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Push-Pull Output

Represent CMOS complementary output stage behaviorally

Library

Integrated Circuits

Description

The Push-Pull Output block represents a CMOS complementary output stage behaviorally. To improve simulation speed, the block does not model all the internal individual MOSFET devices that make up the gate. You can use this block to create a representative output current-voltage relationship when defining an integrated circuit model behavior with Physical Signal blocks from the Simscape™ Foundation library.

You can choose between are two output current-voltage relationships:

  • Linear — The block represents the output as a voltage source plus series resistance and parallel capacitance, as shown in the following figure. The value you specify for the Output resistance parameter is assigned to the series resistance, and the capacitance values are determined by matching the RC time constant to the Propagation delay parameter value.

    The input to the Controlled Voltage Source block is limited to be between the supply rails, and it is also inverted by subtraction from the supply voltage. The inversion makes it behave like a complementary output stage, with a high gate-source voltage resulting in a low output.

  • Quadratic — The output stage is modeled by the two MOSFETs that constitute the complementary pair. The MOSFET parameters are derived from the output resistance values and short-circuit currents that you specify as mask parameters. The gate input demand is lagged to approximate the Propagation delay parameter value.

Both Linear and Quadratic output models add an offset and scale the physical input X so that the gate voltage is given by:

Vg = k · ( X + c )

where

  • k is the input signal scaling.

  • c is the input signal offset.

The offset and scaling can be used, for example, to match logical values for X (that is, range [0,1] ) to [V-, V+] at the output pin. For example, if V+ = 10V and V- = 0, then to match the signal logical values to this voltage range, set c = -1 and k = -10.

For both Linear and Quadratic output models, the protection diodes D1 and D2 act to limit the output voltage range. These diodes are Diode blocks from the Simscape Foundation library, that is, piecewise linear diodes defined by their forward voltage and on resistance. If the voltage across D1 rises above the forward voltage, then the diode starts to conduct, and provided that the on resistance is low, it effectively prevents the output rising above V+ plus the diode forward voltage drop. An equivalent behavior results if the output voltage drops too low.

The output model is very similar to that used for the logic blocks. For a plot of a typical output V-I characteristic when using the Quadratic output model, see Selecting the Output Model for Logic Blocks.

Basic Assumptions and Limitations

The Push-Pull Output block has the following limitations:

  • The block does not accurately model dynamic response.

  • The Quadratic output model does not model any output capacitance effects. Add output capacitance externally to the block if required.

Dialog Box and Parameters

Input Scaling Tab

Input signal scaling, k

The input physical signal X is mapped to the gate voltage by Vg = k · ( X + c ), where k is the input signal scaling. Use this parameter in conjunction with the Input signal offset, c to map the range of X to the voltage range defined by the power supply. The default value is 1 V.

Input signal offset, c

The input physical signal X is mapped to the gate voltage by Vg = k · ( X + c ), where c is the input signal offset. Use this parameter in conjunction with the Input signal scaling, k to map the range of X to the voltage range defined by the power supply. The default value is 0.

Output Characteristics Tab

Output current-voltage relationship

Select the output model, Linear or Quadratic output model options. If Linear, the output voltage drops linearly with output current. If you select Quadratic, then the output voltage dependency on output current is defined by the quadratic I-V characteristics of the two output MOSFET devices. The default value is Linear.

Output resistance

Defines one over the slope of the output I-V characteristic. This parameter is available when you select the Linear option for the Output current-voltage relationship parameter. The default value is 25 Ω.

Power rail voltages, [V- V+], used for measurements

Defines the rail voltages for which mask data output resistances and currents are defined. This parameter is available when you select the Quadratic option for the Output current-voltage relationship parameter. The default value is [ 0 5 ] V.

Output resistance values at zero output current and at I_OH when Vg=V-

A row vector [ R_OH1 R_OH2 ] of two resistance values. The first value R_OH1 is the gradient of the output voltage-current relationship when the complementary pair output is HIGH (Vg=V-) and there is no output current. The second value R_OH2 is the gradient of the output voltage-current relationship when the output is HIGH and the output current is I_OH. This parameter is available when you select the Quadratic option for the Output current-voltage relationship parameter. The default value is [ 25 250 ] Ω.

Output current I_OH when output is shorted to V- and Vg=V-

The resulting current when the output is HIGH (Vg=V-), but the load forces the output voltage to the negative supply rail. This parameter is available when you select the Quadratic option for the Output current-voltage relationship parameter. The default value is 63 mA.

Output resistance values at zero output current and at I_OL when Vg=V+

A row vector [ R_OL1 R_OL2 ] of two resistance values. The first value R_OL1 is the gradient of the output voltage-current relationship when the complementary pair output is LOW (Vg=V+) and there is no output current. The second value R_OL2 is the gradient of the output voltage-current relationship when the output is LOW and the output current is I_OL. This parameter is available when you select the Quadratic option for the Output current-voltage relationship parameter. The default value is [ 30 800 ] Ω.

Output current I_OL when output is shorted to V+ and Vg=V+

The resulting current when the output is LOW (Vg=V+), but the load forces the output voltage to the positive supply voltage. This parameter is available when you select the Quadratic option for the Output current-voltage relationship parameter. The default value is -45 mA.

Propagation delay

Time it takes for the output to reach 63.2% of its final value following a step change in the input, X. For Quadratic output, it is implemented by the lagged gate input demand. The default value is 25 ns.

Protection diode on resistance

The gradient of the voltage-current relationship for the protection diodes when forward biased. The default value is 5 Ω.

Protection diode forward voltage

The voltage above which the protection diode is turned on. The default value is 0.6 V.

Supply Voltage Tab

Negative power rail voltage, V-

Negative power supply voltage applied to the N-channel MOSFET source pin. The default value is 0 V.

Positive power rail voltage, V+

Positive power supply voltage applied to the P-channel MOSFET source pin. The default value is 5 V.

Initial Conditions Tab

Initial output voltage

This parameter is visible when you select the Linear option for the Output current-voltage relationship parameter on the Output Characteristics tab. The parameter is used to set the voltage on the output capacitors so that the output voltage is initialized to the parameter's value. The default value is 0 V.

Initial input signal

This parameter is visible when you select the Quadratic option for the Output current-voltage relationship parameter on the Output Characteristics tab. The parameter is used to initialize the propagation delay first-order lag such that there is no transient at time zero. The default value is 0 V.

Ports

The block has one input physical signal port X and one electrical conserving port that outputs the resulting voltage.

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