Patent Description:
A low drop-out (LDO) regulator is a linear regulator that uses a transistor to generate a regulated output voltage with a low differential between the input voltage and the output voltage. Commonly, in battery powered devices, a switching regulator (such as a buck regulator) is between the battery and an LDO regulator. This circuit arrangement combines the efficiency of a switching regulator and the fast response of a LDO regulator. For further improvements in efficiency, the output voltage from the switching regulator usually is set near the desired regulated output voltage from the LDO regulator. The gate-to-source voltage (to operate the main power transistor in an LDO regulator) is limited by the magnitude of the input voltage to the LDO regulator. <CIT> discloses a threshold voltage adjustment for MOS devices.

Described examples include a voltage regulator having a pass transistor coupled to an input voltage node and an output voltage node. The voltage regulator also includes a drive transistor coupled to a control input of the pass transistor and a first resistor coupled between a source and a back gate of the drive transistor. The voltage regulator further includes a complementary to absolute temperature (CTAT) current generator circuit coupled to the resistor and configured to generate a CTAT current to bias the first resistor.

In described examples, the pass transistor includes a p-type metal oxide semiconductor field effect transistor (MOSFET) including a gate, a source, a drain and a back gate. The source is connected to an input voltage node and to the back gate, and the drain is connected to an output voltage node. The voltage regulator also includes a drive transistor coupled to the gate of the pass transistor, and a first resistor connected between a source and a back gate of the drive transistor. A CTAT current generator circuit is coupled to the resistor. The CTAT current generator circuit is configured to generate a CTAT current that is used to bias the first resistor.

In this description, the term "couple" or "couples" means either an indirect or direct wired or wireless connection. Thus, if a first device couples to a second device, that connection may be through a direct connection or through an indirect connection via other devices and connections. The recitation "based on" means "based at least in part on. " Therefore, if X is based on Y, then X may be a function of Y and any number of other factors.

A voltage regulator (such as a low drop-out (LDO) regulator) is described herein, which includes a drive transistor that drives a signal to a power transistor. The power transistor provides an output voltage from the voltage regulator to a load. In described embodiments, the drive transistor includes a source that is connected to the back gate by way of a resistor. A current flows through the resistor to thereby bias the back gate of the drive transistor. By biasing the drive transistor's back gate, the threshold voltage of the drive transistor can be lowered. Lowering the drive transistor's threshold voltage permits the drive transistor to be turned on with a lower gate-to-source voltage, which thereby permits an increase of load current for the same input voltage to the voltage regulator, increases the available voltage headroom for turning on the power transistor for a given power supply voltage, or which permits the same load current for a smaller input voltage. Further, the potential for a latch-up condition is reduced.

In some embodiments, the current generated within the LDO regulator to bias the drive transistor's back gate is generated by a complementary to absolute temperature (CTAT) current generator. This current generator generates a CTAT current, which is a current that varies inversely with temperature. The drive transistor may comprise a p-type metal oxide semiconductor field effect transistor (PMOS), and the threshold voltage of the PMOS varies inversely with temperature. Because a CTAT current is used to bias the drive transistor's back gate and the threshold voltage is proportional to the back gate voltage, the threshold voltage and the back gate voltage generally track each other with temperature, so they vary in the same direction with temperature.

<FIG> illustrates a system in which a switching regulator <NUM> is coupled to an LDO regulator <NUM> (also termed a "voltage regulator") for providing an output voltage (Vout) to a load <NUM>. The output voltage comprises the operating voltage for the load <NUM>. The load <NUM> may comprise any passive or active electrical circuit or device that performs one or more desired functions. For example, the load <NUM> may comprise circuitry within a computing device such as notebook computer, tablet device or smart phone. The input voltage to the switching regulator <NUM> is designated as VINA, and the output voltage from the switching regulator <NUM> is designated as VINB. Generally, VINB is lower than VINA. As a low drop-out regulator, LDO regulator <NUM> is able to generate a regulated output voltage, Vout, with little headroom between VINB and Vout.

The LDO regulator <NUM> includes an error amplifier (EA) <NUM>, a drive transistor <NUM>, a pass transistor <NUM>, resistors R1 and R2, and a CTAT current generator <NUM>. The resistors R1 and R2 are connected in series between the output voltage node <NUM> and ground, thereby forming a voltage divider. The connection point between the resistors R1 and R2 provides a scaled down version of Vout and is used as a feedback voltage (VFB) to the error amplifier <NUM>. The error amplifier <NUM> amplifies the difference between VFB and a reference voltage, VREF. The output signal <NUM> from the error amplifier <NUM> is provided to the drive transistor <NUM> to turn the drive transistor <NUM> on and off to thereby control the state of the pass transistor <NUM>. Thus, the pass transistor <NUM> is controlled based on the feedback voltage, VFB, to maintain the output voltage, Vout, on output voltage node <NUM> at a regulated level.

A resistor is connected between the source (S) and the back gate (BG) of the drive transistor <NUM>. The resistor is designated as RSB, and can be trimmable as indicated by the arrow through the resistor symbol and as described hereinbelow. The CTAT current generator <NUM> generates a current that varies inversely with temperature. The current produced by the CTAT current generator <NUM> flows through RSB and thus is used to bias the drive transistor's back gate (BG).

<FIG> shows an embodiment of a portion of the LDO regulator <NUM> coupled to a portion of an output stage <NUM> of the error amplifier <NUM>. The error amplifier's output stage <NUM> includes a current source <NUM> coupled to two transistor switches <NUM> and <NUM>. The LDO regulator <NUM> in this embodiment includes the drive transistor MDRV (shown as drive transistor <NUM> in <FIG>), the pass transistor MPWR (shown as pass transistor <NUM> in <FIG>), resistors R1, R2, and RSB, current sources I1 and I2, and CTAT current sources (ICTAT). Current sources I1 and I2 may be equal (i.e., same current). In this embodiment, MDRV and MPWR comprise pMOS transistors, and each has a gate (G), a source (S), a drain (D) and a back gate (BG). The back gate may also be referred to as a bulk connection. The gates of the transistors represent control inputs for the transistors.

The pass transistor MPWR couples to an input voltage node <NUM> and the output voltage node <NUM>. In this configuration, the source of the pass transistor MPWR connects to the input voltage node <NUM> and the drain connects to the output voltage node <NUM>. Further, the back gate of the pass transistor MPWR connects to the source thereby shorting the source to the back gate. The series-connected resistors R1 and R2 connect between the drain of the pass transistor MPWR and ground as shown.

The drive and pass transistors MDRV and MPWR are matched, meaning that they are formed from a common semiconductor substrate and process. The drive transistor MDRV may have a physical size that is smaller than the pass transistor MPWR. Transistors MDRV and MPWR may be chosen to be the same transistor component from a library of components. The device sizes expressed in the general form N*(W/L) (where W is width and L is length) are designed such that L_MDRV = L_MPWR and W_MDRV = W_MPWR. The number of fingers are designed such that N_MPWR = K * N_MDRV where K >> <NUM>. This choice enables the MDRV transistor device parameters to closely track MPWR device parameters across large sample sizes of integrated circuits and across temperature and semiconductor process variations.

The gate of the drive transistor MDRV is coupled to the error amplifier output stage <NUM> as shown and receives the output signal <NUM> from the error amplifier. The current sources I1 and I2 function to drive current the source to drain channel of the drive transistor MDRV. The source of the drive transistor MDRV connects to the current source I1 and the gate of the pass transistor MPWR.

Resistor RSB couples between the source and the back gate of the drive transistor MDRV. Current flowing through resistor RSB biases the back gate of the drive transistor MDRV relative to the source. For example, the back gate voltage is less than the source voltage due to the voltage drop across resistor RSB. The threshold voltage of the transistor MDRV is a function of the source-to-back gate voltage as is shown by the following equation: <MAT> which can be written in a simpler form as: <MAT> where γ is the body effect parameter <MAT> <MAT> where VFB is the flatband voltage, <NUM>φF is the surface potential, εS is the permittivity of silicon, Nd is the doping concentration, and Cox is the gate oxide concentration. In described embodiments, the back gate of the transistor MDRV is biased, which thus reduces the threshold voltage of the transistor.

The current used to bias the back gate through resistor RSB varies inversely with temperature as described hereinabove and is generated by the ICTAT current sources, which comprise the CTAT current generator <NUM> of <FIG>.

<FIG> shows an example of the implementation of the CTAT current generator <NUM>. In this example, the CTAT current generator <NUM> includes a current mirror <NUM>, a bipolar junction transistor (BJT) <NUM>, a resistor R3, transistors <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, and a current source I3. The BJT includes a base (B), collect (C), and an emitter (E). The BJT <NUM> provides a voltage produced across a p-n junction comprising the base and emitter. In other embodiments, other types of p-n junctions can be included other than a BJT. Current source I3 produces a current that causes transistor <NUM> to turn on, thereby causing the BJT to conduct and produce the base-to-emitter voltage. Resistor R3 connects between the base and emitter of the BJT as shown and thus receives the base-to-emitter voltage produced by the BJT <NUM>. As a result, a current flows through resistor R3. The base-to-emitter voltage of the BJT <NUM> varies inversely with temperature, the current through R3 also varies inversely with temperature, thereby representing the ICTAT current.

The current mirror <NUM> (comprising transistors <NUM>, <NUM> and <NUM>) mirrors the ICTAT current into resistor RSB. Accordingly, the voltage generated across RSB is (VBE/R3)xRSB, where VBE/R3 represents the current through resistor R3. If the resistance values of R3 and RSB are equal, then the source-to-back gate bias voltage across resistor RSB will equal the CTAT base-to-emitter voltage of the BJT <NUM>. In some embodiments, the resistance value of RSB is n/R<NUM>, where <NUM><n<<NUM>. Accordingly, the source-to-back gate bias voltage across resistor RSB is less than or equal to the base-to-emitter voltage of the BJT <NUM> and is related to the base-to-emitter voltage of the BJT <NUM> by the ratio of RSB to R3. In some embodiments, RSB and R3 are matched, meaning that they are (a) fabricated using the same steps or using the same component from a design library, (b) have the same dimensions of width and length, and (c) are closely located, and their fingers (if using polysilicon resistors) are evenly spaced. Based on these characteristics, the resistors RSB and R3 are expected to track each other's resistance value across process and temperature variations, such that their ratio RSB/R3 is equal to a design target at all times.

In the example of <FIG>, the CTAT current generator also includes an enable input. The enable input is provided to a switch to selectively configure the CTAT current generator circuit to be in: (a) an active state, in which the CTAT current generator circuit provides the CTAT current to resistor RSB; or (b) an inactive state, in which CTAT current is not provided to the resistor. Accordingly, the CTAT current generation capability of the LDO regulator can be disabled. For example, for a battery operated device to save power, it might be desired to disable the CTAT current generation capability of the LDO regulator. The LDO regulator will otherwise continue to operate, but without the back gate of the drive transistor MDRV being biased with respect to the source.

In the example of <FIG>, transistors <NUM>-<NUM> and <NUM> can be turned on and off by an enable signal (EN) or its complementary signal (ENB). For example, if EN is high and ENB is low, then transistors <NUM> and <NUM> are on and transistors <NUM> and <NUM> are off, thereby permitting the CTAT current generator to bias the drive transistor's back gate with a CTAT current. In contrast, if EN is low and ENB is high, then transistors <NUM> and <NUM> are off and transistors <NUM> and <NUM> are on, thereby preventing the CTAT current generator from biasing the drive transistor's back gate with a CTAT current.

In some embodiments, resistor RSB is trimmable to provide control over the source-to-back gate voltage of the drive transistor MDRV. RSB can be programmable by fabricating RSB using a series of segments and shorting or opening transistor switches across segments. For example, <FIG> illustrates an implementation of resistor RSB as a series of resistors RSB1, RSB2, RSB3, RSB4, RSB5 and RSB6. Resistors RSB1 and RSB6 are always included in the circuit, but resistors RSB2-RSB5 can be individually included or removed from the circuit. A switch across each resistor can be opened or closed by a trim signal. Opening a switch causes the corresponding resistor to be included, and closing the switch shorts the resistor. Switch SW1 permits resistor RSB2 to be included or shorted. Switch SW2 permits resistor RSB3 to be included or shorted. Switch SW3 permits resistor RSB4 to be included or shorted. Switch SW4 permits resistor RSB5 to be included or shorted. The trim signals are shown as T0, T1, T2 and T3. The trim signals may be generated upon power up of the LDO regulator <NUM>, such as (in this example) based on a two-bit trim value stored in a non-volatile memory. With two bits, the trim value can be used to generate four different combinations of trim signals T0-T3, with each trim signal being a high or a low signal to open or close the corresponding switch.

The switches can be programmed using a communication interface, such as the Inter-Integrated Circuit (I<NUM>C) interface or the Serial Peripheral Interface (SPI) in the factory, and the optimal settings can be burned into a non-volatile memory. One trimming method may include:.

Another indirect trimming method could be as follows:.

Claim 1:
A low drop-out voltage regulator, comprising:
a pass transistor (<NUM>) coupled to an input voltage node (VINB), which is coupleable to the output of a switching regulator (<NUM>), and an output voltage node (VOUT), which is coupled to an output voltage divider (R1, R2) and coupleable to a load (<NUM>);
the voltage divider (R1, R2);
a drive transistor (<NUM>), wherein a source (S) of the drive transistor (<NUM>) is coupled to a control input of the pass transistor (<NUM>);
a first resistor (RSB) coupled between the source (S) and a back gate (BG) of the drive transistor (<NUM>);
a complementary to absolute temperature CTAT current generator (<NUM>) circuit coupled between the input voltage node (VINB) and the source (S) of the drive transistor (<NUM>) and configured to generate a CTAT current through the first resistor (RSB) to bias the first resistor (RSB); and
an error amplifier (<NUM>) with a first input coupled to the voltage divider (R1, R2) to receive a feedback voltage (VFB) and a second input for receiving a reference voltage (VREF) and an output coupled to the drive transistor (<NUM>) to turn the drive transistor (<NUM>) on and off.