Abstract:
A low-dropout (LDO) voltage regulator for generating an output voltage is disclosed. The voltage regulator includes a startup circuit, a curvature corrected bandgap circuit, an error amplifier, a metal oxide semiconductor (MOS) pass device and a voltage slew rate efficient transient response boost circuit. The MOS pass device has a gate node which is coupled to the output of the error amplifier, and a drain node for generating the output voltage. The voltage slew rate efficient transient response boost circuit applies a voltage to the gate node of the MOS pass device to accelerate the response time of the error amplifier in enabling the LDO voltage regulator to reach its final regulated output voltage when an output voltage drop occurs in the LDO voltage regulator.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 11/406,172, filed Apr. 18, 2006, which issued as U.S. Pat. No. 7,199,565 on Apr. 3, 2007 and is incorporated by reference as if fully set forth. 

   FIELD OF INVENTION 
   The present invention is related to voltage regulation circuits. More particularly, the present invention is related to a voltage regulator that uses semiconductor devices to provide generally fixed output voltages over varying loads with minimal voltage dropout on the output. 
   BACKGROUND 
   Low-dropout (LDO) voltage regulators have gained popularity with the growth of battery-powered equipment. Portable electronic equipment including cellular telephones, pagers, laptop computers and a variety of handheld electronic devices has increased the need for efficient voltage regulation to prolong battery life. LDO voltage regulators are typically packaged as an integrated circuit (IC) to provide generally fixed output voltages over varying loads with minimal voltage dropout on the output in a battery-powered device. Furthermore, performance of LDO voltage regulators is optimized by taking into consideration standby and quiescent current flow, and stability of the output voltage. 
     FIG. 1  is a schematic diagram of a conventional LDO voltage regulator  100  including a startup circuit  105 , a curvature corrected bandgap circuit  110 , an error amplifier  115 , a metal oxide semiconductor (MOS) pass device  120 , (e.g., a positive channel MOS (PMOS) pass device, a negative channel MOS (NMOS) pass device), resistors  125 ,  130 , and a decoupling capacitor  135  having a capacitance COUT. The LDO voltage regulator  100  outputs an output voltage, V out ,  145 . 
   The curvature corrected bandgap circuit  110  is electrically coupled to the startup circuit  105  and the error amplifier  115 . The startup circuit  105  provides the curvature corrected bandgap circuit  110  with current when no current is flowing through the LDO voltage regulator  100  during a supply increase or startup phase until the bandgap voltage is high enough to allow the curvature corrected bandgap circuit  110  to be self-sustaining. The curvature corrected bandgap circuit  110  generates a reference voltage  152  which is input to a positive input  150  of the error amplifier  115 , and a reference current  154  which is input to a reference current input  158  of the error amplifier  115 . Generally, the reference current  154  is a proportional to absolute temperature (PTAT) current generated by the curvature corrected bandgap circuit  110 . 
   The error amplifier  115  includes a positive input  150  coupled to the curvature corrected bandgap circuit  110  for receiving the reference voltage  152 , a reference current input  158  for receiving the reference current  154 , a negative input  155 , and an amplifier output  160 . 
   The MOS pass device  120  includes a gate node  165 , a source node  170  and a drain node  175 . The MOS pass device  120  may be either a PMOS or an NMOS pass device. The gate node  165  of the MOS pass device  120  is coupled to the amplifier output  160  of the error amplifier  115 . The source node  170  of the MOS pass device  120  is coupled to a supply voltage, V s . The drain node  175  of the MOS pass device  120  generates the output voltage, V out ,  145  of the LDO voltage regulator  100 . The resistors  125  and  130  are connected in series to form a resistor bridge. One end of the resistor  125  is coupled to the drain node  175  of the MOS pass device  120  and the other end of the resistor  125  is coupled to both the negative input  155  of the error amplifier  115  and one end of the resistor  130 . Thus an error correction loop  180  is formed. The other end of resistor  130  is coupled to ground. The decoupling capacitor  135  is coupled between V out  and ground. 
   In the conventional LDO voltage regulator  100 , a capacitance CMOS associated with the gate node  165  of the MOS pass device  120  and the decoupling capacitor  135  cause the slew rate and bandwidth of the error amplifier  115  to be limited. The conventional LDO voltage regulator  100  provides a fixed output voltage, but is constrained by others specifications such as voltage drop, gain and transient response. When a current step occurs, (due to the load of a circuit coupled to the output voltage, V out ,  145 ), the output voltage, V out ,  145  decreases first and, after an error correction loop delay Tfb occurs, the gate node  165  of the MOS pass device  120  is adjusted by the error amplifier  115  to provide the requested output current. 
     FIG. 2  shows a graphical representation of the output voltage, V out ,  145  of the conventional LDO voltage regulator  100  shown in  FIG. 1  during a maximum current step required by the load of a circuit coupled to the voltage output, V out ,  145 . The delay Tfb corresponds to the minimum error correction loop delay to ensure voltage regulation. This delay is proportional to the bandwidth of the error amplifier  115  and may be calculated in accordance with the following Equation (1): 
                   Tfb   =     1   fu       ;           Equation   ⁢           ⁢     (   1   )                 
where Tfb is the delay and fu is the unity gain frequency of the error amplifier  115 .
 
   The voltage drop during this delay may be approximated in accordance with the following Equation (2): 
                   δ   ⁢           ⁢   V     =         I   max       C   out       ⁢   Tfb             Equation   ⁢           ⁢     (   2   )                 
where δV is the voltage drop, I max  is the maximum output current required by the load of a circuit coupled to the voltage output, V out ,  145 , C out  is the capacitance of the decoupling capacitor  135  and Tfb is the error correction loop delay.
 
   Referring to  FIGS. 1 and 2 , the error correction loop  180  provides voltage regulation after the Tfb delay and modifies the voltage of the gate node  165  of the MOS pass device  120  in order to switch on the MOS pass device  120 . The output voltage, V out ,  145  is adjusted until the full load regulated value is reached. The time needed to recover the final value, T reg , may be approximated in accordance with the following Equation (3): 
                   T   reg     =         C   OUT         I   pass     -     I   max         ×     V   drop               Equation   ⁢           ⁢     (   3   )                 
where C out  is the capacitance of the decoupling capacitor  135 , I pass  is the current of the MOS pass device  120 , I max  is the maximum output current required by the load of a circuit coupled to the voltage output, V out ,  145 , and V drop  is the maximum voltage drop.
 
   After T reg , the voltage of the gate node  165  of the PMOS pass device  120 , V gsmax , provides sufficient current through the PMOS pass device  120  to ensure output voltage stability. However, a significant voltage drop and a delay in reaching the final regulated output voltage occurs. 
   It would be desirable to modify the LDO voltage regulator  100  of  FIG. 1  such that it is able to more rapidly set the voltage of the gate node  165  of the PMOS pass device  120  to the V gsmax  voltage (or lower) in order to reduce output voltage drops and delays in reaching the final regulated output voltage, V out ,  145 . 
   SUMMARY 
   The present invention is related to an LDO voltage regulator for generating an output voltage. The voltage regulator includes a startup circuit, a curvature corrected bandgap circuit, an error amplifier, a MOS pass device and a voltage slew rate efficient transient response boost circuit. The MOS pass device has a gate node which is coupled to the output of the error amplifier, and a drain node for generating the output voltage. The voltage slew rate efficient transient response boost circuit applies a voltage to the gate node of the MOS pass device to accelerate the response time of the error amplifier in enabling the LDO voltage regulator to reach its final regulated output voltage when an output voltage drop occurs in the LDO voltage regulator. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more detailed understanding of the invention may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein: 
       FIG. 1  is a schematic diagram of a conventional LDO voltage regulator; 
       FIG. 2  is a graphical representation of the output voltage transient response to a maximum output current step in the conventional LDO voltage regulator of  FIG. 1 ; 
       FIG. 3  is a schematic diagram of an LDO voltage regulator with a voltage slew rate efficient transient response boost circuit configured in accordance with the present invention; 
       FIG. 4  is a graphical representation of the output voltage transient response of the LDO voltage regulator of  FIG. 3  when a transient response boost voltage, Vb, is set to zero volts (ground); 
       FIG. 5  is a graphical representation of the output voltage transient response of the LDO voltage regulator of  FIG. 3  when Vb is set to V gsmax ; and 
       FIG. 6  is a flow diagram of a process of regulating an output voltage implemented by the LDO voltage regulator of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is incorporated in a novel voltage regulator which provides a simple solution to increase voltage regulator performance while reducing output voltage drop. This solution includes a voltage slew rate efficient transient response boost circuit that is configured in accordance with the present invention. The present invention can also be applied to any known voltage regulator structure by incorporating a voltage slew rate efficient transient response boost circuit which provides a simple solution to increase voltage regulator performance. 
   In one embodiment, the gate node of a PMOS pass device is rapidly set to the V gsmax  voltage (or lower) in order to avoid voltage drops and to reduce delays between the output current step and the final regulated output voltage. When the output voltage falls below a predefined threshold, the gate node of the MOS pass device is coupled to V gsmax  (or lower). 
   Referring now to  FIG. 3 , a schematic diagram of an LDO voltage regulator  300  configured in accordance with the present invention is shown. The LDO voltage regulator  300  includes a startup circuit  305 , a curvature corrected bandgap circuit  310 , an error amplifier  315 , a MOS pass device  320 , a resistor bridge  325  including resistors  325 A,  325 B,  325 C, a decoupling capacitor  330  having a capacitance C out , a comparator  335  and a MOS switch device  340 . The LDO voltage regulator  300  generates an output voltage, V out ,  345 . The resistor bridge  325 , the comparator  335  and the MOS switch device  340  form a slew rate efficient transient response boost circuit. The MOS pass device  320  may be either a PMOS or an NMOS pass device. The MOS switch device  340  may be either a PMOS or an NMOS switch device. 
   The curvature corrected bandgap circuit  310  is electrically coupled to the startup circuit  305  and the error amplifier  315 . The startup circuit  305  provides the curvature corrected bandgap circuit  310  with current when no current is flowing through the LDO voltage regulator  300  during a supply increase or startup phase until the bandgap voltage is high enough to allow the curvature corrected bandgap circuit  310  to be self-sustaining. The curvature corrected bandgap circuit  310  generates a bandgap reference voltage  352  which is input to a positive input  350  of the error amplifier  315  and a negative input  355  of the comparator  335 . The curvature corrected bandgap circuit  310  also generates a reference current  354  which is input to a reference current input  358  of the error amplifier  315 . Generally, the reference current  354  is a PTAT current generated by the curvature corrected bandgap circuit  310 . 
   The error amplifier  315  includes a positive input  350  coupled to the curvature corrected bandgap circuit  310  for receiving the bandgap reference voltage  352 , a reference current input  358  for receiving the bandgap reference current  354 , a negative input  360  for receiving an error correction voltage  359  from the resistor bridge  325 , and an amplifier output  365 . 
   The MOS pass device  320  includes a gate node  370 , a source node  372  and a drain node  374 . The gate node  370  of the MOS pass device  320  is coupled to the amplifier output  365 , which outputs a pass device control signal. The source node  372  of the MOS pass device  320  is coupled to a supply voltage, V s . The drain node  374  of the MOS pass device  320  generates the output voltage, V out ,  345  of the LDO voltage regulator  300 . The resistors  325 A,  325 B,  325 C are connected in series to form a resistor bridge  325 . One end of the resistor  325 A is coupled to the drain node  374  of the MOS pass device  320  and the other end of the resistor  325 A is coupled to both a positive input  376  of the comparator  335  and one end of the resistor  325 B. The other end of the resistor  325 B is coupled to the negative input  360  of the error amplifier  315  and to one end of the resistor  325 C. The other end of the resistor  325 C is coupled to ground. The decoupling capacitor  330  is coupled between V out    345  and ground. 
   Still referring to  FIG. 3 , the MOS switch device  340  includes a gate node  380 , a source node  382  and a drain node  384 . An output  378  of the comparator  335  is coupled to the gate node  380  of the MOS switch device  340 . The output  378  generates a switch device control signal. The drain node  384  is coupled to the output  365  of the error amplifier  315  and the gate node of the MOS pass device  320 . The source node  382  of the MOS switch device  340  is coupled to a transient response boost voltage, Vb, which may be generated, for example, by an output current monitoring unit coupled to the voltage output, V out ,  345 . 
   The positive input  376  of the comparator  335  receives a threshold voltage, Vt,  326  from the junction between the resistors  325 A and  325 B. The value of Vt may be calculated in accordance with the following Equation (4): 
                 Vt   =       V   out     -     (       V   drop     -         I   max       C   out       ×     τ   de         )               Equation   ⁢           ⁢     (   4   )                 
where Vt is the threshold voltage of the comparator  335 , V out  is the regulated output voltage, V drop  is the maximum voltage drop allowed, I max  is the maximum output current, C out  is the value of the decoupling capacitor  330  and τ de  is the internal delay of the comparator  335 .
 
   The MOS switch device  340  is a small and fast device having a drain node  384  coupled to the gate node  370  of the MOS pass device  320  and coupled to a transient response boost voltage, Vb, that is set to a “final value” between zero volts, (i.e., a ground value), and a maximum voltage, V gsmax . The purpose of the MOS switch device  340  is to rapidly set a final value on the gate node  370  of the MOS pass device  320  in order to permit the MOS pass device  320  to deliver the maximum output current to V out    145 . 
   As shown in  FIG. 4 , the output voltage transient response of the present invention has the same error correction loop delay Tfb as that in the transient response of the conventional LDO voltage regulator  100  shown in  FIG. 1 . By switching the MOS switch device  340  on, Vb is set to a ground value which results in a high output current and a fast output voltage rising edge. The comparator  335  then switches off the NMOS switch device  340  until the next voltage drop. The output  378  of the comparator  335  is either zero volts, (i.e., a ground value), which turns off the MOS switch device  340 , or V s  which turns on the MOS switch device  340 . During this time, some oscillations may be present due to the multiple comparator switching but the maximum voltage drop is reduced. After the error correction loop delay Tfb, the error correction voltage  359  is provided by the resistor bridge  325  to the negative input  360  of the error amplifier  315 , which provides output voltage regulation and adjusts the output voltage on the gate node  370  of the MOS pass device  320  to the final value. 
   In another embodiment, the transient response boost voltage, Vb, is set exactly to V gsmax . The comparator  335  switches on the MOS switch device  340 , thus coupling the gate node  370  of the MOS pass device  320  to V gsmax , whereby the output current is exactly the same as the load current. Thus, output voltage, V out ,  345  is immediately regulated, as shown in  FIG. 5 . When the voltage drop exceeds Vt, the gate node  370  of the PMOS pass device  320  is immediately coupled to its final value and then the LDO voltage regulator  300  is set to a full load regulated voltage mode. By setting the voltage of the gate node  370  of the MOS pass device using the MOS switch device  340 , instead of waiting for the error amplifier  325  to do it, the error amplifier response time is increased and the voltage output  345  is regulated and the voltage drop of V out    345  is greatly reduced. 
   In accordance with the present invention, a process  600  of regulating an output voltage, V out ,  345  is implemented using the LDO voltage regulator  300 . Referring to  FIGS. 3 and 6 , a bandgap reference voltage  352  is received at the positive input  350  of the error amplifier  315 , a bandgap reference current  354  is received at the reference current input  358  of the error amplifier  315 , and an error correction voltage  359  derived from the output voltage, V out ,  345  is received at the negative input  360  of the error amplifier  315  (step  605 ). The error amplifier  315  generates a pass device control signal which closes the pass device  320  based on the bandgap reference voltage  352 , the bandgap reference current  354  and the error correction voltage  359  to adjust the output voltage, V out ,  345  to a full load regulated value (step  610 ). In step  615 , the transient response boost voltage, Vb, is generated. In step  620 , the bandgap reference voltage  352  is compared by the comparator  335  to a threshold voltage, Vt,  326  derived from the output voltage, V out ,  345 . The comparator  335  generates a switch device control signal which closes the switch device  340  based on the comparison of step  620  to selectively apply the transient response boost voltage, Vb, to the pass device control signal to accelerate the rate at which the output voltage, V out ,  345  is adjusted to the full load regulated value (step  625 ). The transient response boost voltage, Vb, is applied to the pass device control signal when a drop in the output voltage, V out ,  345  occurs. 
   Although the features and elements of the present invention are described in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements of the present invention.