BUCK无隔离电路设计分析
– limits the load current
• Thermal shutdown
– turns the device off if the temperature exceeds a specified threshold
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– Losses and efficiency will be discussed
• Converters generate switching noise • Discrete filter components (L, C) are required • Higher switching frequency => smaller L, C
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Efficiency
Ig Io
Power supply
Vg + –
+ Vo _
µP/DSP core
output DC power Po Vo I o η= = = input DC power Pg Vg I g
0 1 D
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switch duty cycle
Switch-Mode Power Supplies
• Step-up, step-down and inverting configurations available • Switching converters are ideally 100% efficient • Real efficiency can be close to 100%; depends on operating conditions and implementation
Magnetic Buck Converters for Portable Applications
Frank De Stasi Mathew Jacob
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Outline
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. Why use Switching Regulators? Common Device/Converter Specifications Buck Converter Analysis CCM/DCM modes Selection of L and C Synchronous Buck Converters Conduction and Switching Losses Efficiency improvement using PWM/PFM/LDO modes Control Approaches Current Mode Models and Compensation Guidelines Transient Measurement Techniques Layout Guidelines
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Device/Converter Specifications
• Overvoltage protection
– prevents the output voltage from rising above a specified limit
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Impact of efficiency: a system example
uP/DSP core mode % of time in this mode Load current Io [mA] Linear regulator Efficiency [%] Battery current Ig [mA] Average Ig in this mode [mA] 4.45 Efficiency [%] Battery current Ig [mA] Average Ig in this mode [mA] 2.12 29.1 0.14 0.13 78.4 0.53 0.02 93.7 4.45 0.13 93.0 44.82 1.12 87.7 142.60 0.71 Stand-by 90.0 0.1 34.7 0.12 0.11 Wait 4.0 1.0 40.9 1.02 0.04 Run1 3.0 10.0 41.6 10.02 0.30 Run2 2.5 100.0 41.7 100.02 2.50 FullRun 0.5 300.0 41.7 300.02 1.50
• Dynamic voltage regulation
– “Load transient response,” including peak output voltage variation and settling time for a step load transient – “Line transient response,” including output voltage variation and settling time for a step input voltage transient
– Component selection will be discussed
• Duty cycle is the control variable • Closed-loop output voltage control is usually applied
– Dynamic models and control will be discussed
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Device/Converter Specifications
• Static voltage regulation
– DC output voltage precision, i.e., % variation with respect to the nominal value over: • input voltage range (“line regulation”) • output load range (“load regulation”) • temperature
Device/Converter Specifications
• Frequency synchronization
– allows synchronization of the switching frequency to an external system clock
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Linear voltage regulator as power supply
Series pass transistor
Q Iload + Vg + – C Vo – +
Load
-
Vref
Bandgap reference
Advantages of using SMPS over Linear regulators
• SMPS results in significantly lower average battery current • High efficiency over a wide range of loads and output voltages is achieved with a SMPS • SMPS with low quiescent current modes provide longer battery life for mobile systems that spend most of their time in “stand-by”
Linear regulator power model
Ig Rs Io + Vg + – IQ Vo –
Bias current
I g = Io + IQ
Efficiency:
Vo ?< Linear regulator efficiency cannot be greater Vg than the ratio of the output and the input voltage
Total linear reg average Ig [mA] SMPS
Total SMPS average Ig [mA]
Example: • Vg = 3.6 V • Vo = 1.5 V • 0 < Io < 300 mA
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Buck regulators in the system
Power distribution : Vg = 2.8-5.5 V
PS 3.6 V PS 2.5 V PS 1.5 V
Battery
Charger
Buck SMPS regulators
• Simple, low noise, small footprint area • Output voltage lower than the battery voltage • High efficiency only if Vo is close to Vg
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