Driving high power LED from DC
LEDs current regulation
Voltage regulation is not
practicable because of the
exponential, temperature
dependent, voltage-current
LED characteristic.
With voltage regulation
4% voltage change
3.21V to 3.34V
 50% current change
400mA to 600mA
With voltage regulation
20C temp. change
 100mA current change
Constant current or constant
voltage?
• Constant Voltage
– Inexpensive
– Simple control
– Poor control of lumens output
• Constant Current
– Best way to control and maintain lumens
output
Driving LEDs – series, parallel or
both?



Easiest to implement and
Control
Limited by converter
topology or possibly Safety
voltage limitations
Current matching



Low Cost
Poor Regulation
Lossy w/o Op Amp




Best Parallel Control
Most Expensive
Need matching MOSFETs
Linear (lossy)
To be avoided:
• Possible strings brighter
than others due to ΣVf
variations
• At low brightness level,
some LEDs will end up
output no light before
others randomly in
strings.
LED post regulation
• DC - DC LEDs driver
• Linear LEDs Driver
• DC-DC design example
Selecting DC- DC driver LEDs
Requested lighting
performances
Determine LEDs:
• Requested lumens
• Electrical characteristics
• Vf and If
• Color
• # of LEDs
Note: Vo = (#of LEDs x VF) + VSNS
Determinare le features:
• Vin range
• Dimming (PWM or analog)
• Thermal feedback
• Constant frequency
• Constant ripple
• … etc
BOOST:
Vo > Vin
Determinare driver
Topology:
• # di path( IF, current
matching, high Vin)
BUCK:
Vo < Vin
BUCK-BOOST:
Vo<Vin and Vo>Vin
Controller or Regulator:
• Output current
• Efficiency
• Cost
• Solution Size
Topologie DC-DC per LED driver
• Boost: (step-up)
– Regulator
– Controller
• Buck: (step-down)
– Regulator
– Controller
– Floating
• Buck-boost
Boost Controller: quando Vin <
Vout
L
Io
Nella configurazione Boost
la minima tensione della
serie di LEDs (Vout =Sum
Vf led) deve essere più
grande della tensione di
ingresso.
Δ IL
CO
C
+
+
+
VF
Δ IC
-
Il Dimming dei led in questo
tipo di configurazione può
risultare complesso.
VIN
R FB
Utilizzo consigliato: - lunghe serie di LEDs, Backlighting, camera Flash
Pro:
- Numero di LEDs in Out “indipendente” dalla Vin
- Soluzione “elastica” per corrente di Uscita
Contro:
- Efficienza < 85%
- Nessun controllo sul corto LEDs
+
V FB
-
Vo
Boost Converter: quando Vin < Vout
L
Nella configurazione Boost
la minima tensione della
serie di LEDs (Vout =Sum Vf
led) deve essere più grande
della tensione di ingresso.
Io
Δ IL
Ctrl
C
+
CO
+
+
VF
Δ IC
-
Il Dimming dei led in questo
tipo di configurazione può
risultare complesso.
VIN
R FB
+
V FB
-
Utilizzo consigliato: - Lunghe serie di LEDs, Backlighting, camera Flash
Pro:
- Numero di LEDs in Out “indipendente” dalla Vin
Contro:
- Efficienza < 85%
- Nessun controllo sul corto LEDs
- Potenza limitata
Vo
Buck Controller: Vin > Vout
Non-Synchronous
Synchronous
Utilizzo consigliato: - Serie limitate di LEDs, pilotaggio singolo LED.
Pro:
- Controllo della Iout più accurato
- “Protezione al corto” sui LED
- Efficienza Elevata > 85%
- Soluzione “elastica” per corrente di Uscita
Contro:
- # LED pilotabile limitato dalla Vin
Buck Converter: Vin > Vout
Non-Synchronous
Synchronous
(Internal-FET) Regulators
Utilizzo consigliato: - Serie limitate di LEDs, pilotaggio singolo LED.
Pro:
- Controllo della Iout più accurato
- “Protezione al corto” sui LED
- Efficienza Elevata > 85%
Contro:
- # LED pilotabile limitato dalla Vin
- Potenza Limitata
DC/DC Solutions for Every Need
Plug In Power Modules - PTH series
PTH
• Complete DC/DC solution
• Second sourced footprint and functionality –
POLA alliance
• Fastest time to market
Integration
SWIFT™
And
TPS60K™
Ease of Use
POL
Modules
Integrated FET Regulators (SWIFT and TPS60K)
External
FET
System Cost
TPS40KTM
Flexibility
Integrated FET
• Integrated power MOSFETs simplify design
and consume less board space
• Fewer components reduce bill of material
• Easy-to-use software tool saves development
time
External FET Regulators (TPS40K)
• Application and design flexibility available to
the user
• Excellent total systems cost/value
• Easy-to-use software tool saves development
time
Modified Buck: Vin
(anche 220V)
> Vout
Io
L
VIN
+
-
D
Rsense
CO
+
+
Vo
Δ IL
+
Vsense
-
Utilizzo consigliato: - Lunghe Serie limitate di LEDs
Pro:
- Soluzione Economica quando Vin alta
e Isolamento elettrico non necessario
- Dimmerabile
CONTRO:
- Controllo di corrente non accurato
- Efficienza fortemente influenzata dal # LEDs in Out
Buck- Boost: Vout ≤ Vin ≤ Vout
Ctrl
VIN
+
-
Io
C
L
+
VF
CO +
R
FB
+
V FB
-
Vo
Utilizzo consigliato: - Serie di LEDs ove la Sum di Vf è comparabile alla Vin
Pro:
- Flessibilità di configurazione
- Protezione al corto sui Leds
- Dimmerabile
Contro:
- Efficienza fortemente influenzata dal # LEDs in Out
- Costo
Different dimming
3) PWM to ext. Switch
max 50kHz
1)
PWM to Enable
Max 100 Hz
2)
PWM to FB
max 5kHz
TPS40200 Wide Input driver
Features
Benefits
• 4.5V to 52V operation
• Voltage Mode Control with Feed Forward
Compensation
• 700mV Voltage Reference - 1% accuracy
• Internal Under-Voltage Lockout
• Programmable Frequency (35kHz-500 kHz)
• Programmable Overcurrent Protection
• Frequency Synchronization
• Closed Loop Soft Start
• Integrated Driver
• Package - 8 pin SOIC
• Wide input range for use in many
applications
• Voltage feed forward – great line regulation,
fast transient response
• Programmable features allows flexible
design; frequency, overcurrent protection,
under voltage lockout
• Softstart provides smooth, well controlled
power up
• Simple configuration- minimal external
components
1
RC
2
SS/SD
3
COMP
4
FB
TPS40200
VIN
VIN
8
ISNS
7
Rsense
GDRV 6
VOUT
GND
5
Buck Converter example
• LED current 500mA
• Vf LED 5V
• Wide Input Range
(4.5V to 52V)
• Low Cost Design
• Few Components
• Highly Flexible
• 90% Max Duty Cycle
• Simple Low Cost
Solution
Emergency Flash HB LED driver
Buck-boost with the TPS40200
Vin= 4.5V to 52V
Boost Topology for longer
LEDs Strings
LED Current = 500mA
LED Vf = 5V
PWM Dimming Capabilities
Wide Vin Buck Boost LED driver
TPS40211 Boost LED driver
•
•
•
•
•
•
•
•
•
•
•
•
4.5V ≤ VIN ≤ 52V
Current Mode Control (with
Slope Compensation)
Programmable Frequency
(35k to 1MHz)
Frequency Synchronization
(requires external
components)
Closed Loop Soft Start
260mV Voltage Reference
Internal Under-Voltage
Lockout
– 300mV Hysteresis
Integrated Low Side Driver
Programmable Overcurrent
Protection
• Low Side Sensing
• Leading Edge
Blanking
8V LDO for external circuit
biasing (<5mA)
Low Current Shutdown (10uA
typical)
10L MSOP PPad, 10L 3x3 SON
Boost & SEPIC LED drivers
Monolithic step-up portfolio
TPS61175
high-voltage, serial Boost DC/DC
Single LED, high-current Boost DC/DC
38V
3A Switch
Multiple LED, parallel Charge Pump
Multiple LED, parallel Linear
TPS61165
TPS61081
27V
1.2A Switch
3x3 QFN
TPS61140
27V
0.7A Switch
Dual Output
(OLED+6 LED)
0.5A Switch
2.0A Switch
I2C
CSP
TPS61045
38V
0.7A Switch
TPS61150A
26V
0.7A Switch
Dual Output
(2x6 LED max)
TPS61058
1.1A Switch
TPS61045
OLED
26V
0.7A Switch
27V
0.7A Switch
Dual Output
(OLED+6 LED)
TPS61061
0.4A Switch
CSP
27V
0.5A Switch
28V
0.45A Switch
TPS61161
TPS61140
1.5A Switch
TPS61080
TPS61160
0.45A Switch
TPS61059
TPS61170
37V
0.5A Switch
2x2 SON
38V
1.2A Switch
TPS61043
TPS61050
1 LED
Flashlight/
high-power
TPS61060
TPS75105
TPS61062
TPS61042
TPS60251
0.4A Switch
CSP
Dual/Quad Output
CSP
0.4A Switch
CSP
0.5A Switch
Dual Output
I2C
TPS60231
TPS61041
TPS60230
TPS61040
TPS60250
0.4A Switch
no OVP
Triple Output
I2C
0.25A Switch
no OVP
3
4
5
6
# of LEDs Backlight
7
10
12
TPS61165 Monolithic boost LED
driver
Features
Benefits
•
3-V to 18-V input voltage range
•
Mid range input supply
•
38-V open LED protection
•
Ultra-small package for high brightness LED
•
200mV reference voltage with +/-2% accuracy
•
No audible noise during brightness control
•
1.2-MHz switching frequency
•
1.2-A switch current limit
•
One wire dimming interface EasyScaleTM
•
PWM brightness control (10kHz to 100kHz)
•
Build-in soft start-up
•
2mm x 2mm x 0.8 Thin-QFN package
Vin
5
12
Vout
12
24
Idrive
350mA
350mA
LED post regulation
• DC- DC LEDs driver
• Linear LEDs Driver
• DC-DC design example
Lineari
TOPOLOGIE
• Direttamente connessi alla
linea di Alimentazione
• Controllo lineare di corrente
che sostenga il drop di
tensione
• Connessi in cascata a DC/DC
per connessione a lunghe
stringhe di LEDs
Batteria o Linea di
alimentazione
Batteria o Linea di
alimentazione
Linear
reg.
Linear
reg.
DC/DC
Boost
Linear
reg.
Linear
reg.
nx
50mA
to
350mA
strings with individual linear
curent control and PWM
input
up to 14
nx
50mA
to
350mA
Boost + long strings with
individual linear curent
control and PWM possibility
TL4242 500mA LED Driver
Features
Benefits
• Single Output 500mA LED Drive
• PWM Input
• Optimized for backlight
applications
• Short-Circuit Proof
• PWM input for brightness control
• Over-Temperature Protection
• Drive high power LEDs
• Reverse-Polarity Proof
• Simple programmability with
single Resistor
• Open-Load Detection
• Wide Temperature Range (-40C to
+150° C)
• Status output for error reporting
• Drive LEDs with up to 42V
• Wide Operating Voltage
Applications
• Lighting
• Display backlighting
• Gaming
http://focus.ti.com/docs/prod/folders/print/tl4242.html
http://www.ti.com/lighting
LED post regulation
• DC- DC LEDs driver
• Linear LEDs Driver
• DC-DC design example
Design Target
Design Example:
•
•
•
•
Input Voltage: 16-36V
Minimum output voltage: 3.3V
Output Current: 1A
Switching Frequencies
– 300kHz
– 700kHz
– 1.5MHz
• Space optimized designed
Selected Converter : TPS54160
•
•
Supports up to 2.5MHz
Marketed as small sized design for high-density HV Industrial
applications
TPS54160 – 3.5V-60V - 1.5A
converter
Features
•
3.5V to 60V Input Voltage Range
•
200 mOhm High Side MOSFET
•
High Efficiency at Light Loads with a Pulse
VIN
TPS54160
EN
Skipping Eco-Mode™
•
116 uA Operating Quiescent Current
•
1.3 uA Shutdown Current
•
300 kHz to 2.5 MHz Switching Frequency (RT)
•
300 kHz to 2.2 MHz Switching Frequency (CLK)
•
Synchronizes to External Clock
•
Adjustable Slow Start/Sequencing
•
UV and OV Power Good Output
•
Adjustable UVLO Voltage and Hysteresis
•
0.8V Internal Voltage Reference
•
- 40° to +150°C Operating Tj
•
MSOP10 Package with Power Pad
PWRGD
BOOT
PH
SS/TR
RT/CLK
COMP
VSENSE
GND
TPS54160 – functional block
diagram
PWRGD 6
EN 3
VIN 2
Shutdown
UV
Thermal
Shutdown
Enable
Comparator
Logic
UVLO
Shutdown
Shutdown
Logic
OV
Enable
Threshold
Boot
Charge
Voltage
Reference
Minimum
Clamp
Pulse
Skip
ERROR
AMPLIFIER
Boot
UVLO
PWM
Comparator
VSENSE
7
SS/TR
4
Current
Sense
BOOT
1
Logic
And
PWM Latch
Shutdown
Slope
Compensation
PH
10
COMP
8
Frequency
Shift
Overload
Recovery
Maximum
Clamp
Oscillator
with PLL
RT/CLK 5
POWERPAD
11
TPS54160 Block Diagram
GND
9
TPS54160 – adjusting soft start
EN
SS/TR
VSENSE
VOUT
Css (nF ) 
Tss (ms )  Iss (uA)
Vref (V )  0.8
Css=0.1uF
TPS54160 – current limit
• TPS54160 implements Cycle by Cycle Current Limit with Frequency Shift
• More Robust than Hiccup or Latch Off Current Limit Methods
• Switching Frequency changes as a function of VSENSE pin voltage
• Limits Usable Maximum Switching Frequency at High Input Voltage
Switching Frequency vs VSENSE
Switching Frequency (%)
100
75
50
VIN=12V
TJ=25C
25
0
0
0.2
0.4
VSENSE (V)
0.6
0.8
TPS54160 – adjusting switching
freq.
206033
RT ( kOhm) 
fsw( kHz )1.0888
Sw itching Frequency vs RT/CLK Resistance
High Frequency Range
Sw itching Frequency vs RT/CLK Resistance
Low Frequency Range
1000
VIN=12V
TJ =25°C
2000
Switching Frequency
(kHz)
Switching Frequency
(kHz)
2500
1500
1000
500
0
0
25
50
75
100
125
RT (kOhm )
150
175
200
VIN=12V
TJ =25°C
800
600
400
200
0
100
200
300
400
500
600
RT (kOhm )
700
800
900 1000
TPS54160 – switching freq.
selection
• Using RT mode, fsw adjustable from 300 kHz to 2500 kHz
• Using CLK mode, fsw adjustable from 300 kHz to 2200 kHz
• Solution Size, Performance and Efficiency Tradeoffs
• Maximum Input Voltage, Output Voltage and the minimum controllable on time limit fsw
• Operating Area for proper Frequency Shift protection limits fsw
RT ( kOhm) 
206033
fsw(kHz )1.0888
TPS54160 – minimum on time
• DC/DC Converters won’t always work over a wide Vout/Vin Range. Check the
minimum controllable on time in the datasheet, not the maximum oscillator
frequency.
• From TPS54160 datasheet: Minimum Controllable On Time' of 130ns.
• Check if the Vout/Vin (duty cycle) combination will work when the switching
frequency and the minimum controllable on time are known.
Min. Duty Cycle = Min. On time * Switching Frequency
36Vin, 3.3Vout nom 130ns min. on-time examples:
Frequency
300 kHz
700 kHz
1.5 MHz
Min. Duty Cycle
10.6%
10.6%
10.6%
On Time min
289nSec
122nSec
56nSec
Minimum Vout
OK
OK
pulse skipping
TPS54160 – minimum on time issue
Pulse skipping happens when the DC/DC converter cannot
extinguish the gate drive pulses fast enough to maintain the desired
duty cycle. The power supply will still regulate the output voltage,
but…
•
•
•
•
Ripple voltage increases due to the pulses being further apart
Frequency is no longer fixed
Current limit may no longer work since the IC cannot respond in time
Control loop may be unstable. Transient response is also affected
V
Min on-time within spec
t
t
Min on-time out of spec
Controller misses a pulse to maintain desired regulation. Drop the Vin.
Filter Size Consideration
Consider Capacitor Package
• Capacitors are available in fixed sizes
• ESR must be maintained as the case size is reduced
Consider Inductor Volume
• Taller inductors may take up less board area, but pose a height issue
• Inductor Volume = L * i2 / 5x10-3
Area
Passives
IC
Frequency
TPS54160 - inductor selection
• Inductor ripple current > 100 mA for dependable operation.
• For adequate slope compensation in CCM at duty cycles > 50%, keep Kind ≤ 0.4
Lo min 
Vin max  Vout
Vout

Io  Kind
Vin max fsw
Iripple  Io  Kind
0.2  Kind  0.4
300kHz
L ≥ 39 µH
700kHz
L ≥ 18 µH
1.5MHz
L ≥ 8.2 µH
Inductor Size Comparison
Wurth
744773082
1500kHz
8.2uH
116 mΩ
Wurth
744777118
700kHz
18uH
80 mΩ
Wurth
744775139
300kHz
39uH
145mΩ
TPS54160 – output capacitor
selection
2  Iout
Co 
fsw  Vout
Ioh
Co  Lo 
Vf
Co 
2
 Iol 2
2
 Vi 2

Load Transient

1
1

8  fsw Voripple
Iripple
Voripple
R _ esr 
Iripple
Unload Transient
Voltage Ripple
Voltage Ripple
TPS54160 – capacitor size
comparison
300 kHz Design
8X100uF
3X100uF
2.5mm
3.2mm
1500 kHz Design
700 kHz Design
2X100uF
2.5mm
3.2mm
2.5mm
3.2mm
Frequency
Capacitor
Land-pad size
Inductor
Size
Filter Area
(w/o spacing)
Total Converter
area
300kHz
700kHz
1500kHz
80mm2
30mm2
20mm2
54mm2
30mm2
18mm2
134mm2
60mm2
38mm2
341mm2
204mm2
176mm2
Efficiency
Power dissipation comes from several places including:
• FET driving loss (Qg * V * F):
– FET drive loss is proportional to the switching frequency
• FET switching loss function of Vin, Iout, Ton/off, f
– Switching loss is proportional to the switching frequency
• FET resistance ( I2 * Rds(on))
– Same FET – little variance in these examples
• Inductor loss (I2 * DCR + Core losses):
– DCR losses may be reduced when Inductance is smaller
– AC losses are proportional to the frequency
• Capacitor loss (IRMS2 * ESR):
– Capacitor loss is negligible
• IC loss (Iq):
– Same IC – no variance in these design examples
Power dissipation contributors
Pcon  Io 2  Rdson 
Vo
Vin
Conduction Loss
Psw  Vin 2  fsw  Io  0.25 10 9
Switching Loss
Pgd  Vin  3  10 9  fsw
Gate Drive Loss
Pq  116  10 6  Vin
Supply Loss
Total device power dissipation is:
Ptot  Pcon  Psw  Pgd  Pq
TJ  TA  Rth  Ptot
TA max  TJ max  Rth  Ptot
Efficiency Results
300kHz 75%
700kHz 65%
1500kHz 52%
Dissipated power turns into heat:
Frequency
300kHz
700kHz
1500kHz
IC temperature
53° C
89° C
161°C
IC Dissipation
531mW
1200mW
2400mW
TPS54160 – peak current mode
TPS54160 – simplified model
VO
VC
Adc
RESR
gmps
RL
fp
CO
fz

s 
1 

2  fz 
vo
 Adc  
vc

s 
1 

 2  fp 
Adc  gm ps  RL
fp 
fz 
1
C O  R L  2
1
CO  RESR  2
TPS54160 – compensation
Can compensate for any type of output capacitor
by using Type 2A, 2B, or 1 Frequency Compensation
See Datasheet for equations.
TPS54160 – compensation design
Gmod fc 
6.6  Rload  2  Fc  Cout  Resr  1
2  Fc  Cout  Rload  Resr   1
For ceramic
R3 
For tantalum and aluminum
Vo
G mod fc  80 10  6
C1 
1
  R3  fp mod
Co  Re sr
C2 
R3
R3 
Vo  fc
G mod fc  fz mod 80 10 6
C1 
1
  R3  fp mod
C2 
1
2    R3  fz mod
Basic Stability Criterion
Reading Bode Plot
1) Find Cross Over
Frequency at 0dB
• 21 kHz Cross Over
2) Find Phase Margin at the
Cross Over Frequency
• 45° at 21kHz
3) Goal is to have Phase
Margin in between 45° and
90° to maintained a welldamped transient
response
Measured Stability
Measured Stability
Design tools
Summary
Performance
• Higher switching frequency allows for a higher cross over frequency and a faster transient response. Is
this necessary?
• High frequency devices are more expensive. Make sure you really need the benefits and can handle
the trade offs (heat and efficiency).
Noise - EMI
• Sometimes an application specific frequency band must be avoided (AM band, ADSL, IF,)
• Alternative - Synchronize the frequency in an area of the band that is not so sensitive
Minimum on time:
• Vout is limited by the controllable on time and frequency
• Take the input voltage and switching frequency tolerances into consideration
• Check to see if the current limit circuit will respond fast enough
Efficiency
• Efficiency decreases as switching frequency increases mainly due to FET switching losses
• More copper, board area or additional heat-sinking may be needed
Size
• 350kHz power supply is 260mm2 which saves about 35% of the board area when moving to 1600kHz.
(100mm2 savings).
• More heat in a smaller area