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LTC1159 Datasheet(PDF) 11 Page - Linear Technology |
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LTC1159 Datasheet(HTML) 11 Page - Linear Technology |
11 / 20 page 11 LTC1159 LTC1159-3.3/LTC1159-5 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. Line Transient Response The LTC1159 has better than 60dB line rejection and is generally impervious to large positive or negative line voltage transients. However, one rarely occurring condi- tion can cause the output voltage to overshoot if the proper precautions are not observed. This condition is a negative VIN transition of several volts followed within 100µs by a positive transition of greater than 0.5V/ µs slew rate. The reason this condition rarely occurs is because it takes tens of amps to slew the regulator input capacitor at this rate! The solution is to add a diode between the cap and VIN pins of the LTC1159 as shown in several of the typical application circuits. If you think your system could have this problem, add the diode. Note that in surface mount applications it can be combined with the P-gate diode by using a low cost common cathode dual diode. EXTVCC Pin Connection The LTC1159 contains an internal PNP switch connected between the EXTVCC and VCC pins. The switch closes and supplies the VCC power whenever the EXTVCC pin is higher in voltage than the 4.5V internal regulator. This allows the MOSFET driver and control power to be derived from the output during normal operation and from the internal regulator when the output is out of regulation (start-up, short circuit). Significant efficiency gains can be realized by powering VCC from the output, since the VIN current resulting from the driver and control currents will be scaled by a factor of (Duty Cycle)/(Efficiency). For 5V regulators this simply means connecting the EXTVCC pin directly to VOUT. How- ever, for 3.3V and other low voltage regulators, additional circuitry is required to derive VCC power from the output. The following list summarizes the four possible connec- tions for EXTVCC: 1. EXTVCC Left Open. This will cause VCC to be powered only from the internal 4.5V regulator resulting in reduced MOSFET gate drive levels and an efficiency penalty of up to 10% at high input voltages. 2. EXTVCC Connected Directly to VOUT. This is the normal connection for a 5V regulator and provides the highest efficiency. 3. EXTVCC Connected to an Output-Derived Boost Net- work. For 3.3V and other low voltage regulators, efficiency gains can still be realized by connecting EXTVCC to an output-derived voltage which has been boosted to greater than 4.5V. This can be done either with the inductive boost winding shown in Figure 5a or the capacitive charge pump shown in Figure 5b. The charge pump has the advantage of simple magnetics and generally provides the highest effi- ciency at the expense of a slightly higher parts count. VIN P-DRIVE LTC1159-3.3 P-GATE N-CH VIN BAT85 • • P-CH CIN RSENSE COUT 1 µF VOUT LTC1159 • F05a N-GATE P-GND EXTVCC L 1:1 + + + 4 3 2 1 Figure 5a. Inductive Boost Circuit for EXTVCC Figure 5b. Capacitive Charge Pump for EXTVCC VIN P-DRIVE LTC1159-3.3 P-GATE N-CH BAT85 0.22 µF 1 µF BAT85 BAT85 VN2222LL VIN P-CH CIN RSENSE COUT VOUT LTC1159 • F05b N-GATE P-GND EXTVCC L + + + APPLICATIO S I FOR ATIO |
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Similar Description - LTC1159 |
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