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PB50 Datasheet(PDF) 4 Page - Cirrus Logic |
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PB50 Datasheet(HTML) 4 Page - Cirrus Logic |
4 / 5 page P r o d u c t I n n o v a t i o n F r o m PB50 4 PB50U GENERAL Please read Application Note 1 "General Operating Con- siderations" which covers stability, supplies, heat sinking, mounting, current limit, SOA interpretation, and specification interpretation. Visit www.Cirrus.com for design tools that help automate tasks such as calculations for stability, internal power dissipation, current limit; heat sink selection; Apex Precision Power’s completeApplication Notes library;Technical Seminar Workbook; and Evaluation Kits. CURRENTLIMIT For proper operation, the current limit resistor (R CL) must be connected as shown in the external connection diagram. The minimum value is 0.27Ω with a maximum practical value of 47Ω. For optimum reliability the resistor value should be set as high as possible. The value is calculated as follows: +I L= .65/R CL + .010, –IL = .65/RCL. SAFEOPERATINGAREA(SOA) NOTE:The output stage is protected against transient flyback. However, for protection against sustained, high energy flyback, external fast-recovery diodes should be used. COMPOSITEAMPLIFIERCONSIDERATIONS Cascading two amplifiers within a feedback loop has many advantages, but also requires careful consideration of several amplifier and system parameters.The most important of these are gain, stability, slew rate, and output swing of the driver. Operating the booster amplifier in higher gains results in a higher slew rate and lower output swing requirement for the driver, but makes stability more difficult to achieve. GAINSET R G = [ (Av-1) • 3.1K] – 6.2K R G + 6.2K Av = +1 3.1K The booster’s closed-loop gain is given by the equation above.The composite amplifier’s closed loop gain is determined by the feedback network, that is: –Rf/Ri (inverting) or 1+Rf/Ri (non-inverting). The driver amplifier’s “effective gain” is equal to the composite gain divided by the booster gain. Example: Inverting configuration (figure 1) with R i = 2K, R f = 60K, R g = 0 : Av (booster) = (6.2K/3.1K) + 1 = 3 Av (composite) = 60K/2K = - 30 Av (driver) = - 30/3 = -10 STABILITY Stability can be maximized by observing the following guidelines: 1. Operate the booster in the lowest practical gain. 2. Operate the driver amplifier in the highest practical effective gain. 3. Keep gain-bandwidth product of the driver lower than the closed loop bandwidth of the booster. 4. Minimize phase shift within the loop. A good compromise for (1) and (2) is to set booster gain from 3 to 10 with total (composite) gain at least a factor of 3 times booster gain. Guideline (3) implies compensating the driver as required in low composite gain configurations. Phase shift within the loop (4) is minimized through use of booster and loop compensation capacitors Cc and Cf when required. Typical values are 5pF to 33pF. Stability is the most difficult to achieve in a configuration where driver effective gain is unity (ie; total gain = booster gain). For this situation, Table 1 gives compensation values for optimum square wave response with the op amp drivers listed. DRIVER C CH C F C C FPBW SR OP07 - 22p 22p 4kHz 1.5 741 - 18p 10p 20kHz 7 LF155 - 4.7p 10p 60kHz >60 LF156 - 4.7p 10p 80kHz >60 TL070 22p 15p 10p 80kHz >60 For: R F = 33K, RI = 3.3K, RG = 22K Table 1: Typical values for case where op amp effective gain = 1. Figure 2. Non-inverting composite amplifier. SLEWRATE The slew rate of the composite amplifier is equal to the slew rate of the driver times the booster gain, with a maximum value equal to the booster slew rate. OUTPUTSWING The maximum output voltage swing required from the driver op amp is equal to the maximum output swing from the booster divided by the booster gain. The Vos of the booster must also be supplied by the driver, and should be subtracted from the available swing range of the driver. Note also that effects of Vos drift and booster gain accuracy should be considered when calculating maximum available driver swing. 10 20 30 40 50 100 200 300 SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE V S — VO (V) 1 2 3 .1 STEADY STATE T = 85°C C t = 50ms t = 100ms STEADY STATE T = 25°C C STEADY STATE T = 125°C C t = 200ms SOA OP AMP PB50 C F R F R I +15V –15V IN COM +Vs –Vs R CL OUT C C R G R L V IN GAIN COMP C CH |
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