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NMOS与PMOS LDO的概述
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To be able to explain the differences between NMOS and PMOS Linear Voltage Regulators; their basic operation, advantages and limitations, as well as identifing applications where one, or the other, would be appropriate choice.
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NMOS and PMOS Linear Voltage Regulators
Course Objective:
To be able to explain the differences between NMOS and PMOS Linear Voltage Regulators; their basic operation, advantages and limitations, as
well as identifing applications where one, or the other, would be appropriate choice.
Course Map/Table of Contents
1. Course Navigation
1.1 Course Navigation1.
2. Linear Voltage Regulator Basics
2.1 Introduction1.
2.2 The Linear Voltage Regulator2.
2.3 Simple Model3.
2.4 Simple Model, with variables4.
2.5 Simple Model, with values5.
2.6 Simple Model, with a change of Load Current6.
2.7 Simple Model, with a change in Input Voltage7.
2.8 The Control Loop8.
2.9 Simple Model, with Control Loop blocks9.
2.10 The Basic Real-World Model10.
3. So what's the difference?
3.1 The Bipolar model1.
3.2 The Bipolar model2.
3.3 The CMOS model3.
3.4 The CMOS model4.
4. Types of NMOS and PMOS Linear Voltage Regulators
4.1 Two Types1.
4.2 Standard Regulator2.
4.3 LDO Regulators3.
4.4 The Differences to consider4.
5. Standard NMOS Voltage Regulators
5.1 Introduction to the standard NMOS1.
5.2 Losses in the standard NMOS2.
5.3 Simple Model of losses in the standard NMOS3.
5.4 Driving the standard NMOS Pass Element4.
5.5 NMOS Gate Drive vs Low Load Current5.
5.6 NMOS Gate Drive vs High Load Current6.
5.7 Standard NMOS Output Capacitor Requirements7.
5.8 Standard NMOS Summary8.
6. NMOS "Dual-Rail" Voltage regulators
6.1 Introduction to NMOS "Dual-Rail" Voltage Regulator1.
6.2 Losses in the NMOS "Dual-Rail"2.
6.3 Simple Model of Losses in the NMOS "Dual-Rail" LDO3.
6.4 Driving the NMOS "Dual-Rail" Pass Element4.
6.5 Gate Drive vs Low Load Current5.
6.6 Gate Drive vs High Load Current6.
6.7 NMOS 'Dual-Rail' Output Capacitor Requirements7.
6.8 Summary8.
7. PMOS LDO Voltage Regulators
7.1 Introduction to PMOS LDO Voltage Regulators1.
7.2 Losses in the PMOS LDO drive circuitry2.
7.3 Simple Model of Losses in the PMOS LDO Regulator3.
7.4 Driving the PMOS LDO Pass Element4.
7.5 Gate Drive vs Low Load Current5.
7.6 Gate Drive vs High Load Current6.
7.7 PMOS LDO Output Capacitor Requirements7.
NMOS and PMOS Linear Voltage Regulators Copyright © 2010 by National Semiconductor Corporation All rights reserved
7.8 Summary8.
8. Package Thermals
8.1 Package Limitations1.
8.2 Power Dissipation Variables2.
8.3 Power Dissipation Calculation3.
8.4 Junction Temperature Rise4.
8.5 Maximum Ambient Temperature5.
8.6 The Operating Junction Temperature6.
9. Summary
9.1 Summary1.
9.2 Selecting the Best Regulator For Your Application2.
1. Course Navigation
1.1 Course Navigation
1.1 Course Navigation
This course is organized like a book with multiple chapters. Each chapter may have one or more pages.
The previous and next arrows move you forward and back through the course page by page.
The left navigation bar takes you to any chapter. It also contains the bookmarking buttons, 'save' and 'go to.' To save your place
in a course, press the 'save' button. The next time you open the course, clicking on 'go to' will take you to the page you saved or
bookmarked.
The top services bar contains additional information such as glossary of terms, who to go to for help with this subject and an
FAQ. Clicking home on this bar will take you back to the course beginning.
Don't miss the hints, references, exercises and quizzes which appear at the bottom of some pages.
2. Linear Voltage Regulator Basics
This chapter will discuss how a linear voltage regulator works
2.1 Introduction
2.2 The Linear Voltage Regulator
2.3 Simple Model
2.4 Simple Model, with variables
2.5 Simple Model, with values
2.6 Simple Model, with a change of Load Current
2.7 Simple Model, with a change in Input Voltage
2.8 The Control Loop
2.9 Simple Model, with Control Loop blocks
2.10 The Basic Real-World Model
2.1 Introduction
What is a voltage regulator?
Every electronic circuit is designed to operate off of some supply voltage, which is usually assumed to be constant.
A Linear Voltage Regulator provides this constant DC output voltage and contains circuitry that continuously holds the output voltage at
the design value regardless of changes in load current or input voltage.
This assumes that the load current and input voltage are within the specified operating range for the device.
The linear regulator is the basic building block of nearly every power supply used in electronics.
The IC linear regulator is so easy to use that it is virtually foolproof, and so inexpensive that it is usually one of the cheapest
components in an electronic assembly.
Linear Voltage Regulators are 'step-down' devices, that is the output voltage is always lower than the input voltage.
2.2 The Linear Voltage Regulator
What is a Linear Voltage Regulator?
A linear regulator operates by using a voltage-controlled current source to force a fixed voltage to appear at the regulator output
terminal.
The control circuitry continuosly monitors (senses) the output voltage, and adjusts the current source (as required by the load) to hold
the output voltage at the desired value.
The design limit of the current source defines the maximum load current the regulator can source and still maintain regulation.
The output voltage is controlled using a feedback loop, which requires some type of compensation to assure loop stability.
Most linear regulators have built-in compensation, and are completely stable without external components.
Some regulators (like Low-Dropout types), do require some external capacitance connected from the output lead to ground to assure
regulator stability.
2.3 Simple Model
A basic (first order) linear voltage regulator can be modeled with two resistors and a power supply for V
IN
.
Knowing the required output voltage (V
OUT
), the load current (I
LOAD
), along with the input voltage (V
IN
), is essential to creating the
model.
2.4 Simple Model, with variables
In reality, the only constant is the output voltage, V
OUT
.
Everything else can, and will, be constantly changing.
The input voltage may have changes due to outside influences, the load current may change due to a dynamic change in the behaviour of the
load.
Changes in these variables can all happen simultaneously, and the value needed for R
PASS
to hold V
OUT
at a constant value will need to change
as well.
2.5 Simple Model, with values
For the first example, we will assign typical operating values and calculate the value needed for the series pass element R
PASS
.
V
IN
= 12V
V
OUT
= 5V
I
LOAD
= 50 mA
With V
IN
= 12V and V
OUT
= 5V, the voltage across R
PASS
= (12V - 5V) = 7V
With the current through R
PASS
= I
LOAD
= 50 mA, the needed resistance for R
PASS
= (7V / 50mA)= 140 Ohms
2.6 Simple Model, with a change of Load Current
For the second example, we will change the load current from 50mA to 500mA and calculate the value needed for the series pass element R
PASS
.
V
IN
= 12V
V
OUT
= 5V
I
LOAD
= 500 mA
With V
IN
= 12V and V
OUT
= 5V, the voltage across R
PASS
= (12V - 5V)= 7V
With the current through R
PASS
= I
LOAD
= 500 mA, the needed resistance for R
PASS
= (7V / 500mA)= 14 Ohms
2.7 Simple Model, with a change in Input Voltage
For the third example, we will change the input voltage from 12V to 22V and calculate the value needed for the series pass element R
PASS
.
V
IN
= 12V
V
OUT
= 5V
I
LOAD
= 50 mA
With V
IN
= 12V and V
OUT
= 5V, the voltage across R
PASS
= (12V - 5V)= 7V
With the current through R
PASS
= I
LOAD
= 50 mA, the needed resistance for R
PASS
= (7V / 50mA) = 140 Ohms
2.8 The Control Loop
It has been shown that the resistance of series pass element, R
PASS
, needs to change as the operating conditions change.
This is accomplished with a control loop.
The error amplifier monitors the sampled output voltage, compares it to a known reference voltage, and actively changes R
PASS
to keep V
OUT
constant.
A characteristic of any linear voltage regulator is that it requires a finite amount of time to "correct" the output voltage after a change in
load current demand.
This "time lag" defines the characteristic called transient response, which is a measure of how fast the regulator returns to
steady-state conditions after a load change.
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