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TPS799L57YZYR Fiches technique(PDF) 9 Page - Texas Instruments |
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TPS799L57YZYR Fiches technique(HTML) 9 Page - Texas Instruments |
9 / 14 page TPS799L www.ti.com SBVS191 – APRIL 2012 DROPOUT VOLTAGE TRANSIENT RESPONSE The TPS799Lxx uses a PMOS pass transistor to As with any regulator, increasing the size of the achieve a low dropout voltage. When (VIN – VOUT) is output capacitor reduces over/undershoot magnitude, less than the dropout voltage (VDO), the PMOS pass but increases the duration of the transient response. device is in its linear region of operation and rDS(on) of The transient response of the TPS799Lxx is the PMOS pass element is the input-to-output enhanced by an active pull-down device that engages resistance. Because the PMOS device behaves like a when the output overshoots by approximately 5% or resistor in dropout, VDO approximately scales with the more when the device is enabled. When enabled, the output current. pull-down device behaves like a 350- Ω resistor to ground. As with any linear regulator, PSRR degrades as (VIN – VOUT) approaches dropout. This effect is shown UNDERVOLTAGE LOCKOUT (UVLO) in Figure 9 through Figure 14 in the Typical Characteristics section. The TPS799Lxx uses an undervoltage lockout circuit to keep the output shut off until internal circuitry is START-UP operating properly. The UVLO circuit has a deglitch feature so that it typically ignores undershoot The TPS799Lxx uses a start-up circuit to quickly transients on the input if they are less than 50 μs in charge the noise reduction capacitor, CNR, if present duration. (see Functional Block Diagrams, Figure 1). This circuit allows for the combination of very low output MINIMUM LOAD noise and fast start-up times. The NR pin is high impedance so a low leakage CNR capacitor must be The TPS799Lxx is stable with no output load. To used; most ceramic capacitors are appropriate for this meet the specified accuracy, a minimum load of 500 configuration. μA is required. With loads less than 500 μA at junction temperatures near +125°C, the output can Note that for fastest start-up, apply VIN first, and then drift up enough to cause the output pull-down device drive the enable pin (EN) high. If EN is tied to IN, to turn on. The output pull-down device limits voltage start-up is somewhat slower. The start-up switch is drift to 5% typically; however, ground current can closed for approximately 135 μs. To ensure that CNR increase by approximately 50 μA. In typical is fully charged during start-up, use a 0.01- μF or applications, the junction cannot reach high smaller capacitor. temperatures at light loads because there is no noticeable dissipated power. The specified ground current is then valid at no load in most applications. Copyright © 2012, Texas Instruments Incorporated Submit Documentation Feedback 9 |
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