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LTC1435A Datasheet(PDF) 14 Page - Linear Technology |
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LTC1435A Datasheet(HTML) 14 Page - Linear Technology |
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14 / 20 page  14 LTC1435A APPLICATIONS INFORMATION By powering EXTVCCfromanoutput-derivedsource,the additional VIN current resulting from the driver and control currents will be scaled by a factor of Duty Cycle/Efficiency. For example, in a 20V to 5V ap- plication, 10mA of INTVCC current results in approxi- mately 3mA of VIN current. This reduces the midcurrent loss from 10% or more (if the driver was powered di- rectly from VIN) to only a few percent. 3. I2R losses are predicted from the DC resistances of the MOSFET, inductor and current shunt. In continuous mode the average output current flows through L and RSENSE, but is “chopped” between the topside main MOSFET and the synchronous MOSFET. If the two MOSFETs have approximately the same RDS(ON), then the resistance of one MOSFET can simply be summed with the resistances of L and RSENSE to obtain I2R losses. For example, if each RDS(ON) = 0.05Ω, RL = 0.15Ω, and RSENSE = 0.05Ω, then the total resis- tance is 0.25 Ω. This results in losses ranging from 3% to 10% as the output current increases from 0.5A to 2A. I2R losses cause the efficiency to drop at high output currents. 4. Transition losses apply only to the topside MOSFET(s), and only when operating at high input voltages (typically 20V or greater). Transition losses can be estimated from: Transition Loss = 2.5 (VIN)1.85(IMAX)(CRSS)(f) Other losses, including CIN and COUT ESR dissipative losses, Schottky conduction losses during dead-time, and inductor core losses, generally account for less than 2% total additional loss. Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take sev- eral cycles to respond to a step in DC (resistive) load cur- rent. When a load step occurs, VOUT immediately shifts by an amount equal to ( ∆ILOAD)(ESR), where ESR is the ef- fective series resistance of COUT. ∆ILOAD also begins to charge or discharge COUTwhichgeneratesafeedbackerror signal. The regulator loop then acts to return VOUT to its steady-state value. During this recovery time VOUT can be monitored for overshoot or ringing, which would indicate a stability problem. The ITH external components shown in the Figure 1 circuit will provide adequate compensation for most applications. A second, more severe transient is caused by switching in loads with large (>1 µF) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive so that the load rise time is limited to approximately (25)(CLOAD). Thus a 10 µF capacitor would require a 250µs rise time, limiting the charging current to about 200mA. Automotive Considerations: Plugging into the Cigarette Lighter As battery-powered devices go mobile, there is a natural interest in plugging into the cigarette lighter in order to conserve or even recharge battery packs during operation. But before you connect, be advised: you are plugging into the supply from hell. The main battery line in an automo- bile is the source of a number of nasty potential transients, including load dump, reverse battery and double battery. Load dump is the result of a loose battery cable. When the cable breaks connection, the field collapse in the alternator can cause a positive spike as high as 60V which takes several hundred milliseconds to decay. Reverse battery is just what it says, while double battery is a consequence of tow truck operators finding that a 24V jump start cranks cold engines faster than 12V. The network shown in Figure 9 is the most straightforward approach to protect a DC/DC converter from the ravages of an automotive battery line. The series diode prevents current from flowing during reverse battery, while the transient suppressor clamps the input voltage during load dump. Note that the transient suppressor should not Figure 9. Automotive Application Protection 1435A F09 50A IPK RATING LTC1435A TRANSIENT VOLTAGE SUPPRESSOR GENERAL INSTRUMENT 1.5KA24A VIN 12V |
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