SBVS074L January 2007 – March 2017
The TPS74801 belongs to a family of low-dropout regulators that feature soft-start capability. These regulators use a low current bias input to power all internal control circuitry, allowing the NMOS pass transistor to regulate very low input and output voltages.
The use of an NMOS-pass FET offers several critical advantages for many applications. Unlike a PMOS topology device, the output capacitor has little effect on loop stability. This architecture allows the TPS74801 to be stable with any capacitor type of value 2.2 μF or greater. Transient response is also superior to PMOS topologies, particularly for low VIN applications.
The TPS74801 features a programmable voltage-controlled soft-start circuit that provides a smooth, monotonic start-up and limits startup inrush currents that may be caused by large capacitive loads. A power good (PG) output is available to allow supply monitoring and sequencing of other supplies. An enable (EN) pin with hysteresis and deglitch allows slow-ramping signals to be used for sequencing the device. The low VIN and VOUT capability allows for inexpensive, easy-to-design, and efficient linear regulation between the multiple supply voltages often required by processor-intensive systems.
The enable (EN) pin is active high and is compatible with standard digital signaling levels. VEN below 0.4 V turns the regulator off, while VEN above 1.1 V turns the regulator on. Unlike many regulators, the enable circuitry has hysteresis and deglitching for use with relatively slowly ramping analog signals. This configuration allows the TPS748 to be enabled by connecting the output of another supply to the EN pin. The enable circuitry typically has 50 mV of hysteresis and a deglitch circuit to help avoid on-off cycling as a result of small glitches in the VEN signal.
The enable threshold is typically 0.8 V and varies with temperature and process variations. Temperature variation is approximately –1 mV/°C; process variation accounts for most of the rest of the variation to the 0.4-V and 1.1-V limits. If precise turnon timing is required, a fast rise-time signal must be used to enable the TPS748.
If not used, EN can be connected to either IN or BIAS. If EN is connected to IN, it should be connected as close as possible to the largest capacitance on the input to prevent voltage droops on that line from triggering the enable circuit.
The TPS748 has an internal active pulldown circuit that connects the output to GND through an 833-Ω resistor when the device is disabled. This resistor discharges the output with a time constant of:
The power good (PG) pin is an open-drain output and can be connected to any 5.5-V or lower rail through an external pull-up resistor. This pin requires at least 1.1 V on VBIAS in order to have a valid output. The PG output is high-impedance when VOUT is greater than VIT + VHYS. If VOUT drops below VIT or if VBIAS drops below 1.9 V, the open-drain output turns on and pulls the PG output low. The PG pin also asserts when the device is disabled. The recommended operating condition of PG pin sink current is up to 1 mA, so the pull-up resistor for PG should be in the range of 10 kΩ to 1 MΩ. If output voltage monitoring is not needed, the PG pin can be left floating.
The TPS748 features a factory-trimmed current limit that is flat over temperature and supply voltage. The current limit allows the device to supply surges of up to 2 A and maintain regulation. The current limit responds in approximately 10 μs to reduce the current during a short-circuit fault.
The internal current limit protection circuitry of the TPS748 is designed to protect against overload conditions. It is not intended to allow operation above the rated current of the device. Continuously running the TPS748 above the rated current degrades device reliability.
Thermal protection disables the output when the junction temperature rises to approximately 160°C, allowing the device to cool. When the junction temperature cools to approximately 140°C, the output circuitry is enabled. Depending on power dissipation, thermal resistance, and ambient temperature the thermal protection circuit may cycle on and off. This cycling limits the dissipation of the regulator, protecting it from damage as a result of overheating.
Activation of the thermal protection circuit indicates excessive power dissipation or inadequate heatsinking. For reliable operation, junction temperature should be limited to 125°C maximum. To estimate the margin of safety in a complete design (including heatsink), increase the ambient temperature until thermal protection is triggered; use worst-case loads and signal conditions. For good reliability, thermal protection should trigger at least 40°C above the maximum expected ambient condition of the application. This condition produces a worst-case junction temperature of 125°C at the highest expected ambient temperature and worst-case load.
The internal protection circuitry of the TPS748 is designed to protect against overload conditions. It is not intended to replace proper heatsinking. Continuously running the TPS748 into thermal shutdown degrades device reliability.
The device regulates to the nominal output voltage under the following conditions:
If the input voltage is lower than the nominal output voltage plus the specified dropout voltage, but all other conditions are met for normal operation, the device operates in dropout mode. In this condition, the output voltage is the same as the input voltage minus the dropout voltage. The transient performance of the device is significantly degraded because the pass device is in a triode state and no longer controls the current through the LDO. Line or load transients in dropout can result in large output voltage deviations.
The device is disabled under the following conditions:
Table 1 shows the conditions that lead to the different modes of operation.
|Normal mode||VIN > VOUT(nom) + VDO (VIN)||VEN > VEN(high)||VBIAS ≥ VOUT + 1.6 V||I OUT < ICL||T J < 125°C|
|Dropout mode||VIN < VOUT(nom) + VDO (VIN)||VEN > VEN(high)||VBIAS < VOUT + 1.6 V||—||TJ < 125°C|
| Disabled mode
(any true condition disables the device)
|VIN < VIN(min)||VEN < VEN(low)||VBIAS < VBIAS(min)||—||TJ > 165°C|
The TPS748 features a programmable, monotonic, voltage-controlled soft-start that is set with an external capacitor (CSS). This feature is important for many applications because it eliminates power-up initialization problems when powering FPGAs, DSPs, or other processors. The controlled voltage ramp of the output also reduces peak inrush current during start-up, minimizing start-up transient events to the input power bus.
To achieve a linear and monotonic soft-start, the TPS748 error amplifier tracks the voltage ramp of the external soft-start capacitor until the voltage exceeds the internal reference. The soft-start ramp time depends on the soft-start charging current (ISS), soft-start capacitance (CSS), and the internal reference voltage (VREF), and can be calculated using Equation 2:
If large output capacitors are used, the device current limit (ICL) and the output capacitor may set the start-up time. In this case, the start-up time is given by Equation 3:
The maximum recommended soft-start capacitor is 15 nF. Larger soft-start capacitors can be used and do not damage the device; however, the soft-start capacitor discharge circuit may not be able to fully discharge the soft-start capacitor when enabled. Soft-start capacitors larger than 15 nF could be a problem in applications where it is necessary to rapidly pulse the enable pin and still require the device to soft-start from ground. CSS must be low-leakage; X7R, X5R, or C0G dielectric materials are preferred. Refer to Table 2 for suggested soft-start capacitor values.
|270 pF||0.5 ms|
|560 pF||1 ms|
|2.7 nF||5 ms|
|5.6 nF||10 ms|
|10 nF||18 ms|
Another option to set the start-up rate is to use a feedforward capacitor; see the Pros and Cons of Using a Feedforward Capacitor with a Low-Dropout Regulator application report for more information.
VIN, VBIAS, and VEN can be sequenced in any order without causing damage to the device. However, for the soft-start function to work as intended, certain sequencing rules must be applied. Connecting EN to IN is acceptable for most applications, as long as VIN is greater than 1.1 V and the ramp rate of VIN and VBIAS is faster than the set soft-start ramp rate.
There are several different start-up responses that are possible, but not typical:
Figure 23 shows the use of an RC-delay circuit to hold off VEN until VBIAS has ramped. This technique can also be used to drive EN from VIN. An external control signal can also be used to enable the device after VIN and VBIAS are present.