Abstract:
Transition delays in a level shift circuit are equalized by generating a first signal related to the state of the input signal, a second signal inversely related to the state of the input signal, and a third signal that is reciprocal to the second signal. Upon transition of the input signal from a high state to a low state, the third signal is selected for controlling the output until the first signal attains a high state. The first signal is selected for controlling the output when it has reached a high state after the input signal transition. The first signal remains selected upon transition of the input signal from a high state to a low state. Thus, output delays are equalized and reduced to the shortest delay.

Description:
TECHNICAL FIELD 
     The present disclosure relates to level shifters, more particularly to the control of output delays incurred upon transitioning between level states. 
     BACKGROUND 
     Level shifters are utilized in a variety of applications in which it is desired to couple a circuit node to either of two voltage levels in dependence upon the state of an input control signal.  FIG. 1  is a simplified illustration of a driver applied to a high side FET of a buck converter. The converter comprises FETs  10  and  12 , inductor  14  and capacitor  16 . The junction of FETs  10  and  12  and inductor  14  is indicated as SW. Various other converter and controller elements are not shown as they are not necessary for explanation of the broad operation. Output V OUT  of the converter is maintained at a lower voltage level than the input voltage V IN . FETs  10  and  12  are alternatively activated in succession. As V IN  is greater than V OUT , activation of FET  10  permits charging of output capacitor  16  through inductor  14  as current builds in the inductor. When FET  12  is activated, current through inductor begins to decrease. The time activation of the FET switches is regulated to maintain the output voltage at the desired level. 
     Activation and deactivation of FET  10  is under the control of driver  18 . When FET  10  is in the conductive state, the voltage at its gate exceeds the voltage at the inductor side source. For non-conduction, the gate voltage should not exceed the source voltage. In response to controlled input signals, driver  18  shifts the level of voltage applied to the gate of FET  10  between V BOOST  and V SW  to control the states of the FET. Capacitor  17  is coupled between the V BOOST  and V SW  nodes. In a first state, V BOOST  is applied to the gate of FET  10 . The gate to source voltage, V BOOST −V SW  activates FET  10  to a conductive condition. In a second state, V SW  is applied to the gate of FET  10 . As there is no potential difference between the gate and the source, FET  10  is not conductive. 
     A known level shifting circuit is illustrated in  FIG. 2 . An input signal transmission stage  20  comprises PMOS FETs  22  and  24 , NMOS FETs  26  and  28  and inverter  30 . FETs  22  and  26  are coupled in series across voltage reference line  32  and ground, as are series FETs  24  and  28 . The gate of FET  22  is coupled to the drain of FET  28 . The gate of FET  24  is coupled to the drain of FET  26 . The gate of FET  26  is coupled to a select signal input. The gate of FET  28  is coupled to the select signal input through inverter  30 . The output, line N 1 , of stage  20  is coupled to the junction of the gate of FET  24  and the drain of FET  26 . 
     An output stage  40  comprises PMOS FET  42  coupled in series with NMOS FET  44  across voltage reference line  32  and reference line  34 . The junction  36  of FETs  42  and  44  is coupled to the gate of FET  10 . In response to controlled input signals, driver  18  shifts the level of voltage V 36  applied to the gate-source of FET  10  between a level of V BOOST −V SW  volts and zero volts. Junction  36  will either be coupled to line  32 , V BOOST , via FET  42  or to line  34 , V SW , via FET  44 . 
     Coupled between signal transmission stage  20  and output stage  40  is logic circuit  50 . Inverter  52  is coupled between line N 1  and one input of NAND gate  54 . Inverter  52  is also coupled to one input of NAND gate  56  via inverter  58 . The output of NAND gate  54  is coupled to the other input of NAND gate  56 . The output of NAND gate  56  is coupled to the other input of NAND gate  54  via inverters  60  and  62 . The output of NAND gate  54  is coupled to the gate of FET  42 . Inverter  60  is coupled to the gate of FET  44 . Logic circuit  50  is responsive to the state of the signal at N 1  to provide a gating signal to one of the output stage FETs and prevents simultaneous conduction of both output stage FETs. 
     With a high level input signal V SIG  at stage  20 , FET  26  is biased conductive, FET  28  is biased non-conductive, FET  24  is biased conductive, and FET  22  is biased non-conductive. Line N 1  is at a low level state under these conditions. The coupling of N 1  by inverters  52  and  58  imposes a low level signal to the input of NAND gate  56 . NAND gate  56  outputs a high level signal that is inverted by inverter  60 . The low level output is applied to FET  44  to bias it to a non-conductive state. Both inputs of NAND gate  54  are at a high level by virtue of inverters  52  and  62 . NAND gate  54  outputs a low signal that biases FET  42  to a conductive state. Output line  36  thus is coupled to line  32 . 
     With a low level input signal at stage  20 , FET  26  is biased non-conductive, FET  28  is biased conductive, FET  24  is biased non-conductive, and FET  22  is biased conductive. Line N 1  is at a high level state under these conditions. The coupling of N 1  by inverter  52  imposes a low level signal to the input of NAND gate  54 . NAND gate  54  outputs a high level signal to bias FET  42  to a non-conductive state. Both inputs of NAND gate  56  are high to produce a low level output, which is inverted by inverter  60  to bias FET  44  to a conductive state. Output line  36  is thus coupled to line  34 . 
     The level of the input signal to input stage  20  thus selects whether line  36  is coupled to line  32  or line  34 . When line  36  is coupled to line  32 , output FET is conductive, and when line  36  is coupled to line  34 , FET is non-conductive. When the input transitions between level states, delays occur in the input stage  20 , as illustrated in  FIG. 3 . The voltage input signal, V SIG , is exemplified by a square wave waveform. Corresponding waveforms for the signals at N 1  (V N1 −V SW ) and line  36  (V 36 −V SW ) are represented. At time t 1 , the input signal changes from a low level state to a high level state. In response, N 1  changes state from a high level to a low level with a slight delay incurred by the transition of input stage FET  26  to the conductive state. The state of output line  36  then changes from the lower level to the higher level. The total delay between the rising transition of the input signal and the time t 2 , at which the output line  36  reaches its high state, is relatively short. At time t 3 , the input signal reverts to the low level state. Input stage FET  22  is responsive to this change in input signal, via interaction of inverter  30  and FET  28 , to conduct to provide a high level at N 1 . Full conduction occurs with a delay that is significantly greater than the transition of FET  22  to non-conduction. The state of output line  36  then changes from the higher level to the lower level at time t 4 . 
     The delay of the output in response in response to an input signal transition from high level to low level is significantly greater than the delay in response to the low to high input signal transition. The conventional level shifter produces faster High-to-Low transitions than Low-to-High transitions because FETs  22  and  24  must be made weaker devices than FETs  26  and  28  for proper operation. The prior art circuit thus does not produce a level shift functionality that satisfactorily equalizes transition delays in both directions. A level shift circuit that provides the same rising and falling delay is needed. Capability of making both delays equal to the shorter delay of the prior art circuit would be particularly desirable. 
     SUMMARY OF THE DISCLOSURE 
     Transition delays incurred by a level shift circuit can be equalized by generating a first signal having a state directly related to the state of the input signal, generating a second signal having a state inversely related to the state of the input signal, and generating a third signal having a state that is inverse to the state of the second signal. Upon transition of the input signal from a high state to a low state, the third signal is selected for controlling the output until the first signal attains a high state. The first signal is selected for controlling the output when it has reached a high state after the input signal transition. The first signal remains selected upon transition of the input signal from a high state to a low state. By substituting the third signal for the first signal during the slow delay period of the first signal, the delay period seen by the output is replaced by a delay period corresponding to the transition of the second signal. This delay is substantially equal to the faster delay period of the first signal. Thus, output delays are equalized and reduced to the shortest delay. 
     Delay equalization can be implemented with a circuit such as the following. Controlled switches respectively couple an output to first and second voltage reference nodes in accordance with a high or low signal state at an input to the circuit. A level shifting circuit is coupled between the input and a delay equalization circuit. A logic circuit is coupled between the delay equalization circuit and the controlled switches. The delay equalization circuit includes a reset dominant latch circuit that is coupled to the level shifting circuit and a data selection circuit that is coupled to the level shifting circuit, the latch circuit, and the logic circuit. The logic circuit prevents the output from being coupled simultaneously to first and second voltage reference nodes. 
     A first variable signal line of the level shifting circuit has a signal state directly related to the signal state of the input. A second variable signal line of the level shifting circuit has a signal state inversely related to the signal state of the input. These signal lines are applied to respective reset and set inputs of the latch circuit. A first input of the data selection circuit receives a state signal having a state directly related to the state of the first variable signal line. A second input of the data select circuit receives a state signal having a state related to inverse of the state of second variable signal line. The data select circuit also has an input coupled to an output of the latch circuit and an output coupled to the logic circuit. 
     The first variable signal line incurs a first delay in transitioning from a high state level to a low state level and incurs a greater delay in transitioning from a low state level to a high state level. The output signal of the data select circuit exhibits a delay for transition from both high level input to low level input and low level input to high level input that corresponds to the first delay. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations of the present invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements. 
         FIG. 1  is a simplified schematic diagram of a converter and FET driver. 
         FIG. 2  is a circuit diagram of a prior art level shifting circuit. 
         FIG. 3  is a waveform diagram of signals produced by the circuit of  FIG. 2 . 
         FIG. 4  is a partial block diagram of a level shifting and delay equalization arrangement in accordance with the present invention. 
         FIG. 5  is a waveform diagram for the arrangement of  FIG. 4 . 
         FIG. 6  is a circuit diagram of an example circuit that may be utilized for the delay equalization of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     A delay equalization circuit  70  is shown in  FIG. 4 , interposed between input stage  20  and logic circuit  50  of the level shifting circuit. Input stage  20  and logic circuit  50  may comprise the same circuit element configurations as shown in prior art  FIG. 2 . Circuit  70  uses the fast High-to-Low transitions on N 1  to control the Low-to-High transition of V 36  and the High-to-Low transitions on N 2  to control the High-to-Low transition of V 36 . 
     Circuit  70  comprises reset dominant latch  72  and data selection circuit  74 . Line N 1  is coupled to the reset input of latch  72 . A line N 2  is coupled to the set input of the latch. Line N 2  is also coupled to the drain of FET  24 . FET  24  is conductive when the input signal is at a high state and is nonconductive when the input signal is at a low state. The Q output of latch  72  is coupled to a select input, S, of data selection circuit  74 . A line having the same signal state as N 1  is coupled to a 0 input of the data selection circuit. A line having a signal state that is reciprocal to the signal state of N 2  is coupled to a 1 input of the data selection circuit. The output Y of the data selection circuit is coupled to the input of the logic circuit  50 . 
     Operation of the arrangement of  FIG. 4  is described with respect to the waveforms of  FIG. 5 . Depicted therein are waveforms for N 1  (V N1 ), N 2  (V N2 ), Y (V Y ) and the output V 36 , as they correspond to the states of the square wave input signal V IN . When the input signal V SIG  is at a low state, N 1  is high and N 2  is low. Latch  72  is reset by the high N 1  input. The high signal coupled to the S input of data selection circuit  74  incurs application of the high N 1  input to the Y line output. Logic circuit  50  outputs high level signals to FETs  42  and  44 , thereby coupling line  36  to the low reference line  34 . 
     At time t 1 , the input signal changes from a low level state to a high level state. In response, N 1  changes state from the high level to a low level with a slight delay, at t 2 , incurred by the transition of input stage FET  26  to the conductive state. N 2  reaches a high level at a later time, t 3 , when FET  24  is fully conductive. Latch  72  remains reset until N 2  goes high. The high to low transition of N 1  applied by the data selection circuit  74  is applied to the Y line during the reset time. At time t 3 , latch  72  is set, Q becomes low and /N 2  is output at the Y line. As /N 2  is low, Y remains low until the next input signal transition. The delay of the N 2  line transition does not affect the Y output. Thus, the state of output line  36  changes from the lower level to the higher level at time t 2 , at only a slight delay after the input signal transition. Logic circuit  50  outputs low level signals to FETs  42  and  44 , thereby coupling line  36  to the high reference line  32 . 
     At time t 4 , the input signal reverts to the low level state. N 2  goes low relatively quickly. N 1  reaches a high level upon full conduction of FET  22 , at time t 6 . As latch  72  remains set until N 1  goes high, /N 2  continues to be applied to the Y line by data selection circuit  74  until t 6 . As the state of /N 2  is the reciprocal of the state of N 2 , /N 2  attains a high state at t 5 , which is applied to the Y line, at only a slight delay after t 4 . Logic circuit  50  outputs high level signals to FETs  42  and  44 , thereby again coupling line  36  to the low reference line  34 . At t 6 , latch  72  is reset and the high N 1  input is output by the data selection circuit. The delay between t 4  and t 5  is of the same duration as the delay between t 1  and t 2 . Delay equalization circuit  70  thus provides for output of a symmetrical square wave waveform that corresponds to the input signal waveform with a minimum delay. 
     One example of circuit elements for implementing the delay equalization circuit is shown in  FIG. 6 . A series arrangement of PMOS FET  80  and NMOS FETS  82  and  84  in latch  72  is coupled across a power source. N 1  is coupled to the gate of FET  80  by the series arrangement of inverters  86 ,  88  and  90 . The gates of the complementary FETs  80  and  82  are connected to each other so that their conduction states are mutually exclusive. N 2  is coupled to the gate of FET  84  by the series arrangement of inverters  92 ,  94 ,  96  and  98 . The plurality of inverters in the N 1  and N 2  couplings are chosen to balance delays in the N 1  and N 2  signals. Coupled between the junction of the drains of FETs  80  and  82  and node Q of latch  72  is the parallel arrangement of inverters  100  and  102 . 
     Data selection circuit  74  includes a first pair of complementary FETs  104  and  106 , coupled in parallel, and a second pair of complementary FETs  108  and  110 , coupled in parallel. The gates of FETs  104  and  110  are connected together. The gates of FETs  106  and  108  are connected together. The Q node is coupled to the gates of FETs  106  and  108  through inverter  112 . Inverter  112  is coupled to the gates of FETs  104  and  110  through inverter  114 . N 1  is coupled to a first junction of FETs  104  and  106  by the series arrangement of inverters  116  and  118 . A second junction of FETs  104  and  106  is coupled to the Y output. N 2  is coupled to a first junction of FETs  108  and  110  by inverter  120 . A second junction of FETs  108  and  110  are coupled to the Y output. 
     The signals from inverters  112  and  114  are complementary to each other. When Q is high, a high signal is applied to the gates of FETs  104  and  110  and a low signal is applied to the gates of FETs  106  and  108 . For these states, the FETs  104  and  106  will be non-conductive and the FETs  108  and  110  will be conductive. /N 2  will be applied to the Y output line. When Q is low, FETs  104  and  106  will be conductive, FETs  108  and  110  will be non-conductive, and N 1  will be applied to the Y output line. 
     Operation of  FIG. 6 , utilized as the delay equalization circuit  70  of  FIG. 4 , corresponding to the waveforms of  FIG. 5 , is as follows. Prior to t 1 , the signal V SIG  is low. N 1  is at high level due to the conductive state of FET  22 . FET  80  is conductive by reason of the low level signal at its gate. Latch output Q is at the reset low level. FETs  104  and  106  are rendered conductive to apply the high N 1  signal to the Y output. The signal V 36  at line  36  is low as logic circuit  50  renders FET  44  conductive. 
     At time t 1 , V SIG  changes from a low to high. In response, N 1  changes state from high to low with a slight delay at t 2 . N 2  reaches a high level at a later time, t 3 , when FET  24  is fully conductive. During this time, the voltage level of N 2  is not sufficiently high to render FET  84  conductive and the latch output Q remains in the low reset state. FETs  104  and  106  remain conductive and the high to low transition of N 1  is applied to the Y output line. Logic circuit  50  renders FET  42  conductive, thus transitioning V 36  to the high state. At time t 3 , both FETs  82  and  84  are conductive, thus setting the latch to provide a high level at Q. FETs  104  and  106  are rendered non-conductive and FETs  108  and  110  are rendered conductive. The low level /N 2  signal is output to the Y line and V 36  remains high. 
     At time t 4 , the input signal reverts to the low level state. N 2  goes low at t 5 . FET  84  is rendered non-conductive. N 1  reaches a high level upon full conduction of FET  22 , at time t 6 . Between t 5  and t 6 , the latch output Q remains at the high set level to maintain application of /N 2  to the Y output line. Y transitions to a high level with the /N 2  transition at t 5 . Logic circuit  50  activates FET  44  to couple output  36  to the low level line  34 . V 36  attains the low state. At t 6 , FET is rendered conductive to reset the latch output Q to low. The high N 1  input is applied to the Y line and the low state of V 36  is maintained. 
     In this disclosure there are shown and described only preferred embodiments of the invention and but a few examples of its versatility. It is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. For example, the delay equalization advantages of the present disclosure are applicable not only to other power converters, such as boost and buck-boost converters, but also to any application in which a level shifting circuit is required. The specifically illustrated logic circuit, latch circuit and data selection circuit can be replaced with equivalent circuits that are operative to produce the functions described.