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Question 22.12: What is the load current in Fig. 22-21? What is the output r...

What is the load current in Fig. 22-21? What is the output ripple?

22.21
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The LM7812 produces a regulated output voltage of +12 V. Therefore, the load current is:

I_{L}=\frac{12 \ V}{100 \ \Omega }=120 \ mA

We can calculate the peak-to-peak input ripple with the equation given in Chap. 4:

V_{R}=\frac{I_L}{fC}=\frac{120 \ mA}{(120 \ Hz)(1000 \ μF) }=1 \ V

Summary Table 22-1 shows a typical ripple rejection of 72 dB for the LM7812. If we convert 72 dB mentally (60 dB + 12 dB), we get approximately 4000. With a scientific calculator, the exact ripple rejection is:

Summary Table 22-1 Typical Parameters of Popular IC Voltage
Regulators at 25°C
Number V_{out}, V   I_{max}, A  Load Reg, mV Line Reg, mV  Rip Rej, dB Dropout, V   R_{out}, mΩ  I_{SL}, A
LM7805 5 1.5 10 3 80 2 8 2.1
LM7806 6 1.5 12 5 75 2 9 0.55
LM7808 8 1.5 12 6 72 2 16 0.45
LM7812 12 1.5 12 4 72 2 18 1.5
LM7815 15 1.5 12 4 70 2 19 1.2
LM7818 18 1.5 12 15 69 2 22 0.2
LM7824 24 1.5 12 18 66 2 28 0.15
LM78L05 5 100 mA 20 18 80 1.7 190 0.14
LM78L12 12 101 mA 30 30 80 1.7 190 0.14
LM2931  3 to 24 102 mA 14 4 80 0.3 200 0.14
LM7905  −5 1.5 10 3 80 2 8 2.1
LM7912  −12 1.5 12 4 72 2 18 1.5
LM7915  −15 1.5 12 4 70 2 19 1.2
LM317  1.2 to 37 1.5 0.30% 0.02%/V 80 2 10 2.2
LM337  −1.2 to −37 1.5 0.30% 0.01%/V 77 2 10 2.2
LM338  1.2 to 32 5 0.30% 0.02%/V 75 2.7 5 8

RR=antilog\frac{72 \ dB}{20}=3981

The peak-to-peak output ripple is approximately:

V_R=\frac{1 \ V}{4000}= 0.25 \ mV

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