Synchronous regulator with switchable current reference source

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

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.

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.

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.

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

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

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

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

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

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 BATT operating at a voltage of about 3.6 volts.

DC to DC converter 104 converts battery supply voltage V BATT received at an input node 200 to an output supply voltage V OUT at 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.

FIG. 2 is a schematic diagram showing DC to DC converter 104 in further detail, including an inductor or coil L 1 , a capacitor C 1 , transistor switches S 1 and S 2 , comparators 220 , 230 and 246 , current sources 242 and 244 , logic blocks 221 and 250 and resistors 251 and 252 . Battery supply voltage V BATT is received at node 200 and output supply voltage V OUT is 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 OUT supplies a load current I LOAD of about fifty milliamperes to operate a smart card included in load 106 . Except for coil L 1 and capacitor C 1 , the components shown in FIG. 2 are formed on a single semiconductor die as an integrated circuit. In one embodiment, coil L 1 has a value of about twenty-two microhenries and capacitor C 1 has a value of about ten microfarads.

Switch S 1 is a standard high current n-channel metal oxide semiconductor field effect transistor (MOSFET) configured to switch a coil current I COIL through coil L 1 at a level of at least one hundred fifty milliamperes. Switch S 2 is a standard high current p-channel MOSFET also capable of supplying coil current I COIL at a level of at least one hundred fifty milliamperes.

Comparator 220 monitors output supply voltage V OUT for comparing with a reference voltage V REF to provide voltage regulation. A digital enabling signal EN is produced on a node 223 with a logic high value to activate switch S 1 and/or switch S 2 through logic blocks 221 and 250 when V OUT is 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 OUT is near its target regulation value.

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 S 1 and S 2 during the standby mode. An output provides an enabling signal ENABLE on a node 262 .

Comparator 230 compares output supply voltage V OUT with 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 BATT is 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 .

Current sources 242 and 244 supply reference currents I REF1 and I REF2 , respectively, that flow through resistor 252 to battery supply voltage V BATT and establish a reference potential V 207 on a node 207 .

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

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 V 206 resulting from the flow of coil current I COIL through resistor 251 . Another input receives reference potential V 207 for comparing with voltage V 206 to produce a digital phase signal CHG/DSCHG at an output on a node 249 to control whether coil current I COIL is charging or discharging, i.e., increasing or decreasing. When I COIL is charging, CHG/DSCHG is logic high to close switch 245 so that V 207 =V CHG =V BATT −R 252 *(I REF1 +I REF2 ). When I COIL is discharging, CHG/DSCHG is logic low to open switch 245 so that V 207 =V DSCHG =V BATT −R 252 *I REF1 . In effect, V CHG functions to set a maximum or upper current I PEAK =(R 252 /R 251 )*(I REF1 +I REF2 ) for I COIL , while V DSCHG functions to set a minimum or lower current limit I VALLEY =(R 252 /R 251 )*I REF1 for I COIL .

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 S 1 and S 2 , 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 BATT or V OUT , as described in detail below. Logic low operates at ground potential.

The detailed operation of converter 104 can be understood by referring to FIG. 3, which shows a waveform of coil current I COIL versus 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.

Assume further that initially, at time T0, output supply voltage V OUT is zero and coil current I COIL is 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 REF1 and I REF2 at node 207 . VS1 is logic low so switch S 1 is off. VS2 is low, so switch S 2 is turned on to form a conduction path between a node 208 and output node 202 . The initial voltage across coil L 1 is (V BATT −V OUT ), which is nearly equal to V BATT . Coil current I COIL increases with time to charge or store magnetic energy in coil L 1 , and is routed through S 2 to output node 202 . This period when I COIL is charging or increasing is designated as the charging phase of operation. Hence, during the charging phase, I COIL flows to capacitor C 1 to begin to develop output supply voltage V OUT . For typical values of C 1 , V OUT increases a few hundred millivolts during the first cycle.

At time T1, I COIL increases 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 PEAK has a value of about one hundred fifty milliamperes. Switch 245 is opened so only reference current I REF1 flows through resistor 252 , setting potential V 207 to 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 S 2 turns on to discharge I COIL into capacitor C 1 to further develop the potential of V OUT . This period during which I COIL is decaying or decreasing is designated as the discharging phase of I COIL .

At time T2, I COIL decays 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 S 1 remains off during both the charging and discharging phases of the above described cycle. Note further that switch S 2 routes I COIL to charge capacitor C 1 during both the charging and discharging phases of I COIL , i.e., when I COIL is both increasing and decaying. This scheme allows V OUT to be developed in fewer cycles and, by switching I COIL only through a single device S 2 , 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 .

When converter 104 is operating as described above as a step down voltage converter in which the target value of V OUT is less than V BATT , the cycles continue to alternate between state I and state II as described until capacitor C 1 has accumulated sufficient charge from I COIL to reach the V OUT target 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 BATT potential, allowing I COIL to decay to zero and deactivating switches S 1 and S 2 . This idle condition is shown in Table I as state III. Converter 104 remains in this idle state until V OUT decays by the amount of hysteresis in comparator 220 , when ENABLE is again set logic high to begin a new cycle.

Now assume converter 104 is operating in a step up converter mode in which 5.0 volts is the target value of V OUT which is higher than V BATT =3.6 volts. ENABLE is high and operation proceeds as described above until V OUT is 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 L 1 . As V OUT reaches 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.

The charging phase, when I COIL is increasing, occurs by setting VS1 to a logic high to turn on switch S1. This mode of operation allows I COIL to increase more rapidly because the voltage across coil L 1 is 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 COIL increases 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 S 1 , and VS2 is made logic low, turning on switch S 2 to discharge I COIL into capacitor C 1 , as shown as state V in Table I. When I COIL decays 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 OUT increases 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.

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 .

Error amplifier 247 is a standard analog amplifier formed in the regulation feedback path to produce an analog error signal V ERR on a node 222 as a difference between output supply voltage V OUT and reference voltage V REF . V ERR is coupled to an input of comparator 220 for comparing with a skip threshold voltage V SKIP to produce digital enabling signal EN, which functions as described above. When V ERR is 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 OUT is at or near its target value, when V ERR is 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 SKIP sets a regulation limit within which converter 104 is idled. In one embodiment, V SKIP has a value of about fifty millivolts, which means converter 104 regulates to within fifty millivolts of its target value.

As a feature of this embodiment, as V OUT approaches its target value, the limit currents I PEAK and I VALLEY have values which are adjusted in an analog fashion by V ERR . Hence, as V OUT approaches its target value, I PEAK and I VALLEY are reduced accordingly to allow smaller corrections to be made to V OUT while maintaining operation over a narrower frequency range. In one embodiment, I REF1 has an adjustment range between about ten and about one hundred microamperes, and I REF2 is adjustable from about twenty to about two hundred microamperes.

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.