CIRCUIT ALLOWS SLEW RATE CONTROL WITH INDEPENDENT RISE AND FALL RATES

This circuit (figure 1) will impose a maximum slew rate on a signal; positive and negative rates can be independently controlled. The circuit is useful in servo applications where the error signal needs to be limited to be within the power rails to ensure predictable operation.

The current supplied to C1 is limited by constraining the voltage across R9 within a fixed range. The extremes of the range determine the positive and negative slew rates. Keeping a constant voltage across R9 causes a constant current to flow into C1 producing a linear ramp in its voltage.

Consider an input step from 0 to 5 volts and V_s of 3 volts: the output of IC1 will approach the positive rail and V_j initially will be limited to 3 volts plus a diode forward voltage drop (V_D), (R8 is included only to ensure that V_j is properly clamped), the current through R9 will therefore be:

		Vs + VD		3 + 0.7
IR9	=		=		mA	=	0.37 mA
		R9		10

and,

dVc		IR9
	=		=	37 Vs-1
dt		C1

As V_c rises it is necessary to add it to V_s so that the voltage across R9 remains constant; this addition is effected by IC2. The ramp action stops when V_c = V_i .

A similar argument can be applied to negative changes in V_i ; in the circuit shown the negative slew rate will be double the positive one because of the x2 inverting amplifier IC6.

The diode forward voltage drops have little effect on the operation and even a small voltage change at the input (<0.7V) will cause heavy forward biasing of one of the diodes provided that the rate of change of V_i exceeds the maximum. However, since the slew rates are related to V_D they will be somewhat temperature dependent; adding two more amplifiers to make precision clamps as shown in figure 2 allows the slew rates to be more accurately controlled since the effective V_D is reduced to almost zero (by a factor equal to the open loop gain of the amplifiers).

 

Figure 1: Basic Slew Rate Limiting Circuit

 

Figure 2: Improved Current Limit