Abstract:
A switching regulator that has first, second, third and fourth terminals, a first power transistor disposed between the first terminal and a first node, a second power transistor disposed between the first node and a second node, a filter including a capacitor and an inductor, and a controller. The first power transistor is partitioned into a plurality of individually-addressable first transistor segments. The second node couples the second and fourth terminals. The second power transistor is partitioned into a plurality of individually-addressable second transistor segments. The inductor is disposed between the first node and the third terminal, and the capacitor is disposed between the third and fourth terminals. The controller is operable in a plurality of modes including a normal mode in which the controller opens and closes all of the first transistor segments and all of the second transistor segments, and a test mode in which the controller opens and closes less than all of the first transistor segments and all of the second transistor segments.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to provisional U.S. Application Serial No. 60/218,433, filed on Jul. 14, 2000. 
     
    
     
       BACKGROUND  
         [0002]    The present invention relates to the design and testing of power management integrated circuits.  
           [0003]    Power amplifiers, low drop-out regulators and voltage regulators are examples of power management integrated circuits that include power transistors. Voltage regulators, such as DC-to-DC converters, are used to provide stable voltage sources for electronic systems. Efficient DC-to-DC converters are particularly needed for battery management in low power devices, such as laptop computers and mobile phones. Switching voltage regulators (or simply “switching regulators”) are known to be an efficient type of DC-to-DC converter. A switching regulator generates an output voltage by converting an input DC voltage into a high frequency voltage, and filtering the high frequency voltage to generate the output DC voltage.  
           [0004]    Conventional synchronous switching regulators include two switches implemented with power transistors. One power transistor is used to alternately couple and decouple an unregulated input DC voltage source, such as a battery, to a load, such as an integrated circuit. The other power transistor is used to alternately couple and decouple the load to ground. An output filter, typically including an inductor and an output capacitor, is coupled between the input voltage source and the load to filter the output of the switches and produce the output DC voltage. The two power transistors are typically controlled by a pulse modulator, such as a pulse width modulator or a pulse frequency modulator.  
           [0005]    Performance of a switching regulator is generally affected by the characteristics of the power transistors. When a power transistor is open, it is desirable to-have the power transistor operate as close to an open circuit as possible, i.e., the off-resistance of the power transistor should be very high (ideally infinite). In the closed position, the power transistor should act as a close to a short circuit as possible, i.e., the on-resistance of the power transistor should be very small (ideally zero). A high on-resistance generally results in increased power dissipation in the power transistor, degrading the efficiency of the regulator.  
           [0006]    One of the challenges in the design of switching regulators with on-chip power transistors is the testability of the device during manufacturing. As just discussed, it is desireable to minimize the on-resistance of the power transistors. Unfortunately, it is difficult to obtain an accurate measurement reading of a low on-resistance value, e.g., tens of mΩ or lower, because the package lead resistance, the socket resistance or the contact resistance of a probe during a wafer probe test or the automatic test equipment (ATE) contactor&#39;s contact resistance can be of the order of the on-resistance value. To create a voltage drop across the power transistor that is measurable at a required degree of accuracy, a substantially large DC current has to be provided to the power transistor. Recent automatic test equipment (ATE) testers that are designed to handle complex mixed-signal IC testing are typically not equipped with integrated instrumentation to measure complex mixed-signal circuits and support high current conduction simultaneously. In addition, the conduction of a high current through wafer probe tips tends to wear out the hardware very quickly, resulting in production delays and increased testing cost.  
           [0007]    Furthermore, to test the closed-loop performance of the regulator using ATE, large currents must be conducted through the ATE, the test probes, and their associated parasitic inductance. Since the parasitic inductance introduced during ATE testing may be an order of magnitude larger than that in the typical application for the regulator, large transient voltages are created on-chip when the power transistors switch, leading to measurement inaccuracies and potentially even permanent damage to the chip.  
         SUMMARY  
         [0008]    In one aspect, the invention is directed to a switching regulator having first, second, third and fourth terminals, a first power transistor disposed between the first terminal and a first node, a second power transistor disposed between the first node and a second node, a filter including a capacitor and an inductor, and a controller. The first power transistor is partitioned into a plurality of individually-addressable first transistor segments. The second node couples the second and fourth terminals. The second power transistor is partitioned into a plurality of individually-addressable second transistor segments. The inductor is disposed between the first node and the third terminal, and the capacitor is disposed between the third and fourth terminals. The controller is operable in a plurality of modes including a normal mode in which the controller opens and closes all of the first transistor segments and all of the second transistor segments, and a test mode in which the controller opens and closes less than all of the first transistor segments and all of the second transistor segments.  
           [0009]    Implementations of the invention may include one or more of the following features.  
           [0010]    Each first transistor segment may have a source coupled to the first terminal, a drain coupled to the first node and a gate coupled to the controller through a segment control line. Each second transistor segment may have a source coupled to the first node, a drain coupled to the second node and a gate coupled to the controller through a segment control line. The controller may operate in the normal mode in response to a substantially constant load. The controller may be configured to switch to the test mode in response to a request to measure an on-resistance of a power transistor. The first power transistor may be a p-channel MOSFET and the second power transistor may be an n-channel MOSFET. All of the segments may have an equivalent transistor width.  
           [0011]    In another aspect, the invention is directed to a method for measuring an on-resistance of a power transistor integrated onto an integrated circuit chip. The power transistor includes a plurality of individually-addressable transistor segments. Less than all of the transistor segments are closed, an on-resistance of the closed transistor segments is measured, and an on-resistance of the power transistor is derived from the on-resistances of the transistor segments.  
           [0012]    Implementations of the invention may include one or more of the following features.  
           [0013]    The transistor segments may be closed one at a time, an on-resistance of each closed transistor segment may be measured; and an on-resistance of the power transistor may be derived by averaging the on-resistances of all of the transistor segments. The transistor segments may have an equivalent width. Each transistor segment may include one or more single transistors connected in parallel.  
           [0014]    In another aspect, the invention is directed to a method of testing a switching regulator with a power transistor on an integrated circuit chip for use with an application board having circuitry that includes a first inductor with a first inductance and a first capacitor with a first capacitance. In the method, a power transistor including a plurality of individually-addressable transistor segments on a chip is provided. The chip is installed on a testing board having circuitry that includes a second inductor with a second inductance greater than the first inductance and a first capacitor with a second capacitance less than the first capacitance. The circuit is operated with the power transistor on the integrated circuit chip using less than all of the transistor segments, and a closed-loop performance characteristic of the switching regulator is measured.  
           [0015]    Implementations of the invention may include one or more of the following features. The performance characteristic may be output voltage, line regulation or load regulation. Measuring the line regulation may include measuring first and second output voltages with different input voltages. Measuring the load regulation may include measuring first and second output voltages with different load currents, e.g., a minimum load current and a modified maximum load current (that is less than a maximum load current for the switching regulator when installed on an application board). The application board may have a first load current, and the test board may have a second load current that is less than the first load current. The chip may be installed on an application board having circuitry that includes the first inductor with the first inductance and the first capacitor with the first capacitance. The power transistor may include N transistor segments, and the operating step may be performed using exactly one of the N transistor segments. The first inductance may be L and the second inductance may be L*N. The first capacitance may be C and the second capacitance may be C/N. An on-resistance of a closed transistor segment may be measured during operation of the circuitry, and an on-resistance of the power transistor may be derived from the on-resistance of the closed transistor segment.  
           [0016]    Advantages that can be seen in implementations of the invention include one or more of the following. The resistance of the power transistors in a switching regulator can be measured accurately. The resistance of the power transistors can be controlled more accurately, leading to more precisely control the switching timing. Switching-related losses can be reduced by decreasing the on-resistance of the power transistors.  
           [0017]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will become apparent from the description, the drawings, and the claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a schematic diagram of a switching regulator.  
         [0019]    [0019]FIG. 2 is a circuit diagram illustrating a power transistor from the switching regulator of FIG. 1. 
     
    
       [0020]    Like reference numbers and designations in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0021]    In a two-switch buck-converter circuit  100  shown in FIG. 1, a switching regulator  102  is coupled to a DC input voltage source  104 , such as a battery, by an input terminal  106 . The switching regulator  102  is also coupled to a load  108 , such as an integrated circuit, by an output terminal  110 . The switching regulator  102  serves as a DC-to-DC converter between the input terminal  106  and the output terminal  110 .  
         [0022]    The switching regulator  102  includes a power switch  112  and a rectifier switch  116 . The power switch  112  is a N-segment p-channel power MOSFET. (“PMOS power transistor”) which alternately couples and decouples the input terminal  106  to an intermediate node  114 ; the rectifier switch  116  is a N-segment n-channel power MOSFET (“NMOS power transistor”) which alternately couples and decouples the intermediate node  114  to ground  118 . The value of N—i.e., the number of segments—can be selected by a circuit designer to achieve a desired measurement accuracy (described below). A filter is configured to convert the rectangular waveform of the intermediate voltage V X  at the intermediate terminal  114  into a substantially DC output voltage V out  at the load  108 . In one implementation, the filter includes an inductor  120  and an output capacitor  122 .  
         [0023]    An implementation of the rectifier switch  116  and its associated driver circuitry is shown in FIG. 2. Each segment  200  of the rectifier switch  116  includes an NMOS transistor  204 . Although each segment Sn(i) is represented in FIG. 2 by a single NMOS transistor  204 , where i=1 . . . N, integer, it should be noted that each segment Sn(i) can be implemented by multiple single NMOS transistors connected in parallel. The segments Sn(i) are coupled such that: (a) all of the segment drains are connected to the intermediate node  114 ; (b) all of the segment sources are connected to ground  118 ; and (c) each of the segment gates are isolated and driven by a separate inverter driver  206 . The input of each inverter driver  206  is coupled to an NAND gate  208 . The power switch  112  can be similarly implemented.  
         [0024]    The switching regulator  102  includes a controller  124  for controlling the operation of the power switch  112  and the rectifier switch  116 . The controller  124  can be implemented using circuitry, software, or a combination of both. One input of each NAND gate  208  is connected by a common control line  126  to the controller, whereas the other input of each NAND gate  208  is connected to the controller by an individual control line  128 . Each segment Sn(i) of the rectifier switch  116  is coupled to the first control line  126  by a sub-control line  202 . The controller  124  controls the switching of each segment Sn(i) in the rectifier switch  116  by applying a common control signal Ctrl(NMOS_switch) to the first control line  126  and an individual control signal Ctrln(i) to each second control line  128 . Thus, a transistor  204  in a specific segment turns on only when both the common control signal Ctrl(NMOS_switch) and the associated individual control Ctrln(i) are high. The switching of each segment Sp(i) in the power switch  112  can be similarly controlled by the controller  124  by applying a common control signal Ctrl(PMOS switch) and an individual control signal Ctrlp(i).  
         [0025]    In a normal mode of operation, the controller  124  alternately opens and closes all of the segments in the power switch  112  and the rectifier switch  116 , such that an intermediate voltage V X  having a rectangular waveform is generated at the intermediate terminal  114 . For example, the controller  124  sets Ctrln( 1 , . . . ,N)=1 and then alternately switches between Ctrl(NMOS_switch)=1 and Ctrl(NMOS_switch)=0 to alternately turn on all of the segments Sn(i) in the rectifier switch  116  simultaneously and then turn off all of the segments Sn(i) in the rectifier switch  116  simultaneously. Again, the power transistor  112  can be controlled similarly.  
         [0026]    The switching regulator  102  can be fabricated such that the power switch  112 , rectifier switch  116 , and load  108  are located on-chip (i.e., on a single chip), and the controller  124 , inductor  120  and output capacitor  122  are discrete component located off-chip.  
         [0027]    The performance of the switching regulator  102  is affected in part by the characteristics of the power switch  112  and the rectifier switch  116 . For example, to achieve high power conversion efficiency in the switching regulator  102 , the on-resistance Rdson total  of each switch should be low to minimize resistive conduction losses. The switching regulator can be operated in a segmentation measurement test mode so that the on-resistance Rdson total [measured] of each switch can be measured. For example, assume the rectifier switch  16  (having a transistor width W ntotal ) is partitioned into N segments, each having an equivalent segment width W ni . The ideal on-resistance Rdson ni  [ideal] of each segment Sn(i) is defined by:  
           Rdson   ni [ideal]= N*Rdson   s [ideal] 
         [0028]    In the segmentation measurement test mode, the following steps are performed to measure the on-resistance Rdson ni  [measured] of each segment Sn(i) of the rectifier switch  116 :  
         [0029]    1. Close a segment. Sn(i) (e.g., to close segment Sn( 2 ) exclusively, the controller  124  sets Ctrl(NMOS_switch)=1, Ctrln( 2 )=1, and Ctrln( 1 , 3 , 4  . . . ,N)=0).  
         [0030]    2. Apply a current I ntest  to the closed segment Sn(i).  
         [0031]    3. Measure a voltage drop V ni  [measured] across the closed segment Sn(i).  
         [0032]    4. Calculate the on-resistance Rdson ni  [measured] of the closed segment Sn(i):  
           Rdson   ni [measured]= V   ni [measured]/ I   ntest    
         [0033]     where V ni  [measured] is the measured voltage drop across the closed segment Sn(i) and I ntest  is the amount of DC current applied to the closed segment Sn(i).  
         [0034]    In practice, when making a measurement Rdson s [measured] of the resistance of the switch with the test equipment,  
           Rdson   s [measured]= Rdson   s [actual]+ R [parasitic] 
         [0035]    where Rdson s  [actual] is the on-resistance of the switch (with all segments closed), and Rdson [parasitic] is the unwanted resistance due to wafer probe contact and automatic test equipment (ATE) contact resistance. However, when making a measurement Rdson ni [measured] of the resistance of a single segment in the switch with the test equipment, it is also generally the case that  
           Rdson   ni [measured]= Rdson   ni [actual]+ R [parasitic] 
         [0036]    where Rdson ni  [actual] is the on-resistance of the segment based on the segment dimensions, and R[parasitic] is the unwanted resistance due to wafer probe contact and automatic test equipment (ATE) contact resistance.  
         [0037]    Once the on-resistance Rdson ni  [measured] of all of the segments Sn(i) of the rectifier switch  116  have been obtained, the average on-resistance Rdson ni [measured]{avg} of the rectifier switch  116  can be calculated:  
           Rdson   ni [measured]{avg}=( Rdson   nN [measured]+ . . . +Rdson n1 [measured])/ N    
         [0038]    We know that  
           Rdson   ni [measured]{avg}= Rdson   ni [ideal]{avg}+ R   i [parasitic]{ avg}   
         [0039]    where  
           Rdson   ni [ideal]{avg}=( Rdson   nN [ideal]+ . . . + Rdson   ni [ideal])/ N    
         and  
           R   ni [parasitic]{avg}=( R   N [parasitic]+ . . . + R   1 [parasitic])/ N    
         [0040]    The on-resistance Rdson s [calculated] of the rectifier switch  116  can be calculated from the following:  
         Rdson s [calculated]=Rdson ni [measured]{avg}/ N    
         [0041]    Since  
           Rdson   ni [measured]{avg}/ N=[Rdson   ni [ideal]{avg}+R ni [parasitic]{avg}]/ N=Rdson   s [ideal]+[ R   ni [parasitic]/N] 
         [0042]    it follows that  
           Rdson   s [calculated]≅ Rdson   s [ideal]; for large N  
         [0043]    The value of N can be selected to achieve a desired measurement accuracy of the voltage drops V ni [measured] across the respective closed segments Sn(i). Once the value of the low on-resistance Rdson ntotal  [measured] of the rectifier switch  116  is obtained, a comparison of the measured on-resistance Rdson ntotal  [measured] and the ideal on-resistance Rdson ntotal  [ideal] can be made. If, Rdson ntotal  [measured]&gt;&gt;Rdson ntotal  [ideal], then a failure analysis of the rectifier switch  116  can be performed by analyzing each segment&#39;s on-resistance Rdson ni  [measured]. In this manner, segments that failed—i.e., segments Sn(i) having Rdson ni [measured]&gt;&gt;Rdson ni [ideal]—can be easily identified. The on-resistance Rdson ptotal [measured] of the power switch  112  can be similarly calculated.  
         [0044]    In high output current applications, the performance of the switching regulator  102  can be evaluated using a closed-loop verification technique. In order to avoid the practical problems introduced by ATE contact inductances (e.g., large transient noise across the switching regulator  102 ), the switching regulator  102  can be placed in a low output current test mode of operation. The closed-loop characteristics of the switching regulator  102  can be easily verified as long as the loop gain and opened-loop pole locations are kept the same. In one implementation, a switching regulator  102  in the low output current test mode is configured as follows:  
         [0045]    1. Partition each switch into N segments having equivalent segment widths W i .  
         [0046]    2. Close one of the N segments in each switch.  
         [0047]    3. Increase the value of the output filter inductor  120  by N times (i.e., replace the discrete inductor component  120  having an inductance of L with a discrete inductor component  120 ′ having an inductance of N*L). This will also serve to reduce AC ripple current passing through the regulator by the factor N.  
         [0048]    4. Reduce the value of the output filter capacitor  122  by N times (i.e., replace the discrete capacitor component  122  having a capacitance of C with a discrete capacitance component  122 ′ having a capacitance of C/N).  
         [0049]    5. Reduce the tested load current range from 0-Imax to 0-Imax/N (Imax is the maximum load current for the switching regulator when using an application board).  
         [0050]    Specifically, a chip with the switching regulator  102  can be tested by installing the chip with the power switches  112  and  116  on a test board that is identical to the application board, except for having a discrete inductor component  120 ′ with an inductance of N*L (where L is the inductance of the discrete inductor component  120  on the application board) and a discrete capacitor component  122 ′ with an inductance of C/N (where C is the capacitance of the discrete capacitor component  122  on the application board) and a maximum load current of Imax/N (where Imax is the maximum load current on the application board). The switching regulator  102  is then run using just one (or less than all) of the N segments. For example, in power switch  112 , just one of the individual control signals Ctrlp(i) is set high, the remainder of the individual control signals Ctrlp(i) are set low, and the active segment is controlled by the common control signal Ctrl(PMOS switch). The rectifier switch  116  can be controlled similarly. While the switching regulator is running, the automatic testing equipment can be used to test the voltage at various points on the circuit and perform a failure analysis. Assuming that the test is successful, the chip with the power switches  112  and  116  is then installed on an application board with the discrete inductor component  120  and discrete capacitor component  122 .  
         [0051]    As part of the testing, the closed-loop performance characteristic of the switching regulator can be measured. Two exemplary performance characteristics are the line regulation and the load regulator. To measure the line regulation, two measurements of the output voltage are performed. The first measurement is performed with the minimum input voltage, and the second measurement is performed with the maximum input voltage. The deviation between the two measurements provides an estimate of the line regulation when the chip is installed on the application board. To measure the load regulation, another two measurements of the output voltage are performed. The first measurement is performed with the minimum load current, e.g., zero, and the second measurement is performed with the maximum load current, e.g., Imax/N. The deviation between the two measurements provides an estimate of the load regulation when the chip is installed on the application board.  
         [0052]    In this manner, the combination of (3) and (5) reduces the maximum current passed through the regulator by N times, the combination of (1), (3), and (5) reduces the voltage overstress by approximately N times, and the combination of (1), (3) and (4) keeps the loop gain and opened-loop dominant pole locations nearly similar to those in the application circuit. By using a fraction of each switch and adjusting the values of the external discrete output filter inductor and capacitor components, the closed-loop characteristics of the switching regulator  102  can be verified.  
         [0053]    In addition, the general functionality of the various circuit blocks within the feedback control loop can be verified without having to measure each of the individual on-chip circuit blocks separately. This results in production test-time reduction and ultimately lower manufacturing costs.  
         [0054]    The invention has been described in terms of particular embodiments. Other embodiments are within the scope of the following claims. For example, the steps of the invention can be performed in a different order and still achieve desirable results.