The output rate of change strongly depends on the input differential voltage. The maximum output rate of change is known as the slew rate. Figure 5-21 shows a generic simplified block diagram of an operational amplifier with the Miller compensation capacitor C_M and measured data of the Output Rate of Change vs Input Differential Voltage (V_ID). With slew boost architectures the compensation capacitor, C_M known as the miller capacitor, is charged in 3 discrete regions with varying current ${\mathrm{I}}_{{\mathrm{C}}_{\mathrm{M}}}$ , where the output rate of change is $\frac{\mathrm{d}{\mathrm{V}}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{\mathrm{d}\mathrm{t}}\approx \frac{{\mathrm{I}}_{{\mathrm{C}}_{\mathrm{M}}}}{{\mathrm{C}}_{\mathrm{M}}}$ . Note ${\mathrm{I}}_{{\mathrm{C}}_{\mathrm{M}}}=\mathrm{ }{\mathrm{I}}_{\mathrm{g}\mathrm{m}\_\mathrm{m}\mathrm{a}\mathrm{x}}\mathrm{ }+\mathrm{ }{\mathrm{I}}_{\mathrm{b}\mathrm{o}\mathrm{o}\mathrm{s}\mathrm{t}}$ .

1. Small signal output rate of change (linear region): $\frac{\mathrm{d}{\mathrm{V}}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{\mathrm{d}\mathrm{t}}\approx \frac{{\mathrm{I}}_{\mathrm{g}\mathrm{m}}}{{\mathrm{C}}_{\mathrm{M}}}\mathrm{ }$ ; ${\mathrm{I}}_{\mathrm{b}\mathrm{o}\mathrm{o}\mathrm{s}\mathrm{t}}=0\mathrm{ }\mathrm{m}\mathrm{A}$ and ${\mathrm{I}}_{\mathrm{g}\mathrm{m}}<\mathrm{ }{\mathrm{I}}_{\mathrm{g}\mathrm{m}\_\mathrm{m}\mathrm{a}\mathrm{x}}\mathrm{ }$
   • Note that the relationship between V_ID and ${\mathrm{I}}_{\mathrm{g}\mathrm{m}}$ is linear in this region. See the green region in Figure 5-21. Typically, small signals are less than 100mV. For this device the small signal region is approximately 20mV and less.
2. Moderate input differential voltages (nonlinear region): $\mathrm{N}\mathrm{a}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{a}\mathrm{l}\_\mathrm{S}\mathrm{R}\approx \frac{{\mathrm{I}}_{\mathrm{g}\mathrm{m}\_\mathrm{m}\mathrm{a}\mathrm{x}}}{{\mathrm{C}}_{\mathrm{M}}}$ ; ${\mathrm{I}}_{\mathrm{b}\mathrm{o}\mathrm{o}\mathrm{s}\mathrm{t}}=0\mathrm{ }\mathrm{m}\mathrm{A}$ and ${\mathrm{I}}_{\mathrm{g}\mathrm{m}}=\mathrm{ }{\mathrm{I}}_{\mathrm{g}\mathrm{m}\_\mathrm{m}\mathrm{a}\mathrm{x}}$
   • There is a rising slope as ${\mathrm{I}}_{\mathrm{g}\mathrm{m}}\mathrm{ }\mathrm{a}\mathrm{p}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{a}\mathrm{c}\mathrm{h}\mathrm{e}\mathrm{s}\mathrm{ }{\mathrm{I}}_{\mathrm{g}\mathrm{m}\_\mathrm{M}\mathrm{a}\mathrm{x}}$ . See the blue region in Figure 5-21. For this device the natural slew is 0.5V/μs and occurs for input signals of 20mV to 150mV.
3. Large input differential voltages (nonlinear region):$\mathrm{B}\mathrm{o}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{d}\_\mathrm{S}\mathrm{R}\approx \frac{{\mathrm{I}}_{\mathrm{g}\mathrm{m}\_\mathrm{m}\mathrm{a}\mathrm{x}}\mathrm{ }+\mathrm{ }{\mathrm{I}}_{\mathrm{b}\mathrm{o}\mathrm{o}\mathrm{s}\mathrm{t}}}{{\mathrm{C}}_{\mathrm{M}}}$ .
   • The slew boost circuitry is activated and additional current ${\mathrm{I}}_{\mathrm{b}\mathrm{o}\mathrm{o}\mathrm{s}\mathrm{t}}$ helps charge the compensation capacitor quickly. See the red region in Figure 5-21. For this device the boosted slew rate is 30V/µs and occurs for input signals greater than 150mV. Technically the boosted slew is increasing from approximately 150mV to 1V, and for input signals greater than 1V the max the slew boost is achieved.

Figure 5-22 below shows the Output Rate of Change vs Input Differential Voltage comparison of the New device vs the Old device.