The TPS748A-Q1 offers very low dropout performance, making the device designed for high-current, low V_IN and low V_OUT applications. The low dropout allows the device to be used in place of a dc/dc converter and still achieve good efficiency. Equation 4 provides a quick estimate of the efficiency.

Equation 4.

This efficiency provides designers with the power architecture for applications to achieve the smallest, simplest, and lowest cost solutions.

For this architecture, there are two different specifications for dropout voltage. The first specification (see Figure 6-11 ) is referred to as V_IN dropout and is used when an external bias voltage is applied to achieve low dropout. This specification assumes that V_BIAS is at least 2.8 V above V_OUT, which is the case for V_BIAS when powered by a 5.0-V rail with 5% tolerance and with V_OUT = 1.5 V. If V_BIAS is higher than V_OUT + 2.8 V, the V_IN dropout is less than specified.

The second specification (illustrated in Figure 6-12 ) is referred to as V_BIAS 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 V_BIAS provides the gate drive to the pass transistor; therefore, V_BIAS must be 1.9 V above V_OUT. Because of this usage, having IN and BIAS tied together become a highly inefficient solution that can consume large amounts of power. Pay attention not to exceed the power rating of the device package.