전자부품 데이터시트 검색엔진 |
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LM1950 데이터시트(PDF) 4 Page - National Semiconductor (TI) |
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LM1950 데이터시트(HTML) 4 Page - National Semiconductor (TI) |
4 / 6 page Application Hints HIGH CURRENT OUTPUT The 750 mA output is fault protected against overvoltage If the supply voltage rises above approximately 30V the out- put will automatically shut down This protects the internal circuitry and enables the IC to survive higher voltage tran- sients than would otherwise be expected The LM1950 will survive transients and DC voltages up to 60V 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 circuits to either ground or supply voltage Standard thermal shutdown circuits are employed to protect the LM1950 from over heat- ing FLYBACK RESPONSE Since the LM1950 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 Waveform 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 drive 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 LM1950 The minimum output break- down voltage of the IC output is rated at b54V with respect to the supply voltage The LM1950 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 Output clamp voltages ranging from b40V to b120V (with VCC supply of 14V) will be en- countered 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 x L x f(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 For frequency stability of the switch a 01 mF or larger out- put bypass capacitor is required TLH11237 – 4 FIGURE 1 TLH11237 – 5 FIGURE 2 Diode Clamp TLH11237 – 6 FIGURE 3 Zener Clamp for Rapid Controlled Current Decay 4 |
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