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TPS64200 Datasheet(PDF) 11 Page - Texas Instruments |
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TPS64200 Datasheet(HTML) 11 Page - Texas Instruments |
11 / 29 page TPS64200,TPS64201 TPS64202,TPS64203 SLVS485 − AUGUST 2003 www.ti.com 11 Soft Start The TPS6420x has an internal soft start circuit that limits the inrush current during start up. This prevents possible voltage drops of the input voltage in case a battery or a high impedance power source is connected to the input of the TPS6420x. During soft start the current limit is increased from 25% of its maximum to the maximum within about 250 µs. 100% Duty Cycle Low Dropout Operation The TPS6420x offers the lowest possible input to output voltage difference while still maintaining regulation with the use of the 100% duty cycle mode. In this mode the P-channel switch is constantly turned on. This is particularly useful in battery powered applications to achieve longest operation time by taking full advantage of the whole battery voltage range. Enable A voltage higher than the EN trip point of 1.3 V up to the input voltage forces the TPS64200 into shutdown. In shutdown, the power switch, drivers, voltage reference, oscillator, and all other functions are turned off. The supply current is reduced to less than 1 µA in shutdown. Pulling enable low starts up the TPS64200 with the softstart as described under the chapter softstart. Undervoltage Lockout The undervoltage lockout circuit prevents the device from misoperation at low input voltages. Basically, it prevents the converter from turning on the external PMOS under undefined conditions. Current Limit The ISENSE input is used to set the current limit for the external PMOS. The sense resistor must be connected between VI and source of the external PMOS. The ISENSE pin is connected to the source of the external PMOS. The maximum current is calculated by: I (cur lim) + V (ISENSE) R S (1) For low cost solutions the rDS(on) of the external PMOS can also be used to set the current limit. In this case the ISENSE pin is connected to the drain of the PMOS. The current in the PMOS is automatically sampled by the TPS6420x some 10 ns after the PMOS is turned on. The ISENSE pin should always be connected to either the source of the PMOS or the drain if an additional sense resistor is used. Otherwise there is no working overcurrent protection and no soft start in the system. The maximum drain current if the rDS(on) is used as a sense resistor is calculated by: I (cur lim) + V (ISENSE) r DS(on) (2) Short-Circuit Protection With a controller only limited short circuit protection is possible because the temperature of the external components is not supervised. In an overload condition, the current in the external diode may exceed the maximum rating. To protect the diode against overcurrent, the off-time of the TPS6420x is increased when the voltage at the feedback pin is lower than its nominal value. The off-time when the output is shorted (feedback voltage is zero) is about 4 µs. This allows the current in the external diode to drop until the PMOS is turned on again and the overcurrent protection switches off the PMOS again. The off-time is directly proportional to the voltage at feedback. |
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