Post on 03-Jan-2016
Technical Review of Linear Regulator Operation & Performance1.Dropout Volta
ge4.Efficiency 7.Transient Resp
onse
2.Quiescent current & Ground cu
rrent
5.Load Regulator
8.Frequency
Response
3.LDO Topology
6.Line Regulation
9.Range of ESR
1.Dropout Voltage
• Dropout voltage is the input-to-output voltage difference at which the circuit cease to “regulate” against further reductions in input voltage.
2.Quiescent Current & Ground Current
• Quiescent current or ground current is the difference between input and output currents.
• Quiescent consists of bias current & drive current of pass element.– Bias current
• Band-gap reference.• Sample resistor.• Error amplifier.
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Summary
• PNP bipolar good for low dropout application,but it got high quiescent current and low efficiency.
• NMOS most advantage because of its low on resistance.gate drive difficulties make it less than ideal.
• PMOS has been highly developed,and now the performance level exceeding most bipolar devices.
5.Load Regulation
• Load regulation is a measure of the circuit ability to maintain the o/p under varying load condition.
Load Regulation
• Since load regulation is steady-state parameter,all frequency components are neglected.
• The load regulation is limited by the open loop current gain of the system.
6.Line Regulation
• Line regulation is a measure of the circuit ability to maintain the o/p under varying line input voltage.
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Line Regulation
• Since line regulation is steady-state parameter,all frequency components are neglected.
• Increase dc open loop gain increase line regulation.
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7.Transient Response
• Transient response is the maximum allowable o/p volt variation for a load current step change.
• The transient response is a function of output Cap value(Co),the ESR ,the bypass Cap(Cb),and maximum load current Io(max).
In addition,another pole exist from the input impedanceOf the pass element,and the o/p impedance of the errorAmplifier.Rpar,Cpar.
9.Range of ESR
• A LDO needs would require an o/p cap and an o/p ESR to stabilize the control loop.
The linear regulator is unstableBecause the phase shift at unit gain is –180,to make it stable A zero must be added.
With External Compensation
• The zero produced by the ESR locates before the UGF,so,the phase shift at UGF will be around –90.