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TPS73225 Datasheet(PDF) 13 Page - Texas Instruments |
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TPS73225 Datasheet(HTML) 13 Page - Texas Instruments |
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13 / 18 page  www.ti.com DROPOUT VOLTAGE BOARD LAYOUT RECOMMENDATION TO TRANSIENT RESPONSE INTERNAL CURRENT LIMIT SHUTDOWN dV dt + V OUT C OUT 80kW (4) TPS73201, TPS73215, TPS73216 TPS73218, TPS73225, TPS73230 TPS73233, TPS73250 SBVS037F – AUGUST 2003 – REVISED SEPTEMBER 2004 This noise reduction effect is shown as RMS Noise Voltage vs CNR in the Typical Characteristics section. The TPS732xx uses an NMOS pass transistor to The TPS73201 adjustable version does not have the achieve extremely low dropout. When (VIN - VOUT) is noise-reduction pin available. However, connecting a less than the dropout voltage (VDO), the NMOS pass feedback capacitor, CFB, from the output to the FB pin device is in its linear region of operation and the will reduce output noise and improve load transient input-to-output resistance is the RDS-ON of the NMOS performance. pass element. The TPS732xx uses an internal charge pump to For large step changes in load current, the TPS732xx develop an internal supply voltage sufficient to drive requires a larger voltage drop from VIN to VOUT to the gate of the NMOS pass element above VOUT. The avoid degraded transient response. The boundary of charge pump generates ~250µV of switching noise at this transient dropout region is approximately twice ~2MHz; however, charge-pump noise contribution is the dc dropout. Values of VIN - VOUT above this line negligible at the output of the regulator for most insure normal transient response. values of IOUT and COUT. Operating in the transient dropout region can cause an increase in recovery time. The time required to recover from a load transient is a function of the IMPROVE PSRR AND NOISE PERFORMANCE magnitude of the change in load current rate, the rate of change in load current, and the available head- To improve ac performance such as PSRR, output room (VIN to VOUT voltage drop). Under worst-case noise, and transient response, it is recommended that conditions [full-scale instantaneous load change with the PCB be designed with separate ground planes for (VIN - VOUT) close to dc dropout levels], the TPS732xx VIN and VOUT, with each ground plane connected only can take a couple of hundred microseconds to return at the GND pin of the device. In addition, the ground to the specified regulation accuracy. connection for the bypass capacitor should connect directly to the GND pin of the device. The low open-loop output impedance provided by the NMOS pass element in a voltage follower configur- The TPS732xx internal current limit helps protect the ation allows operation without an output capacitor for regulator during fault conditions. Foldback helps to many applications. As with any regulator, the addition protect the regulator from damage during output of a capacitor (nominal value 1µF) from the output pin short-circuit conditions by reducing current limit when to ground will reduce undershoot magnitude but VOUT drops below 0.5V. See Figure 11 in the Typical increase duration. In the adjustable version, the Characteristics section for a graph of IOUT vs VOUT. addition of a capacitor, CFB, from the output to the adjust pin will also improve the transient response. The TPS732xx does not have active pull-down when The Enable pin is active high and is compatible with the output is over-voltage. This allows applications standard TTL-CMOS levels. VEN below 0.5V (max) that connect higher voltage sources, such as alter- turns the regulator off and drops the ground pin nate power supplies, to the output. This also results current to approximately 10nA. When shutdown capa- in an output overshoot of several percent if the load bility is not required, the Enable pin can be connected current quickly drops to zero when a capacitor is to VIN. When a pull-up resistor is used, and operation connected to the output. The duration of overshoot down to 1.8V is required, use pull-up resistor values can be reduced by adding a load resistor. The below 50 k Ω. overshoot decays at a rate determined by output capacitor COUT and the internal/external load resist- ance. The rate of decay is given by: (Fixed voltage version) 13 |
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Similar Description - TPS73225 |
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