Abstract:
A switch mode power supply ( 30 ) having a power-good function ( 34 ) which senses whether a switch mode converter ( 32 ) is switching. The power-good function determines and indicates that the switch mode power supply is within tolerance when the switch mode converter is switching, i.e., a duty cycle is between 0 and 100%. Conversely, has a duty cycle determined to be 0% or 100%, the power-good function determines and indicates that the power supply is outside tolerance. The power-good function circuitry senses the output directly, (mode A) wherein the switching converter is driven by an integrator consisting of an error amplifier within the feedback loop. The power-good function is simple, accurate, and saves valuable silicon space, especially when implemented in switch mode power supplies that are programmable and have multiple output lines.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    Cross-reference is made to commonly assigned U.S. patent application Attorney&#39;s Docket Number TI-33249, filed herewith, the teachings of which are incorporated herein by reference.  
         FIELD OF THE INVENTION  
         [0002]    The present invention is generally related to the field of switch mode power supplies, and more particularly to the detection of when a switch mode power supply is within regulation, sometimes known as a “power-good” function.  
         BACKGROUND OF THE INVENTION  
         [0003]    Switch mode power supplies typically include a switching converter having a varying duty cycle. Conventionally, the switching converter is responsively driven by an integrator, such as an error amplifier configured in a feedback loop. To achieve regulation, the integrator typically increases or decreases the duty cycle of the switching converter until the power supply is within regulation, that is, having an output voltage, such as a dc voltage, within tolerance of a specification.  
           [0004]    Conventional switch mode power supplies also typically have a “power-good” function. A “power-good indication” is typically an indication of whether the power supply is operating within intolerance. Conventional methods of providing this “power-good” function require excessive amount of circuitry, especially since these types of power supplies typically have multiple outputs which can be programmed to multiple output voltages, thus necessitating a separate “power-good indication” for each of the multiple outputs. In addition, the presence of multiple out requires that each circuit be very accurate to reduce the probability of yield loss at test.  
           [0005]    [0005]FIG. 1 illustrates one conventional approach at  10  providing a power good indication. This approach is seen to include a fixed reference voltage provided to a pair of comparators, each comparator tapping off a high precision resistive divide network configured to sense the output of the switch mode converter. This approach is typically referred to as a “window comparator”, as it includes circuitry determining whether the output voltage of the switch mode converter is within a “window” determined to be acceptable and within tolerance. The inherent disadvantages of this approach is the need for an accurate resistive divide network, requiring precision resistors. The need for comparators and resistors, including high accuracy resistors, takes up valuable real estate in the silicon, necessitating complex circuitry to achieve accuracy.  
           [0006]    Another conventional approach is shown at  20  in FIG. 2, whereby the window comparator, includes a PWM comparator and additional pairs of comparators for each output power line. Again, the additional need of comparators takes up valuable silicon space, is complex, and is subject to accuracy problems.  
           [0007]    There is desired an improved switch mode power supply having an improved “power-good” function that reduces the amount of circuitry required, especially for power supplies having multiple programmable output lines.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention achieves technical advantages as a switch mode power supply having a “power-good” function that determines whether or not the switching converter is switching to determine if the power supply is within regulation. It is recognized by the present invention that a switch mode power supply has a switch mode converter operating somewhere between a zero and 100% duty cycle if it is within regulation. Thus, the present invention takes advantage of this operation by detecting whether or not the duty cycle is 0% or 100% which indicates whether the control loop is keeping the power supply in regulation. Advantageously, the present invention uses an existing accurate error amplifier to determine whether or not the switching converter is switching. A binary or digital output is provided which indicates whether or not the power supply is within regulation. The power good function of the present invention conserves valuable silicon die space due to its inherent simple design, and is highly accurate. There is no need for accurate high precision resistive divide networks, or additional amplifiers.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    For a more complete understanding of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings wherein:  
         [0010]    [0010]FIG. 1 is a prior art window comparator circuit sensing a switch mode converter using a resistive divide network including high precision resistors and a plurality of additional comparators;  
         [0011]    [0011]FIG. 2 is also a prior art approach depicting a window comparator including a PWM comparator and additional comparators to provide a power good function;  
         [0012]    [0012]FIG. 3 is a schematic of the present invention including a switch mode converter and a power good function sensing the switching of the switch mode converter;  
         [0013]    [0013]FIG. 4 is a waveform diagram depicting pulsed signal at mode B when the comparator is switching;  
         [0014]    [0014]FIG. 5 is a waveform diagram depicting when the comparator is high all the time such that signal line B is always high;  
         [0015]    [0015]FIG. 6 is a waveform diagram depicting when the comparator is low all the time such that signal line B is also being high all the time;  
         [0016]    [0016]FIG. 7 is a waveform diagram depicting when the switching comparator is not switching and then when the switching comparator is switching such that the output line F is indicative of whether or not the comparator is switching;  
         [0017]    [0017]FIG. 8 is a functional block diagram of a switching power supply incorporating the power good function of the present invention;  
         [0018]    [0018]FIG. 9 is a serial control interface timing diagram of the power supply of FIG. 8;  
         [0019]    [0019]FIG. 10 is an eight channel application circuit schematic including two power supplies coupled to a common serial control interface; and  
         [0020]    [0020]FIG. 11 is a block diagram of an eight channel AC5 line cord; implementing the power supply.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    Referring now to FIG. 3, there is depicted generally at  30  a switch mode converter circuit including an integrator  32  and an error amplifier  34  in a feedback loop. If the output  36  of the converter circuit  30  is not held in regulation, the integrator  32  will eventually cause the duty cycle of the converter circuit  30  to reach either 100% or 0%.  
         [0022]    The present invention includes a simple, small, digital circuit  40  used to detect when the duty cycle of the converter circuit is at either 100% or 0%. If the duty cycle is determined by circuit  40  to be at 0% or 100%, the circuit  40  indicates an output  42  reports that the converter  30  is not regulating within tolerance. Circuit  40  is seen to include D-type latches  46 ,  48  and  50 , as well as inverters  52  and  54 , as well as resistor R 0  and capacitor C 0 . When the switching converter circuit  30  is running at a duty cycle other than 0% or 100%, there is a continual stream of pulses provided from node A to the clock input of latch  46 . Latch  46 , resistor R 0 , capacitor C 0  and inverters  52  and  54  are configured to form a one-shot timer. As long as the converter  30  does not have a 0% or 100% duty cycle, there is a stream of regular negative going pulses at the QZ output of the latch  46 , as depicted in FIG. 4.  
         [0023]    The continual stream of pulses from latch  46  resets the shift registers  48  and  50 , so that the “switch” output signal shown at  42 , depicted as waveform F, remains high and which is indicative that the converter  30  is within tolerance.  
         [0024]    If the stream of reset pulses stops at node B, as depicted in FIG. 5 and FIG. 6, a logic “1” is clocked through registers  48  and  50  and the switch output signal  42  goes low, indicating that the converter circuit  30  has stopped switching and thus the converter  30  is at either 0% or 100% duty cycle, which indicates that the converter  30  is not in regulation.  
         [0025]    [0025]FIG. 5 depicts when the comparator  32  is high all the time, and the series of pulses at node B stop. FIG. 6 indicates when the comparator  32  has an output that is low all the time, also causing the output signal line B to remain high all the time.  
         [0026]    As depicted in the left half of FIG. 7, when no pulses are provided at node B, a logic “1” clocks through registers  48  and  50 , and the signal at node E goes logic high after 2 clock cycles at node C, indicating that the converter  30  is not switching.  
         [0027]    The right side of the diagram in FIG. 7 illustrates that when a regular series of pulses are provided at node B, these pulses continually reset registers  48  ad  50  so that node E stays low, causing node F to go high and correspondingly indicating that the converter is switching.  
         [0028]    The present invention achieves technical advantages by providing a binary output signal on signal output line  42  indicative of whether or not the switching converter  36  is switching. The present invention assumes that while the switching converter  32  is switching, the power supply is within regulation. Conversely, when the converter stops switching  32 , it is determined that the power supply is not within regulation. This corresponding output signal is provided as a logic high on output line  42  when the converter  33  switching, and providing a logic low on output line  42  when the converter is not switching.  
         [0029]    The present invention achieves technical advantages by using an existing accurate error amplifier  34  to directly detect whether or not the control loop is within regulation. No additional precision resistors or comparators are required, thus realizing a savings in silicon space. The present invention is highly accurate and applicable to converters for all output voltages being provided.  
         [0030]    Referring now to FIG. 8, there is shown at  60  a four channel, voltage mode step-down converter incorporating the present invention, providing four independently controllable output voltages. Each regulated channel includes a high-side PMOSFET switch with a typical R Ds  (ON) of one ohm, which makes it suitable for high efficiency, low current applications. Commands sent to the converter  60  over the four wire serial port program the outputs independently or globally to supply voltages from 7.5 Volts to 13.1 Volts in 0.4V increments. When the input voltage is desired at an output, a bypass mode can be activated which fully enhances the PMOSFET switch and disables the switching circuitry of the selected channel.  
         [0031]    The converter  60  is an ideal companion device to power Texas Instruments THS7102 ADSL line drivers as a part of the AC5 Central Office ADSL chipset. With the AC5 chipset controlling the converter  60  output voltages, significant power savings are realized by reducing the excess supply headroom on a per line basis.  
         [0032]    Pin Assignments  
         [0033]    Pin assignments of a 16 pin package are defined in Table 1 below.  
                                                           TABLE 1                           PIN                    NAME   NO.   PIN DESCRIPTION   FUNCTIONAL DESCRIPTION                    LX0   1   Channel 0 switch output   Output to inductor and catch diode       LX1   2   Channel 1 switch output       GND   3   Ground   Power and Analog Ground       SFS   4   Frame sync input   Read/Write frame start strobe       SDI   5   Serial data in   8 bit address/16-bit data word signal       EN   6   Enable   EN &lt; V il : Disable all channels,                   EN &gt; V il : Enable activates                   outputs (see text)       FB0   7   Channel 0 feedback input   Feedback from L-C filter output       FB1   8   Channel 1 feedback input       FB2   9   Channel 2 feedback input       FB3   10   Channel 3 feedback input       CBS   11   Channel bank select   Assigns internal channels to                   respond to serial               address bit ADR2= 0 when                   CBS &lt;V il , or to                   ADR2 = 1 when CBS &gt; V ih         SCLK   12   Serial clock input   Serial clock/                   synchronization signal       SDO   13   Serial data out   Status data output signal       VIN   14   Input supply voltage   Chip supply                   and channel 0-3 switch input       LX2   15   Channel 2 switch output   Output to inductor and catch diode       LX3   16   Channel 3 switch output                  
 
         [0034]    With continued reference to FIG. 8, various functional blocks will now be discussed.  
         [0035]    Reference System/Voltage Divider &amp; Multiplexer  
         [0036]    The reference system consists of a band-gap circuit, four digital to analog converter outputs (DACs), and smoothing filters. The reference system provides independent set-point voltages to the PWM control loops of each channel, and are programmed via the 4 wire serial port. Output control of the regulators is provided in 15 steps with 400-millivolt resolution over a range of 7.5V to 13.1V. The DACs can also be programmed to force the PMOSFETs into the fully “on” pass-through or bypass mode to pass the input voltage to any output.  
         [0037]    UVLO Circuit &amp; Power-Up State  
         [0038]    The Under-Voltage Lockout (UVLO) circuit controls device operation when the input voltage is below the UVLO threshold such as during power-up or power-down. Hysteresis built in to the UVLO detection circuit reduces sensitivity to noise and ripple on the power supply inputs to the converter  30 . Prior to reaching the UVLO threshold, the ramp oscillator is disabled so that no switching occurs in the converter  10 , the PMOS transistors are forced into the off state, and the registers and DACs are reset. Once the UVLO threshold is reached, the soft-start sequence begins. If the input voltage falls below the UVLO threshold after the device is programmed and operating, all four outputs are disabled, the DACs are set to zero volts and the programming registers are reset. Subsequently returning VIN above the UVLO threshold will require re-initialization of the phase stagger and channel voltage programming.  
         [0039]    Soft-Start Sequence and Voltage Transitioning  
         [0040]    When the supply voltage exceeds the UVLO threshold, the converter  30  is ready to be programmed via the serial interface. As each channel is programmed and enabled with a voltage code, the channel DACs begin stepping the output up from zero volts to the target voltage in 200-millivolt increments. If the target voltage is 15 Volts (i.e.pass-through mode) the DAC will continue to increment in 200-millivolt steps between 13.1 Volts and the fully “on” state. When a channel is commanded to transition from one voltage level to another, the output steps up (or down) to the new level in 200-millivolt increments. The period between each DAC increment is approximately 250 microseconds when the SCLK frequency equals 4.416 MHz. This results in a maximum ramp-up time of 8 milliseconds when stepping from OV to 15V, and a maximum transition time between max and min regulation voltages (7.5V, 13.1V) of 4 milliseconds. The use of small stop increments provides a smooth predictable ramp and prevents inadvertent tripping of the overcurrent limit.  
         [0041]    Note that while an output is transitioning to the new target voltage, its voltage code register is protected from being overwritten. During this transition period, the channel status may be read via the 4 wire serial port using the read protocol. The data returned will be non-zero while channel is transitioning.  
         [0042]    Oscillator, Divider &amp; Sync Circuit  
         [0043]    The converter  30  has a free-running internal ramp oscillator that operates at a nominal frequency of 450 kHz. When the 4.416 MHz SCLK signal is present, a synchronous divide-by-eight circuit provides a 552 kHz clock to synchronize the PWM ramp. The start of the ramp is coincident with every eighth rising edge of SCLK. If the converter  30  SCLK pin is driven at a frequency lower than eight times the free-running frequency of the oscillator (f osc ), it may result in chaotic operation. Care should be taken to guarantee that the minimum frequency at the SCLK input is 4.0 Megahertz.  
         [0044]    Phase Stagger Circuit  
         [0045]    When two converter circuits  30  are used as a pair to operate as an 8-channel unit, the PWM ramps in the two devices can be advantageously phase staggered to reduce input ripple and bypass requirements as previously discussed in detail earlier. The initialization command forces the PWM ramp of the converter  30  with its CBS pin tied low to be staggered by four SCLK cycles compared to the device with its CBS pin forced to a logic high. Note that this command clears the voltage programming in both converters  30  and disables the outputs. Voltage programming instructions can be issued immediately following the initialization command.  
         [0046]    Enable (EN)  
         [0047]    If the EN pin is held low when the converter  30  is powered up, the oscillator will start and free-run. Serial commands to initialize the PWM clocks and program the output levels will be accepted, but the outputs will be held off and will not begin regulating until the EN pin is pulled above V ih .  
         [0048]    If the converter  30  is programmed with outputs enabled when EN is pulled LOW, all outputs are shut off and all DACs are reset. The EN pin does not affect the oscillator, which continues to run and maintain PWM phase stagger. The previously programmed channel voltages are also maintained in the registers. If EN is pulled above V ih , the converter  30  channels start up through the soft-start sequence and reach regulation at the previously programmed target voltages.  
         [0049]    Bypass mode may be forced on all outputs by pulling EN above VIN—0.2V. When bypass mode is forced, all four channels step up to 15V in 200-millivolt increments.  
         [0050]    Over Current Protection  
         [0051]    During steady state operation, the overcurrent protection threshold is 150 milliamps minimum, 300 milliamps maximum, sampled approximately 500 nanoseconds after the start of the switching cycle. When overcurrent is sensed in the PMOSFET, the output is disabled for a “hiccup” time of 170 to 360 milliseconds (SCLK=4.416 MHz). In the “pass-through” mode, the overcurrent detection remains active and the “hiccup” behavior is unchanged.  
         [0052]    During the soft-start sequence and voltage transitioning, the currents in the PMOSFET are higher than steady state. The overcurrent trip threshold is increased to prevent inadvertent shut-down &amp; re-start action (hiccupping) in the overcurrent protection circuit.  
         [0053]    Thermal Shutdown  
         [0054]    Thermal shutdown disables the controller if the junction temperature exceeds 150° C. The hysteresis is 10° C. This shuts down off the switching circuitry and resets the soft-start circuitry. If the IC returns to normal temperature, it re-starts and returns to the programmed target voltages.  
         [0055]    Serial Control Interface Timing Diagram  
         [0056]    The serial control interface timing is depicted in FIG. 9.  
         [0057]    Serial Command Bit Assignments  
         [0058]    The serial command bit assignments are depicted in Table 2 below.  
                                           TABLE 2                           SERIAL BIT            POSITION   NAME   DESCRIPTION                    15   R/W*   Set to logic 1 to read from converter, set to logic 0 to write to               converter       14   ADR2   Channel bank select, compared to logic state of CBS pin to               select between two converter devices used in an 8 channel               configuration       13   ADR1   Internal channel select MSB, used with ADRO to select one               of four output channels       12   ADRO   Internal channel select LSB, used with ADRi to select one               of four output channels       11   S3   Device address MSB (S3=1 required to address converter)       10   S2   Device address bit S2=1 required to address converter)       9   S1   Device address bit (S1=1 required to address converter)       8   S0   Device address LSB (SO=1 required to address converter)       7   D7   Voltage programming MSB       6   D6   Voltage programming bit       5   D5   Voltage programming bit       4   D4   Voltage programming LSB       3   D3   Channel enable/disable (D3=0 enables channel(s))       2   D2   Global start       1   D1   Unassigned       0   DO   Initialize counters                  
 
         [0059]    Valid Commands  
         [0060]    Valid commands to the converter are shown in Table 3 below.  
                   TABLE 3                       WORD   DESCRIPTION                   0000111100001001   Initialize PWM clocks with phase stagger and disable all           channels       Oddd1111vvvv0100   Turn on &amp; regulate all channels to voltage code vvvv (see           voltage programming code table)       Oaaa1111vvv0000   Turn on &amp; regulate channel aaa to voltage code vvvv (see           voltage programming code table)       Oaaa111dddd1000   Disable channel aaa       1aaa1111dddddddd   Read channel status from channel aaa                  
 
         [0061]    Voltage Programming Codes  
         [0062]    Voltage programming codes are shown by Table 4 below.  
                                             TABLE 4                       VOLTAGE CODE   OUTPUT   VOLTAGE CODE   OUTPUT       (D4-D7)   VOLTAGE   (D4-D7)   VOLTAGE                                0   7.5   8   10.7       1   7.9   9   11.1       2   8.3   A   11.5       3   8.7   B   11.9       4   9.1   C   12.3       5   9.5   D   12.7       6   9.9   E   13.1       7   10.3   F   Pass through                   mode                  
 
         [0063]    Channel Status Read Back Codes  
         [0064]    Channel status read back codes are shown in Table 5 below.  
                   TABLE 5                       STATUS BYTE VALUE   OUTPUT       (D0-D7)   MEANING                   00h   Channel settled to regulation window       FFh   Channel not settled or fault condition (note 1)                          
 
         [0065]    Serial Interface Protocol  
         [0066]    The serial interface uses SCLK (Serial Clock), SFS (Serial Frame Sync), SDI (Serial Data In) and Bank Select inputs, and outputs device status on SDO (Serial Data Out). SFS and SDI inputs are sampled on the falling edge of SCLK. An SFS pulse indicates that the bus master is ready to transmit a word, and the bit and frame counters in the converter 30 are reset when SFS is high. The first bit (b 15 ) of the 16-bit word is shifted in on the next failing edge of SCLK. The first eight bits of the word are denoted as the Address or Command, and the last eight bits are Data. Refer to the table titled “Serial Command Bit Assignments”.  
         [0067]    The Command consists of three fields—the R/W bit; Channel Select bits ADR2-0; and four Device Select bits S3-SO. The R/W bit determines whether the data portion of the word will be written to the converter  30  or read from the converter  30 . The value in the Channel Select field determines which output channel is to receive programming data. Channel select bit ADR2 is compared to the logic level on the Channel Bank Select input. This allows two distinct converter  30  devices to be addressed as one logical eight-channel unit. The remaining bits ADR1, ADR0 are decoded to select one of the four on chip channels. The third part of the command is the 4-bit Device Select, bits S3-SO. The converter  30  has been assigned a device ID of “F” for S3-S0. This value must be used to address converter devices.  
         [0068]    The data field, D7-D0, is used to program output voltage levels and control converter  10  operation.  
         [0069]    Pass Through Mode  
         [0070]    The pass through mode may be used to force a channel&#39;s PMOSFETs to remain in the fully enhanced “on” state. Use of the pass through mode is desirable under several conditions. First, transmitting high peak-to-peak voltages will require maximum headroom on the line driver supply. Second, if the load current is too small, the Line Ranger circuit will be required to operate in discontinuous mode. The output may ring in response to transient conditions. Low load current conditions may occur if the line driver is idle and the quiescent current has been reduced to conserve power. If the line must remain ready to return to normal operation, the pass through mode is appropriate. If the line is unused or can tolerate start up delays, the channel shut down mode should be considered to conserve additional power.  
         [0071]    Channel Shut Down  
         [0072]    A bit value of 1″ in bit  3  is used to shut down the addressed channel. Shutting down of an unused channel is recommended when power savings warrant complete power down of a line driver, and start up delays in returning to normal operation are not critical.  
         [0073]    Global Program  
         [0074]    Data Bit  2  in the serial word is the “global turn-on and regulate” signal. It is used to program all outputs to the same voltage and start them up at the same time.  
         [0075]    PWM Clock Initialization  
         [0076]    Data Bit  0  is used to initialize the on-board clocks. The signal to initialize the clocks is ANDed with data bit  5  and cannot be given without powering down the converter  36  and going through a complete restart sequence.  
         [0077]    Status Readback  
         [0078]    The converter  36  is designed to monitor its output state and recognize when it has settled into regulation at its programmed value according to the present invention as previously described in detail. The SDO pin, output  42 , reports a channel in a voltage transition or error condition (Channel Not Ready) by returning a data value of FFh. When SDO returns a value of 00h, the channel is in regulation.  
         [0079]    The following conditions will cause a “Channel Not Ready” status to be reported:  
         [0080]    Channel Disabled  
         [0081]    PWM duty factor outside expected range (i.e. 0% or 100% PW)  
         [0082]    Channel in overcurrent  
         [0083]    Channel Transitioning to New Target Value  
         [0084]    Over-temperature Shutdown (affects all four channels)  
         [0085]    Noise immunity circuits in the fault detector introduce a delay in the reporting of the channel status. For instance, if a command to transition to a new target voltage is issued, the output voltage may be stable up to 250 microseconds before the detection circuit reports that the channel is ready. The minimum recommended status polling interval per channel is 500 microseconds.  
         [0086]    Recommended Component Values  
         [0087]    Table 7 depicts recommended component values for use of converter  30 .  
                                   TABLE 7                       Ref.                           Designator   Device   MFR #1   P/N   MFR #2   P/N                   C1-5   10 uF Tantalum Cap   Kemet   T495D106                           M035AS       C6   0.1 uF Ceramic Cap   Kemet       AVX       L1-4   220 uH Inductor   GCI   5143   JW Miller   PM74S-                           221K       D1-4   Schottky Diode   Diodes, Inc.   BAT54AW                  
 
         [0088]    [0088]FIG. 11 depicts a block diagram of an eight channel AC5 line card with a Texas Instruments Line Ranger option and utilizing the power converter  30  of the present invention.  
         [0089]    The numerous innovative teachings of the present application will be described with particular reference to the presently preferred exemplary embodiments. However, it should be understood that this class of embodiments provides only a few examples of the many advantageous uses and innovative teachings herein. In general, statements made in the specification of the present application do not necessarily delimit any of the various claimed inventions. Moreover, some statements may apply to some inventive features, but not to others.  
         [0090]    Although a preferred embodiment of the apparatus and system of the present invention has been illustrated in the accompanied drawings and described in the foregoing Detailed Description, it is understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications, and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.