SBVS099G November 2007 – October 2015 TPS74701
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
R1 and R2 can be calculated for any output voltage using the formula shown in Figure 25. See Table 2 for sample resistor values of common output voltages. To achieve the maximum accuracy specifications, R2 should be less than or equal to 4.99 kΩ.
|R1 (kΩ)||R2 (kΩ)||VOUT (V)|
|270 pF||0.5 ms|
|560 pF||1 ms|
|2.7 nF||5 ms|
|5.6 nF||10 ms|
|0.01 μF||18 ms|
The device is designed to be stable for all available types and values of output capacitors greater than or equal to 2.2 μF. The device is also stable with multiple capacitors in parallel, which can be of any type or value.
The capacitance required on the IN and BIAS pins strongly depends on the input supply source impedance. To counteract any inductance in the input, the minimum recommended capacitor for VIN and VBIAS is 1 μF. If VIN and VBIAS are connected to the same supply, the recommended minimum capacitor for VBIAS is 4.7 μF. Good quality, low ESR capacitors should be used on the input; ceramic X5R and X7R capacitors are preferred. These capacitors should be placed as close the pins as possible for optimum performance.
The TPS74701 was designed to have excellent transient response for most applications with a small amount of output capacitance. In some cases, the transient response may be limited by the transient response of the input supply. This limitation is especially true in applications where the difference between the input and output is less than 300 mV. In this case, adding additional input capacitance improves the transient response much more than just adding additional output capacitance would do. With a solid input supply, adding additional output capacitance reduces undershoot and overshoot during a transient event; see Figure 19 in the Typical Characteristics: VEN = VIN section. Because the TPS74701 is stable with output capacitors as low as 2.2 μF, many applications may then need very little capacitance at the LDO output. For these applications, local bypass capacitance for the powered device may be sufficient to meet the transient requirements of the application. This design reduces the total solution cost by avoiding the need to use expensive, high-value capacitors at the LDO output.
The TPS74701 offers very low dropout performance, making it well-suited for high-current, low VIN/low VOUT applications. The low dropout of the TPS74701 allows the device to be used in place of a DC-DC converter and still achieve good efficiency. This feature provides designers with the power architecture for their applications to achieve the smallest, simplest, and lowest cost solution.
There are two different specifications for dropout voltage with the TPS74701. The first specification (shown in Figure 22) is referred to as VIN Dropout and is used when an external bias voltage is applied to achieve low dropout. This specification assumes that VBIAS is at least 1.39 V
The second specification (shown in Figure 23) is referred to as VBIAS Dropout and applies to applications where IN and BIAS are tied together. This option allows the device to be used in applications where an auxiliary bias voltage is not available or low dropout is not required. Dropout is limited by BIAS in these applications because VBIAS provides the gate drive to the pass FET; therefore, VBIAS must be 1.39-V above VOUT. Because of this usage, IN and BIAS tied together easily consume huge power. Pay attention not to exceed the power rating of the IC package.
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. If the ramp rate of the input sources is slower than the set soft-start time, the output tracks the slower supply minus the dropout voltage until it reaches the set output voltage. If EN is connected to BIAS, the device soft-starts as programmed, provided that VIN is present before VBIAS. If VBIAS and VEN are present before VIN is applied and the set soft-start time has expired, then VOUT tracks VIN. If the soft-start time has not expired, the output tracks VIN until VOUT reaches the value set by the charging soft-start capacitor. Figure 24 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.
When VBIAS and VEN are present and VIN is not supplied, this device outputs approximately 50 μA of current from OUT. Although this condition does not cause any damage to the device, the output current may charge up the OUT node if total resistance between OUT and GND (including external feedback resistors) is greater than 10 kΩ.
The TPS74701 provides low-output noise when a soft-start capacitor is used. When the device reaches the end of the soft-start cycle, the soft-start capacitor serves as a filter for the internal reference. By using a 0.001-μF soft-start capacitor, the output noise is reduced by half and is typically 30 μVRMS for a 1.2-V output (10 Hz to 100 kHz). Further increasing CSS has little effect on noise. Because most of the output noise is generated by the internal reference, the noise is a function of the set output voltage. The RMS noise with a 0.001-μF soft-start capacitor is given in Equation 3:
The low-output noise of the TPS74701 makes it a good choice for powering transceivers, PLLs, or other noise-sensitive circuitry.
Figure 25 illustrates the typical application circuit for the TPS74701 adjustable output device.
Table 4 shows the design parameters for this application.
|DESIGN PARAMETER||EXAMPLE VALUE|
|VIN||1.8 V ± 10%|
|VBIAS||3.3 V ±10%|
|VOUT||1.5 V ± 3%|
|Start-up time||< 2 ms|