DC/DC Converters 101
Understanding Power Supply Basics and Terminology
Brian King
Agenda
• Lecture
• Overview
• Linear Regulators
• Switching Power Supplies
• Topologies
• Synchronous vs. Non-synchronous
• Controller vs. Converter
• Selecting the Best Power Solution
Why should I care about power?
1. Every electronic system uses power.
2. Your power source never matches your system needs.
Power Source
What you need
Typically
5V,12V or 24V
6.0Vdc-16Vdc
40Vdc Surge
3.0Vdc-4.2Vdc
DC/DC Supply
gets you from
here to there
1.2V Core @ 2A
2.5V I/O @ 1.2A
3.3V
5V
+/-12V
3
Linear Regulators vs. Switching Supplies
• Linear Regulator
– Pass element operates in the linear region
– Down conversion only
INPUT
Filtering
Filtering
OUTPUT
Pass Element(s)
• Switching Power Supply
– Pass elements switch, turning fully on/off each cycle
– Filtering includes an inductor
– Multiple topologies (Buck, Boost, Buck-boost…)
4
Linear Regulator
ADVANTAGES:

Low O/P ripple & noise

Fast transient response

Low cost (for low power, at least)

Easy to design

No EMI to worry about
DISADVANTAGES:

Low efficiency at VIN>>VOUT

High dissipation (needs large heat-sink)

VOUT<VIN – always!
APPLICATIONS:

Extremely low ripple & noise apps

Low input to output voltage difference

Tight regulation

Fast transient response
5
Dropout Voltage
• Dropout (headroom): The minimum required voltage
across an LDO to maintain regulation
+ Vdo -
Example:
– Vin = 3.1V to 4.2V
– Vout = 2.5V @ 100mA
– Need at least 600mV headroom
6
Linear Regulator vs LDO
• Linear Regulator has Higher Dropout Voltage.
– Transistor or Darlington pair pass element
– LM317 (1.5A linear regulator)
• 1.5V to 2.5V dropout voltage
• Good for larger Vin to Vout ratios, 12V to 5V output
• CHEAP!!!
• LDO = Low Dropout Regulator
– Typically higher performance
• PSRR, regulation tolerance, transient response, etc
– MOSFET pass element
– TPS72501 (1A LDO)
• 170mV dropout voltage
• Good for 3.3V to 3.0V output
7
Linear Regulator Power Dissipation
Input Current = Output Current
Efficiency

P out
P in

V out  I out
V in  I in

V out
V in
Power Loss = Iout * (Vin – Vout)
• Power loss is usually a limiting factor!
8
Linear Regulator vs Switcher
2.5W LDO +
ground plane
as heat sink
6W Switcher
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Switcher
DC
VIN
DC
VOUT
ADVANTAGES:
 High efficiency
 VOUT>=<VIN
 Wide input voltage range
 Low power dissipation (small heatsink)
 High Watt/cm2
 Isolation possible (with transformer)
 Multiple O/Ps possible (with transformer)
DISADVANTAGES:
 EMI
 Slower transient response
 More difficult to design
 Higher output ripple & noise
APPLICATIONS:
 High efficiency power supplies
 High ambient temperatures
 Large input to output voltage difference
 Space constraints
 High output power
10
Basic Topologies
Buck
VIN
VOUT
V OUT  D  V IN
Boost
VIN
V OUT  V IN ,
VOUT
V OUT  VIN ,
V OUT 
Buck/Boost
VIN
VOUT
VIN
1 D
V OUT  ,   V IN ,
V OUT 
 D  V IN
1 D
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Synchronous vs Non Sync
Non-Synchronous Buck
Non-synchronous
L
Q1
D1
C0
Synchronous Buck
Synchronous
L
Q1
Q2
1. Diode voltage drop is fairly
constant with output current
2. Less efficient
3. Less expensive
4. Used with higher output
voltages
C0
1. MOSFET has lower voltage
drop
2. More efficient
3. Requires additional control
circuitry
4. Costs more
12
Synchronous vs Non Sync
Vin=5V
Vout=1V
Iout=1A
Rdson_sync=0.12ohm
1V Output Synchronous
PFET_SYNC
Iout
PFET_SYNC
1A 
PFET_SYNC
0.096W

88%
2
1  D  Rdson
0.8  0.12
2
Vf_diode=0.5V
1V Output Non-Synchronous
Pdiode
Idiode_avg Vdiode
Pdiode
( 1  D)  Iout 0.5V
Pdiode
0.4W

69.4%
Sync vs Non-sync is less of an issue with higher Vout
Higher duty cycles = less power dissipation in Sync FET or Catch Diode
13
Synchronous vs Non Sync
Power FET
Synchronous FET
14
Synchronous vs Non Sync
Integrated Power FETs
Rectifier Diodes
Integrated Power FET
and synchronous FET
15
Controller vs Converter
• Controller
–
–
–
–
Discrete MOSFETs
Provides the “brains” to control the power stage
More complicated to design
Full control over FET selection, switching frequency, overcurrent,
compensation, softstart
– Can tailor the power supply to meet your specific needs
• Converter (Fully integrated)
–
–
–
–
Integrated switches
“plug and play” design
Limited range of output filter components
Limited control over functionality
• Converter (Partially integrated)
–
–
–
–
May offer full or partial feature set , internal or external compensation
Internal Power FET, external sync-FET or catch diode
Limited control over frequency, overcurrent, softstart, etc
Allows wider range of output filter components
16
Converter (Fully Integrated)
TPS62293
2.3V to 6V input
1A Output Current
2.25MHz
Everything is integrated, minimum external components
17
Converter (Partially Integrated)
TPS54620
4.5V to 17V input
6A Output Current
Internal FETs, external SoftStart, Compensation, Frequency set… more flexibility
Set frequency
Compensation
18
Controller
TPS40303/4/5
3V to 20V input
10A Output Current
300kHz to 1.2MHz
External FETs
Compensation
Softstart
Current limit
19
Size vs. Cost vs. Efficiency
Efficiency
Cost
Synchronous
Non-synchronous
Linear Regulator
Power
Density
Cost
Converter (Fully Integrated)
Converter (Partially Integrated)
Controller
20
Efficiency vs Vout
• Efficiency depends on output voltage?
The datasheet says:
• Why isn’t MY supply 95% efficient?
21
Efficiency vs Vout
Simplified power dissipation
equations assuming no
inductor current ripple
3.3V Output
1V Output
Power FET
Conduction
Losses
Sync FET
Conduction
Losses
Total FET
Losses
0.173 W
0.136 W
(does not include
other circuit losses)
22
Efficiency vs Vout
3.3V Output
1V Output
T P S 6 2 4 0 0 E ffic ie n c y v s Vo u t (Vin =5 V,Io u t=3 0 0 m A )
9 4 .5
94
E ffic ie n c y (% )
9 3 .5
93
9 2 .5
92
9 1 .5
91
9 0 .5
90
8 9 .5
1 .5
2
2 .5
3
3 .5
Vo u t (V)
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PWM vs PFM
• Pulse Width Modulation
– Constant frequency
– Low output voltage ripple
– Used with high output currents
• Pulse Frequency Modulation
– Varying frequency with Vin and load
– Very high efficiency at very light loads
– Higher output voltage ripple
– Potential operation in audio range
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PWM vs PFM
100
90
PFM mode
VO U T 2 = 1 .8 V
80
V IN = 2 .7 V
E ffic ie n c y (% )
70
60
50
V IN = 3 .6 V
V IN = 5 .0 V
VIN = 2 .7 V
P o w e r-S a v e M o d e (P S M )
VIN = 3 .6 V
40
VIN = 5 .0 V
30
PWM mode
F o rc e d P W M M o d e
20
10
0
0 .1
1
10
100
1000
L o a d C u rre n t, I O U T (m A )
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Startup - Softstart
–
–
–
–
Slowly turning on the power supply
Controlled rise of output voltage
Minimizes inrush currents
Minimizes system level voltage drops
• Pulling high currents out of input bus
• High impedance batteries
– Internal vs SS capacitor
• Larger SS capacitor = longer softstart time
26
Startup - Sequencing
• Sequencing
– Controlling the order that different power supplies
are turned on
– Important for uP loads
– Minimizing overall inrush current
Sequential sequencing
27
Startup - Sequencing
• Ratiometric Sequencing
• Simultaneous Sequencing
28
Easy Answers – Power Quick Search
• Provides a list of possible linear regulators, controllers
and converters based on inputs
• Great starting point for selecting a device
29
Easy Answers – Power Quick Search
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More Answers – Browse The Product Tree
31
Easy (Simulated) Answers – WEBench
• Provides a complete design based on inputs
• Best for customers with little or no power background
32
Easy (Real) Answers – TI Designs/PowerLAB
• Searches reference designs based on input
33
THANKS!!
Questions???
ufseniordesignanaloghelp@list.ti.com
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