The new XC6601 LDO regulator from TOREX is ideally suited for use in applications requiring a very low input (down to 1.0V) and output (down to 0.7V) voltage, a characteristic made possible by the use of an NMOS transistor instead of a conventional PMOS. NMOS technology enables higher currents and also features a lower on-state resistance and drop-out voltage. The XC6601 furthermore provides an extremely fast transient response and requires only 25µA of current to operate. Together, these characteristics make the XC6601 the perfect product for low-voltage applications.

For most applications, the XC6601 is intended to serve as a second regulator downstream of a DC/DC converter, with the VIN pin connected directly to the DC/DC converter’s output. A sample application is shown in Figure 1 below, in which a highly efficient synchronous step-down DC/DC converter from TOREX’s 3MHz XC9235/36/37 series is also used. The converter transforms the supplied voltage from the Li-Ion battery to 1.8V, at 150mA, for the processor I/Os. The downstream connected XC6601 takes this 1.8V output and from it generates the 1.5V / 300mA voltage required by the processor core.

Excellent Drop-Out Voltage Performance
One of the key arguments for using the XC6601 in this type of low-voltage application is the new regulator’s exceedingly low drop-out voltage – a mere 38mV at 100mA. In fact, compared to other popular products offering a very low drop-out voltage, such as the XC6210 series, the XC6601’s figure is 50% lower given comparable conditions. Figure 2 clearly shows this advantage.

NMOS vs. PMOS Driver Resistors
Why NMOS? A key reason for choosing the latter is that, for PMOS transistors, the IC supply voltage is identical to the transistor’s drive voltage. The latter thus cannot be lower than the IC supply voltage – which, though already minimal, cannot be made arbitrarily lower. In addition, the voltage VGS for PMOS transistors is formed by the difference between VIN and VSS, so VGS will also go down if VIN is reduced. However, reducing VIN will in turn increase the on-state resistance, so VIN cannot be made arbitrarily small either. The XC6601, on the other hand, has separate pins for the IC supply voltage (VBIAS) and input voltage (VIN). As a result, VIN can be reduced to 1.0V, which is significantly lower than for standard, fast LDOs. As a sample comparison, the minimum voltage for the XC6210 LDO (with a PMOS transistor) has a lower limit of 1.5V.

In this new NMOS architecture, VGS is formed by the difference between VBIAS and VOUT (Fig. 3). Now, if a supply voltage greater than 2.5V is applied to the VBIAS pin, VOUT can be almost arbitrarily small. VGS will not be affected by this value and always remains large enough to ensure a low on-state resistance.Transient Response
Fig. 4 confirms that the XC6601 has a very good transient response performance. For a load of 1mA to 100mA, the output voltage will drop by a mere 45mV for a very short duration and then return to a stable 1.5V.

In a final, factory-performed manufacturing step, the output voltage of the XC6601 can be programmed between 0.7V and 1.8V in 50mV increments, with the required operating voltage lying between 1.0V and 3.0V. The IC supports output currents of up to 400mA and can be operated with low-ESR ceramic capacitors; and its many different features include current limiting and overtemperature protection. The XC6601 is available in the following packages: USP-6C, SOT-25 and SOT89-5.

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