直流电源稳压电路

电路能力测试-2：直流电源稳压电路

Circuit Knowledge Test-2: DC-DC LDO Regulator

1. What will be the DC voltage at pin-1 of IC LM7805?

2. What will be the supply current for DV 5V regulated output on this drawing?

3. If 78M05 with DPAK package dissipation 60°C/W, then what surface temperature will be for IC 78M05 at 25°C room temperature?

4. Same condition as the Question-3: If DC 5V output will supply 150 mA for all circuits, what surface temperature will be for IC 78M05 at 25°C room temperature?

5. What minimum input voltage will be allowed for 5V stabilized output if the minimum dropout voltage for LM78M05 is 1.5 Volt?

6. What will be the suitable resistance value and power rating of RP92 RP93 for lowest LM78M05 surface temperature theoretically at 100 mA supply current and 10 volt low line input?

试题解答

Circuit Knowledge Test-2: DC-DC LDO Regulator

ANS-1:

Vin = 12 – 34 * 0.084 = 12 – 2.86 = 9.14 Volt.

ANS-2:

Same as the input current 84 mA.

ANS-3:

Power loss = (9.14 – 5.0) * 0.084 = 0.35 W. then Tht = 0.35 * 60 = 21 °C

Final junction temperature Tj = 25 + 21 = 46 °C

ANS-4:

Vin = 12 – 34 * 0.15 = 6.9 Volt.

Power loss = (6.9 – 5.0) * 0.15 = 0.285 W. then Tht = 0.285 * 60 = 17 °C

Final junction temperature Tj = 25 + 17 = 42 °C

ANS-5:

Vmin = 34 * 0.084 + 1.5 + 5.0 = 9.36 Volt.

ANS-6:

Vdrop = 10 – 5.0 – 1.5 = 3.5 then R = 3.5/0.1 = 35 Ohm.

RP92 = RP93 = 68 Ohm.