Patent Abstract:
In a voltage regulator including an inductor current flowing through a sense element with a first temperature coefficient, and a current sense circuit for generating a current sense signal related to the first temperature coefficient by sensing the inductor current from the sense element, a temperature compensation device and method determines a second temperature coefficient according to the first temperature coefficient and temperature variation, and produces a compensation signal with the second temperature coefficient to compensate variations in the current sense signal caused by the first temperature coefficient.

Full Description:
FIELD OF THE INVENTION 
   The present invention is related generally to a voltage regulator, and more particularly, to a temperature compensation device and method for a voltage regulator. 
   BACKGROUND OF THE INVENTION 
   Voltage regulators have been applied extensively in various electronic products as power supplies. In state-of-art voltage regulators, in order to prevent load from being damaged due to voltage spike of the output in transients, voltage droop function is adapted for diminishing the voltage spike on the load in transients.  FIG. 1  schematically shows a conventional voltage regulator  100  with voltage droop function, in which driver  104  switches transistors  106  and  108  coupled in series between input voltage Vin and ground GND in response to pulse width modulation (PWM) signal provided by control circuit  102 . Thereby, inductor current IL is generated to charge output capacitor Co so that output voltage Vo is produced. In the control circuit  102 , from the voltage drop across current sense resistor Rs due to the inductor current IL flowing therethrough, current sense circuit  110  produces current sense signal 
                   Ix   =       IL   ×   Rs     K       ,           [     EQ   ⁢     -     ⁢   1     ]               
where K is the equivalent resistance of the current sense circuit  110 . The current sense signal Ix passes through droop resistor R ADJ  and produces load line droop voltage
   Vdroop=Ix×R   ADJ .  [EQ-2] 
Because of virtual ground, the voltage on pin  116  intends to be equal to reference voltage Vref, and therefore the output voltage will be
   Vo=Vref−Vdroop .  [EQ-3] 
Error amplifier  112  generates error signal EA from the difference between its inverting and non-inverting inputs, and PWM comparator  114  compares the error signal EA with ramp signal Vramp to determine the PWM signal for the driver  104 . From the equations EQ-1, EQ-2 and EQ-3, it is known that the output voltage Vo of the regulator  100  will decrease as the inductor current IL increases.
 
     FIG. 2  schematically shows another conventional voltage regulator  200  with voltage droop function, which comprises control circuit  102 , driver  104 , transistors  106  and  108 , current sense circuit  110 , error amplifier  112 , PWM comparator  114 , and current sense resistor Rs as well. However, the reference voltage Vref is coupled to the non-inverting input of the error amplifier  112  via the droop resistor R ADJ  and pin  116 , and the output of the current sense circuit  110  is coupled to the non-inverting input of the error amplifier  112 . When the current sense signal Ix passes through the droop resistor R ADJ , load line droop voltage Vdroop is produced as described in the equation EQ-2. The inverting input of the error amplifier  112  is coupled to the output Vo, and thereby the output voltage Vo follows the equation EQ-3 due to virtual ground. Consequently, according to the equations EQ-1, EQ-2 and EQ-3, it is known that the output voltage Vo of the regulator  200  will decrease as the inductor current IL increases. 
   In a voltage regulator with droop function, the load line droop signal proportional to the output current is sensed by the current sense resistor. Unfortunately, the resistance of an ordinary resistor is a function of temperature, so that the load line is also the function of temperature. Consequently, incorrect operations may happen because the control circuit  102  provides incorrect PWM signal due to the incorrect current sense signal Ix caused by the temperature coefficient of the current sense resistor Rs. In order to prevent mal-operations by the control circuit  102  resulted from temperature variations, the droop resistor R ADJ  with proper negative temperature coefficient is chosen to be positioned in the vicinity of the current sense resistor Rs to compensate the voltage variations caused by temperature changes on the current sense resistor Rs with positive temperature coefficient. Nevertheless, resistors with negative temperature coefficients are not ordinary resistors and thereby are more expensive. In addition, in order to position the droop resistor R ADJ  nearby the current sense resistor Rs, the conductive wire between the pin  116  and droop resistor R ADJ  is lengthened, which makes the pin  116  tend to be affected by switching noises. Moreover, there is always a distance between the resistors R ADJ  and Rs, and hence the temperature changes in the resistors R ADJ  and Rs are different, thereby introducing inaccurate compensation to the current sense signal Ix. Furthermore, the position of the droop resistor R ADJ  in one voltage regulator may be different from that in another, and thereby various resistors with different negative temperature coefficients have to be prepared for the droop resistor R ADJ  in applications of various voltage regulators. 
   SUMMARY OF THE INVENTION 
   One object of the present invention is to provide a temperature compensation device and method for a voltage regulator. 
   Another object of the present invention is to provide a temperature compensation device and method to cancel the temperature coefficient of the load line signal to zero, so as to obtain a temperature-invariant load line. 
   Yet another object of the present invention is to provide a temperature compensation device and method without lengthened conductive wire to avoid noise interferences. 
   Still another object of the present invention is to provide a temperature compensation device and method having tunable temperature coefficient for compensating variations caused by current sense resistor due to temperature changes. 
   In a voltage regulator including an inductor current flowing through a sense element with a first temperature coefficient, and a current sense circuit for generating a current sense signal related to the first temperature coefficient by sensing the inductor current from the sense element, according to the present invention, a temperature compensation device comprises a temperature coefficient tuner to determine a second temperature coefficient according to the first temperature coefficient and a temperature variation, and a compensation signal generator coupled to the temperature coefficient tuner to produce a compensation signal related to the second temperature coefficient to compensate the current sense signal. In one embodiment, the temperature coefficient tuner comprises a thermistor and temperature-invariant resistors to constitute an equivalent resistor with the second temperature coefficient, and the second temperature coefficient is determined by selecting the resistances of the temperature-invariant resistors. In one embodiment of the compensation signal generator, a current source supplies a temperature-invariant current to the temperature coefficient tuner to produce a voltage related to the second temperature coefficient, thereby further generating the compensation signal to compensate variations in the current sense signal caused by the first temperature coefficient. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  schematically shows a conventional voltage regulator with voltage droop function; 
       FIG. 2  schematically shows another conventional voltage regulator with voltage droop function; 
       FIG. 3  schematically shows a voltage regulator according to the present invention; 
       FIG. 4  shows another embodiment for the temperature coefficient tuner in  FIG. 3 ; 
       FIG. 5  schematically shows another voltage regulator according to the present invention; and 
       FIG. 6  schematically shows a voltage regulator with configurable compensation temperature according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   According to the present invention,  FIG. 3  schematically shows a voltage regulator  300 , in which control circuit  302  comprises error amplifier  310  in response to the output voltage Vo of the regulator  300  and reference voltage Vref to produce error signal EA for PWM comparator  312  to compare with ramp signal Vramp to thereby generate PWM signal, and with the PWM signal driver  304  switches transistors  306  and  308  coupled in series between input voltage Vin and ground GND to produce inductor current IL flowing through inductor L to charge output capacitor Co to generate the output voltage Vo. According to the voltage drop across current sense resistor Rs that is coupled to the inductor L, transconductive amplifier  324 , serving as current sense circuit, generates current sense signal Ix 1  supplied to temperature compensation device  305  via sample-and-hold circuit  322 . Owing to the current sense resistor Rs having temperature coefficient TC 1 , the current sense signal Ix 1  will be affected by the temperature coefficient TC 1 . In order to eliminate the influence of the temperature coefficient TC 1  for the control circuit  302  to operate correctly, the temperature compensation device  305  will compensate the current sense signal Ix 1  so that current sense signal Ix 2  invariant to the temperature coefficient TC 1  is obtained to couple to resistor R ADJ  to produce load line droop voltage Vdroop. 
   In the temperature compensation device  305 , temperature coefficient tuner  307  comprises thermistor RTSEN with fixed temperature coefficient, and variable resistors RTJ 1  and RTJ 2  that are temperature invariant. In this embodiment, the thermistor RTSEN is integrated in the chip of the driver  304 . Generally, a chip of the driver  304  has excess pins, and hence the pin count will not increase when the thermistor RTSEN is integrated in the chip of the driver  304 . In addition, the driver  304  is very close to the transistors  306  and  308 , which are dominant heat sources, to the inductor L, as well as to the current sense resistor Rs. Thereby, the difference between the temperature changes in the thermistor RTSEN and current sense resistor Rs is extremely small. Moreover, by integrating the thermistor RTSEN in the chip of the driver  304 , its resistance and temperature coefficient can be made more precisely by using various technologies in the manufacturing process. In the temperature coefficient tuner  307 , the combination of the thermistor RTSEN and resistors RTJ 1  and RTJ 2  constitutes an equivalent resistor with temperature coefficient TC 2 . Those skilled in the art of electronic circuits may know the temperature coefficient is 
                   TC2   =       d   ⁢     {       RTJ1   ×     [     RTJ2   +     R   ⁡     (   T2   )         ]         RTJ1   +     [     RTJ2   +     R   ⁡     (   T2   )         ]         }       dT2       ,           [     EQ   ⁢     -     ⁢   4     ]               
where T 2  is the temperature of the thermistor RTSEN, and R(T 2 ) is the resistance of the thermistor RTSEN at temperature T 2 . In another embodiment, as the temperature coefficient tuner  330  shown in  FIG. 4 , the variable resistor RTJ 1  is coupled in series to the parallel connection of the variable resistor RTJ 2  and thermistor RTSEN, and the temperature coefficient of this equivalent resistor is
 
                 TC2   =         d   ⁡     [     RTJ1   +       RTJ2   ×     R   ⁡     (   T2   )           RTJ2   +     R   ⁡     (   T2   )             ]       dT2     .             [     EQ   ⁢     -     ⁢   5     ]               
From the equation EQ-4 or EQ-5, it is known that the temperature coefficient TC 2  of the equivalent resistor can be tuned by adjusting the resistances of the variable resistors RTJ 1  and RTJ 2 . One skilled in the art knows that in order to compensate voltage changes caused by the temperature coefficient TC 1  of the current sense resistor Rs, it should be satisfied that
   TC 2×Δ T 2=Δ T 1× TC 1,  [EQ-6] 
where ΔT 1  is the temperature change on the current sense resistor Rs, and ΔT 2  is the temperature change on the thermistor RTSEN. According to the equation EQ-6, the temperature coefficient required for the equivalent resistor in the temperature coefficient tuner  307  can be derived as
 
                 TC2   =         Δ   ⁢           ⁢   T1   ×   TC1       Δ   ⁢           ⁢   T2       .             [     EQ   ⁢     -     ⁢   7     ]               
In the regard of current technology, it is not difficult to obtain the temperature changes on the current sense resistor Rs and the thermistor RTSEN. Referring to  FIG. 3 , current source  316  in the compensation signal generator  309  supplies temperature-invariant current Iz to the temperature coefficient tuner  307 , and accordingly the equivalent resistor composed of the resistors RTSEN, RTJ 1  and RTJ 2  will produce the voltage drop
   VTSEN=Iz×R   TT ( TC 2),  [EQ-8] 
where R TT (TC 2 ) is the resistance of the equivalent resistor. By the equation EQ-8, it is shown that the voltage VTSEN is also dependent on the temperature coefficient TC 2 . The non-inverting input of buffer  320  is coupled with the voltage VTSEN, and according to virtual ground, the voltage on the inverting input of the buffer  320  is equal to the voltage VTSEN. Thereby, passing through temperature-invariant resistor RTC, the current serving as the compensation signal is
 
                 ITC   =       VTSEN   RTC     =         Iz   ×       R   TT     ⁡     (   TC2   )         RTC     .               [     EQ   ⁢     -     ⁢   9     ]               
By the equation EQ-9, it is known that the current ITC is dependent on the temperature coefficient TC 2 . Operational circuit  314  divides the current sense signal Ix 1  by the current ITC to result in current sense signal Ix 2  to compensate the influence of the temperature coefficient TC 1  on the current sense signal Ix 1 .
 
   In  FIG. 3 , the temperature compensation device  305  makes use of the variable resistors RTJ 1  and RTJ 2  and the thermistor RTSEN to form an equivalent resistor, and tunes the temperature coefficient of the equivalent resistor by adjusting the resistance of the variable resistors RTJ 1  and RTJ 2 . Consequently, it is not necessary to prepare resistors with various temperature coefficients for various voltage regulator applications, and the temperature coefficient of the thermistor RTSEN can be positive. Thus, the cost can be reduced effectively without the need to use extraordinary resistors with negative temperature coefficients. Furthermore, it is not necessary for the resistor R ADJ  in  FIG. 3  to lengthen the conductive wire for arranging the resistor R ADJ  close to the current sense resistor Rs as the conventional voltage regulator  200  shown in  FIG. 2 . Hence, noise interference caused by lengthened conductive wire can be prevented. 
   In the temperature compensation device  305 , the current sense signal Ix 1  is divided by the current ITC to obtain the current sense signal Ix 2  in order to compensate the influence of the temperature coefficient TC 1  on the current sense signal Ix 1 . Nevertheless, the compensation function can also be achieved by applying other operational methods. As shown in  FIG. 5 , likewise, voltage regulator  400  comprises control circuit  302 , driver  304 , transistors  306  and  308 , error amplifier  310 , PWM comparator  312 , transconductive amplifier  324 , and resistors Rs and R ADJ . In temperature compensation device  402 , likewise, current source  316 , transistor M, buffer  320 , and resistors RTSEN, RTJ 1 , RTJ 2 , and RTC are included. However, in this embodiment, the current sense signal Ix 1  is mirrored by mirror circuit  408 , and two identical current sense signals Ix 1  are produced accordingly, for providing for operational circuits  404  and  406 . The current sense signal Ix 1  supplied to the operational circuit  406  is multiplied by current ITC and divided by temperature-invariant reference current Iref to obtain current signal
 
 I× 3 =I× 1 ×ITC÷Iref   [EQ-10]
 
In the operational circuit  404 , the other current sense signal Ix 1  minus the signal Ix 3  to obtain the current sense signal Ix 2  for compensating variations on the current sense signal Ix 1  caused by the temperature coefficient TC 1 .
 
   Moreover, the temperature compensation device according to the present invention can also be configured with threshold temperature for compensation.  FIG. 6  shows voltage regulator  500  with configurable compensation temperature according to the present invention, which also comprises control circuit  302 , driver  304 , transistors  306  and  308 , error amplifier  310 , PWM comparator  312 , sample-and-hold circuit  322 , transconductive amplifier  324 , and resistors Rs and R ADJ . In temperature compensation device  502 , temperature coefficient tuner  307  also includes resistors RTSEN, RTJ 1 , and RTJ 2  to form an equivalent resistor with temperature coefficient TC 2 . Compensation signal generator  309  supplies current Iz provided by current source  316  to the temperature coefficient tuner  307  to produce voltage VTSEN, and temperature-invariant current Iz 1  provided by current source  504  to temperature-invariant resistor RTSET to produce threshold voltage VTSET. Analog summing circuit  506  has a positive input coupled with the voltage VTSEN and a negative input coupled with the threshold voltage VTSET. When the temperature of the thermistor RTSEN increases, the voltage VTSEN will increase as well. As the temperature of the thermistor RTSEN increases to a predetermined threshold, the voltage VTSEN will be greater or equal to the threshold voltage VTSET, such that the summing circuit  506  produces output
 
 Vcomp=VTSEN−VTSET,   [EQ-11]
 
to the non-inverting input of buffer  320 . Due to virtual ground, the voltage on the inverting input of the buffer  320  is equal to Vcomp, and thereby determining the current passing through resistor RTC for the compensation signal
 
                 ITC   =         V   ⁢           ⁢   comp     RTC     .             [     EQ   ⁢     -     ⁢   12     ]               
Operational circuit  314  receives the current ITC through transistor M, and divides the current sense signal Ix 1  by the current ITC to determine the current sense signal Ix 2  for compensating variations caused by the temperature coefficient TC 1  of the current sense resistor Rs.
 
   In the foregoing embodiments, the resistor R ADJ  is coupled between the current sense signal Ix 2  and reference voltage Vref. Alternatively, the resistor R ADJ  can be coupled between the current sense signal Ix 2  and output Vo of the regulator, as the conventional regulator  100  shown in  FIG. 1  does. In addition, although the method for sensing the inductor current IL in the foregoing embodiments is sensing the voltage drop across the current sense resistor Rs coupled in series to the inductor L, other methods, such as sensing the voltage drop across the parasitic resistor of the inductor L, sensing the voltage drop across the resistor coupled in series to the transistor  308 , and sensing the voltage drop across the transistor  308 , can also be adopted. Single-phase voltage regulator is used in the foregoing embodiments to illustrate the principles of the present invention. In multi-phase voltage regulators, the described principles in the foregoing single-phase voltage regulators can be applied to implement as well. 
   While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.

Technology Classification (CPC): 8