Synchronous regulator with switchable current reference source

[0002] The present invention relates in general to integrated circuits and, more particularly, to up-down voltage converters integrated on a semiconductor substrate.

[0003] Many applications use voltage converters to convert an input voltage at one value to either a higher or lower valued output voltage as needed by a load circuit. For example, portable smart card readers typically use a battery to supply a typical input voltage of about 3.6 volts. A voltage converter in the card reader uses the battery voltage to generate a transformer or coil current that is used to develop an output voltage at either three volts or five volts, depending on the type of smart card which is inserted into the reader. Most smart card readers maintain a low fabrication cost by using a single up-down voltage converter to step the battery voltage to five volts or down to three volts as needed.

[0004] Previous up-down voltage converters use a four transistor bridge arrangement to switch the coil current to develop the output voltage. However, this arrangement routes the coil current serially through two of the transistors, which results in power being dissipated by the resistance of two conducting transistors. To avoid an excessive power loss, larger transistors with lower resistances are used, but the large transistors occupy a large die area, which therefore increases the cost of manufacturing the voltage converters.

[0005] Hence, there is a need for an up-down voltage converter and method of converting a voltage that uses fewer transistors in order to reduce the cost and increase the power efficiency of the voltage converter.

[0006] FIG. 1 is a block diagram of a portable smart card reader using a voltage converter to generate an output voltage;

[0007] FIG. 2 is a schematic diagram showing further detail of the voltage converter;

[0008] FIG. 3 is a timing diagram showing a waveform of the coil current generated by the voltage converter; and

[0009] FIG. 4 is a schematic diagram showing the voltage converter in an alternate embodiment.

[0010] In the figures, elements having the same reference number have similar functionality.

[0011] FIG. 1 is a schematic diagram of a portable smart card reader 10 operating with a load 106 that includes an inserted smart card, including a battery 102 and a direct current (DC) to DC voltage converter 104. In one embodiment, battery 102 provides a battery supply voltage V_BATToperating at a voltage of about 3.6 volts.

[0012] DC to DC converter 104 converts battery supply voltage V_BATTreceived at an input node 200 to an output supply voltage V_OUTat an output node 202. The circuitry designated as load 106 includes a smart card that may be either of two types, i.e., a three volt smart card or a five volt smart card, referring to the supply voltage specified for operating each type of smart card. The type of smart card is detected by system 10 and the corresponding value of VOUT is provided by converter 104. Hence, if a three volt smart card is inserted, converter 104 operates in a step down mode to convert V_BATT=3.6 volts to a lower output supply voltage V_OUT=3.0 volts. If a five volt smart card is inserted, converter 104 operates in a step up mode to convert V_BATT=3.6 volts to a higher output supply voltage V_OUT=5.0 volts.

[0013] FIG. 2 is a schematic diagram showing DC to DC converter 104 in further detail, including an inductor or coil L₁, a capacitor C₁, transistor switches S₁and S₂, comparators 220, 230 and 246, current sources 242 and 244, logic blocks 221 and 250 and resistors 251 and 252. Battery supply voltage V_BATTis received at node 200 and output supply voltage V_OUTis produced at node 202 to have a value of either 3.0 volts or 5.0 volts, depending on the type of smart card. V_OUTsupplies a load current I_LOADof about fifty milliamperes to operate a smart card included in load 106. Except for coil L₁and capacitor C₁, the components shown in FIG. 2 are formed on a single semiconductor die as an integrated circuit. In one embodiment, coil L₁has a value of about twenty-two microhenries and capacitor C₁has a value of about ten microfarads.

[0014] Switch S₁is a standard high current n-channel metal oxide semiconductor field effect transistor (MOSFET) configured to switch a coil current I_COILthrough coil L₁at a level of at least one hundred fifty milliamperes. Switch S₂is a standard high current p-channel MOSFET also capable of supplying coil current I_COILat a level of at least one hundred fifty milliamperes.

[0015] Comparator 220 monitors output supply voltage V_OUTfor comparing with a reference voltage V_REFto provide voltage regulation. A digital enabling signal EN is produced on a node 223 with a logic high value to activate switch S₁and/or switch S₂through logic blocks 221 and 250 when V_OUTis below its predetermined regulation value of either three or five volts. Comparator 220 is referred to as a hysteretic comparator since its inputs typically are configured to have about one hundred millivolts of hysteresis to avoid excessive switching when V_OUTis near its target regulation value.

[0016] Logic block 221 has an input at a node 225 for receiving an ON/OFF enabling signal that activates DC to DC converter 104 when a smart card is inserted in the reader. When no smart card is present, ON/OFF initiates a standby mode which deactivates portions of converter 104 to reduce standby power consumption. Hence, ON/OFF has the effect of disabling switches S₁and S₂during the standby mode. An output provides an enabling signal ENABLE on a node 262.

[0017] Comparator 230 compares output supply voltage V_OUTwith a reference voltage (V_BATT−ΔV) to determine whether converter 104 is operating in an up- or down-conversion mode. The advantages of setting the reference at a few hundred millivolts below V_BATTis described in detail below. In one embodiment, ΔV has a value of about two hundred millivolts. The comparison produces a digital mode signal UP/DOWN on a node 232.

[0018] Current sources 242 and 244 supply reference currents I_REF1and I_REF2, respectively, that flow through resistor 252 to battery supply voltage V_BATTand establish a reference potential V₂₀₇on a node 207.

[0019] Resistors 251-252 are matched or scaled to provide resistances R₂₅₁and R₂₅₂, respectively, in a ratio that is constant over a specified range of operating, processing and environmental conditions. In one embodiment, R₂₅₁is about two ohms and R₂₅₂is about two kilohms to provide a resistance ratio R₂₅₂/R₂₅₁of about 1000:1.

[0020] Comparator 246, in combination with resistors 251-252, current sources 242 and 244 and a switch 245, essentially functions as a current sensor 239. An input coupled to a node 206 receives a voltage V₂₀₆resulting from the flow of coil current I_COILthrough resistor 251. Another input receives reference potential V₂₀₇for comparing with voltage V₂₀₆to produce a digital phase signal CHG/DSCHG at an output on a node 249 to control whether coil current I_COILis charging or discharging, i.e., increasing or decreasing. When I_COILis charging, CHG/DSCHG is logic high to close switch 245 so that V₂₀₇=V_CHG=V_BATT−R₂₅₂*(I_REF1+I_REF2). When I_COILis discharging, CHG/DSCHG is logic low to open switch 245 so that V₂₀₇=V_DSCHG=V_BATT−R₂₅₂*I_REF1. In effect, V_CHGfunctions to set a maximum or upper current I_PEAK=(R₂₅₂/R₂₅₁)*(I_REF1+I_REF2) for I_COIL, while V_DSCHGfunctions to set a minimum or lower current limit I_VALLEY=(R₂₅₂/R₂₅₁)*I_REF1for I_COIL.

[0021] Logic block 250 includes a combinational logic circuit whose inputs receive ENABLE, UP/DOWN and CHG/DSCHG as control signals and whose outputs at nodes 256 and 254 produce drive signals VS1 and VS2 for driving the gates of switches S₁and S₂, respectively. A truth table for logic block 250 is shown in Table I below, where H indicates a logic high level, L indicated a logic low, and X indicates a “don't care” state, i.e., either H or L. Note that logic block 250 includes level shifting circuitry to provide logic high values for VS1 and VS2 at the potential of either V_BATTor V_OUT, as described in detail below. Logic low operates at ground potential.

[0022] The detailed operation of converter 104 can be understood by referring to FIG. 3, which shows a waveform of coil current I_COILversus time over several cycles of operation. First, the down conversion mode is described as follows, assuming that V_BATT=3.6 volts and V_OUT=3.0 volts.

[0023] Assume further that initially, at time T0, output supply voltage V_OUTis zero and coil current I_COILis zero. The logic values for ENABLE, UP/DOWN and CHG/DSCHG are represented in Table 1 as state I. CGH/DSCHG is logic high to close switch 245 to sum reference currents I_REF1and I_REF2at node 207. VS1 is logic low so switch S₁is off. VS2 is low, so switch S₂is turned on to form a conduction path between a node 208 and output node 202. The initial voltage across coil L₁is (V_BATT−V_OUT), which is nearly equal to V_BATT. Coil current I_COILincreases with time to charge or store magnetic energy in coil L₁, and is routed through S₂to output node 202. This period when I_COILis charging or increasing is designated as the charging phase of operation. Hence, during the charging phase, I_COILflows to capacitor C₁to begin to develop output supply voltage V_OUT. For typical values of C₁, V_OUTincreases a few hundred millivolts during the first cycle.

[0024] At time T1, I_COILincreases to its maximum limit value I_PEAK, which causes comparator 246 to switch CHG/DSCHG to a logic low value as shown in state II of Table I. In one embodiment, I_PEAKhas a value of about one hundred fifty milliamperes. Switch 245 is opened so only reference current I_REF1flows through resistor 252, setting potential V₂₀₇to a value representative of minimum current level I_VALLEY. VS2 is set to a logic high value of V_BATT, which allows the potential of node 208 to rise until switch S₂turns on to discharge I_COILinto capacitor C₁to further develop the potential of V_OUT. This period during which I_COILis decaying or decreasing is designated as the discharging phase of I_COIL.

[0025] At time T2, I_COILdecays to a value equal to I_VALLEY, causing comparator 246 to produce CHG/DSCHG with a logic high value and VS2 with a logic low value to begin another cycle indicated in Table I as state I. Note that switch S₁remains off during both the charging and discharging phases of the above described cycle. Note further that switch S₂routes I_COILto charge capacitor C₁during both the charging and discharging phases of I_COIL, i.e., when I_COILis both increasing and decaying. This scheme allows V_OUTto be developed in fewer cycles and, by switching I_COILonly through a single device S₂, converter 104 functions with a low parts count and fabrication cost as well as a high power efficiency. The period from T0 to T2 to complete one cycle typically ranges from about two to ten microseconds, or a frequency between one hundred kilohertz and five hundred kilohertz. It is considered desirable to maintain the period within this time range in order to control electromagnetic interference and to maintain tighter regulation of V_OUT.

[0026] When converter 104 is operating as described above as a step down voltage converter in which the target value of V_OUTis less than V_BATT, the cycles continue to alternate between state I and state II as described until capacitor C₁has accumulated sufficient charge from I_COILto reach the V_OUTtarget value of 3.0 volts. At that point, logic block 221 generates a logic low level for ENABLE, which switches VS2 to a logic high V_BATTpotential, allowing I_COILto decay to zero and deactivating switches S₁and S₂. This idle condition is shown in Table I as state III. Converter 104 remains in this idle state until V_OUTdecays by the amount of hysteresis in comparator 220, when ENABLE is again set logic high to begin a new cycle.

[0027] Now assume converter 104 is operating in a step up converter mode in which 5.0 volts is the target value of V_OUTwhich is higher than V_BATT=3.6 volts. ENABLE is high and operation proceeds as described above until V_OUTis within ΔV volts of V_BATT. At this point, the switching frequency of converter 104 decreases to about one hundred kilohertz because of the low potential (V_BATT−V_OUT) across coil L₁. As V_OUTreaches the value of reference voltage (V_BATT−ΔV), comparator 230 produces UP/DOWN with a logic high value to switch the operation to the step up or up conversion mode shown as state IV in Table I. In one embodiment, ΔV has a value of about two hundred millivolts, so when V_BATT=3.6 volts, reference voltage (V_BATT−ΔV)=3.4 volts approximately. The operation proceeds as follows.

[0028] The charging phase, when I_COILis increasing, occurs by setting VS1 to a logic high to turn on switch S1. This mode of operation allows I_COILto increase more rapidly because the voltage across coil L₁is essentially V_OUT, rather than V_OUT−V_BATT. This technique maintains the operating frequency of converter 104 above at least one hundred kilohertz. I_COILincreases until reaching the value of I_PEAK, at which time comparator 246 generates CHG/DSCH with a logic low, VS1 undergoes a high to low transition to turn off S₁, and VS2 is made logic low, turning on switch S₂to discharge I_COILinto capacitor C₁, as shown as state V in Table I. When I_COILdecays to the value of I_VALLEY, converter 104 returns to state IV of Table I. Converter 104 alternates between states IV and V until V_OUTincreases to its target value, at which point logic block 221 generates ENABLE with a logic low to put converter 104 in an idle state designated as state VI in Table I.

[0029] FIG. 4 is a schematic diagram showing DC to DC converter 104 in an alternate embodiment incorporating an analog type regulation of V_OUT, rather than the hysteretic regulation of the embodiment shown in FIG. 2. The structure and operation of most of the devices are similar to what has been described, except for the addition of an error amplifier 247 and the function of current sources 242 and 244.

[0030] Error amplifier 247 is a standard analog amplifier formed in the regulation feedback path to produce an analog error signal V_ERRon a node 222 as a difference between output supply voltage V_OUTand reference voltage V_REF. V_ERRis coupled to an input of comparator 220 for comparing with a skip threshold voltage V_SKIPto produce digital enabling signal EN, which functions as described above. When V_ERRis larger than V_SKIP, i.e., when converter 104 is essentially out of regulation, EN is logic high. However, in order to avoid excessive switching when V_OUTis at or near its target value, when V_ERRis less than V_SKIP, EN is made logic low to put converter 104 in an idle state, either state III or state VI. In effect, V_SKIPsets a regulation limit within which converter 104 is idled. In one embodiment, V_SKIPhas a value of about fifty millivolts, which means converter 104 regulates to within fifty millivolts of its target value.

[0031] As a feature of this embodiment, as V_OUTapproaches its target value, the limit currents I_PEAKand I_VALLEYhave values which are adjusted in an analog fashion by V_ERR. Hence, as V_OUTapproaches its target value, I_PEAKand I_VALLEYare reduced accordingly to allow smaller corrections to be made to V_OUTwhile maintaining operation over a narrower frequency range. In one embodiment, I_REF1has an adjustment range between about ten and about one hundred microamperes, and I_REF2is adjustable from about twenty to about two hundred microamperes.

[0032] In summary, the present invention provides a voltage converter circuit and method of converting a voltage that has a high performance and a low manufacturing cost. A first transistor is switched on by a first value of a control signal to generate a coil current at a node when the converter's output voltage is greater than a reference voltage. A second switch is turned on by a second value of the control signal to generate the coil current when the output voltage is less than the reference voltage. The second switch is coupled between the node and the output of the converter circuit to discharge the coil current to develop the output voltage with either value of the control signal. The converter adjusts to either a step up or step down conversion mode automatically while using only two high current switching transistors that can be integrated on the same semiconductor die with control circuitry using a standard CMOS process. Since the coil current is routed through only one switching transistor at a time, the voltage converter has a low switching resistance and a high power efficiency. Moreover, by using only two switching transistors to charge and discharge the coil current for up or down conversion, the voltage converter occupies a small die area and consequently has a low fabrication cost. While regulating, the coil current fluctuates between upper and lower current limits, and therefore decays to zero only under a no load or idling condition. By operating with a nonzero coil current, a high switching efficiency is achieved while reducing coil ringing during switching to maintain a low level of electromagnetic interference. In addition, the benefits of the invention are obtained by using an inductor or coil, rather than a more expensive transformer, which further reduces the fabrication cost of a system.