideration however is the additional heat dissipated in the IC as a result This must be added to normal device dissipation when considering junction temperatures and heat sinking requirements Worst case for the additional dissipation can be approximated as Additional PD e I2 xLxf (Watts) where I e peak solenoid current (Amps) L e solenoid inductance (Henries) f e maximum frequency input signal (Hz) For solenoids where the inductance is less than ten milli- henries the additional power dissipation can be ignored Overshoot undershoot and ringing can occur on certain loads The simple solution is to lower the Q of the load by the addition of a resistor in parallel or series with the load A value that draws one tenth of the current or DC voltage of the load is usually sufficient TLH5271 – 10 FIGURE 2 Diode Clamp TLH5271 – 11 FIGURE 3 Zener clamp for rapid controlled current decay 5 |
Similar Part No. - LM1921 |
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Similar Description - LM1921 |
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