Upload
vocong
View
229
Download
0
Embed Size (px)
Citation preview
In partnership with: In partnership with:
LED basics, TPS92512 LED driver and passive components’ selections
Issac Hsu, Worldwide Marketing Engineer, TI
Velibor Radovic, Product Definition Engineer, Wuerth Elektronik
In partnership with:
Abstract
Thank to technological advancements, Light Emitting Diode (LED)
becomes brighter every year and it is not limited to just being used in
indicators. Many applications such as Automotive headlight, Street light,
Traffic light and even commercial and residential lighting fixtures are
using high brightness LEDs. In this webinar, we are going to talk about
the basics of LEDs, TI’s TPS92512 LED driver and Wuerth Elektronik’s
passive components’ selections for LED lighting applications.
In partnership with:
Contents
• What is an LED
• High current step-down regulators
– Introduction of TPS92512
– Design examples
• Passive components selections (Wuerth)
In partnership with:
WHAT IS AN LED?
In partnership with:
What is an LED? Symbol
Light Emitting Diode (LED)
A PN junction semiconductor
device that emits incoherent optical
radiation when forward biased. The
optical emission may be in the
ultraviolet, visible, or infrared
wavelength regions.
In partnership with:
LED are current driven devices
• LEDs are current driven devices with
V-I curves similar to PN junction diodes
• Small changes in VF yield large
changes in IF
• Luminous output more proportional to IF
• Controlling the current through the LED yields much better regulation of the light output
VF
IF
3.2 3.4
In partnership with:
Automotive Most reliable & tight accuracy •AEC-Q100 qualified •±5% LED current accuracy •DRL, fog lamp, HB/LB head-lights
Illumination Whenever there are LEDs, there are needs for LED drivers
General illuminations Tight accuracy & easy to use •AC direct linear •Widest VIN range DC/DC controller / converter •MR-16, AR-111, A19, T8 tubes, downlights, etc.
Special LED usages High efficiency & application oriented •Widest VIN range DC/DC controller / converter •High efficiency •Projector light source, IR LED, UV LED
In partnership with:
TPS92512 HIGH CURRENT LED DRIVERS
In partnership with:
TPS92512/HV 2.5 A Buck Regulator for LED Lighting
Features Benefits
• Input Voltage Range: 4.5 V to 42 V (60V HV) • Suitable for a Wide Variety of LED Applications
• IOUT up to 2.5 A with internal N-channel
MOSFET
• Integrated FET reduces BoM and PCB Space
• Up to 2 MHz Adjustable Switching Frequency • Flexible Inductor Selection (Size vs Ripple)
• Frequency Synchronization Input • Overdrive Internal Oscillator with External Clock
Input - Simplifies EMI in Multi-String Applications
• Analog Dimming (>10:1) • Input for LED Binning and Thermal Control
• PWM Dimming (>100:1) • Accurate PWM Control of Light Intensity
• UVLO, Over-Current, and Over-Temperature
Protection
• Protects IC During Fault and Abnormal Operating
Conditions
Applications
• Industrial: Street Lighting, Emergency/Exit
Lighting, Retail Illumination, Appliance Lighting
• Automotive: Aftermarket Fog, Flood Lights,
Light Bars
NEW
TOOLS
• TPS92512EVM-001
SPICE model available
In partnership with:
TPS92512 pin descriptions NAME NO. TYPE(1) DESCRIPTION
BOOT 1 O A bootstrap capacitor is required between BOOT and PH. If the voltage on this capacitor is below
the minimum required by the output device, the output is forced to switch off until the capacitor is
recharged
COMP 8 O Error amplifier output, and input to the output switch current comparator. Connect frequency
compensation components to this pin
GND 9 G Ground
IADJ 6 I Analog current adjust pin. The voltage applied to this pin will set the current sense (ISENSE pin)
voltage. The range of this ADJ pin is 180mV to 1.8V and the corresponding ISENSE pin voltage is
the IADJ pin voltage divided by 6
ISENSE 7 I Inverting node of the transconductance (gM) error amplifier
PDIM 4 I PWM dimming input pin. The duty cycle of the PWM signal linearly controls the average output
current of the converter
PH 10 O The source of the internal high-side MOSFET
PowerPAD PAD G GND pin must be electrically connected to the exposed pad directly beneath the device on the
printed circuit board for proper operation
RT/CLK 5 I Resistor timing and external clock. An internal amplifier olds this pin at a fixed voltage when using
an external resistor to ground to program the switching frequency. If the pin is pulled above the
PLL upper threshold, a mode change occurs and the pi becomes a synchronization input. The
internal amplifier is disabled and the pin becomes a high impedance clock input to the internal
PLL. If the clock edges stop, the internal amplifier is re-enabled and the mode returns to the
resistor-programmed function
UVLO 3 I Adjustable undervoltage lockout. Set with resistor divider from VIN
VIN 2 P Input supply voltage, 4.5V to 42V or 4.5V to 60V for the HV version
(1) I = Input, O = Output, P = Supply, G = Ground
In partnership with:
Boot capacitor (BOOT and PH pins) – pins 1 and 10 • Put 100nF (minimum voltage rating of 10V) ceramic capacitor between
the BOOT and PH pins
• Provide gate drive voltage for the internal high-side MOSFET
• BOOT voltage drops below 2V, the BOOT UVLO circuit turns off the
MOSFET which allows the BOOT capacitor to be recharged
In partnership with:
Under voltage lock-out setting (UVLO pin) – pin 3 • Internal UVLO on the VIN of the device.
• For device protection only, no hysteresis
• UVLO pin should always be used to set the minimum VIN voltage and
set with 4.5V as minimum
In partnership with:
• Switching frequency determined by RT resistor connected to RT/CLK
pin
• Frequency range from 300 kHz to 2 MHz
Switching frequency setting (RT/CLK pin) – pin 5
In partnership with:
LED current setting (IADJ and ISENSE pins) – pins 6 and 7
In partnership with:
LED brightness dimming (PDIM and IADJ pins) – pins 4 and 6 • Either or both PWM dimming and analog dimming can control LED
brightness
• PWM dimming:
– When PDIM pin is low, the gate driver is disabled and the LED current
quickly reduces to zero
– A square wave of logic 0 < 0.79V and logic 1 > 1.45V is required
– Dimming frequency ranges from 100Hz to 1kHz
– Accurate 100:1 dimming ratio by PWM dimming achievable
• Analog dimming:
– The current sense voltage is most accurate with IADJ voltages between
0.18V and 1.8V to provide an accurate 10:1 dimming ratio
• By using both 2 dimming pins, we are able to achieve an accurate
1,000:1 dimming ratio in total without compromise
In partnership with:
External compensation (COMP pin) – pin 8
• Error amplifier output of TPS92512 is connected to the COMP pin
• Simple to stabilize and only requires a capacitor from the COMP pin to
ground (CCOMP)
• A 100nF capacitor is recommended and will work well for most
applications
• The overall system bandwidth can be approximated by:
In partnership with:
Design example 1
Specification:
• VIN range of 12 V to 48 V
• UVLO set to 12 V with 0.8 V hysteresis
• 3 LED output, 9.7 V stack, VOUT = 10 V
• 1.5 A LED current (at VISENSE = 300 mV for best accuracy)
• Switching frequency of 570 kHz
• LED current ripple of 10 mA or less
In partnership with:
Standard component selections
• Choose a 0.1 μF ceramic capacitor with a 10 V or greater rating for
CCOMP and CBOOT
• Connect IADJ to VIN through a 10 MΩ resistor to clamp it at 1.8 V and
provide an ISENSE voltage regulation point of 300 mV
• Connect a 10 nF capacitor from IADJ to ground
• Connect ISENSE to R(ISENSE) through a 1 kΩ resistor
In partnership with:
Calculate UVLO values
• VSTART = 12V; VSTOP = 11.2V; VHYS = 0.8V
• For the closest 1% accuracy resistance value, 176kW and 19.3 kW
would be used for R1 and R2, respectively
In partnership with:
Setting the switching frequency
• Desired switching frequency at 570kHz
• According to the RRT equations:
• Choose the closet 1% accuracy resistance value, 200kW will be used
In partnership with:
Setting the LED current
• Use a 300mV as sense voltage, the RISENSE for 1.5A is calculated as
follow:
In partnership with:
Selecting the free wheeling diode
• Rectifier diode conducts current during MOSFET off-time
• Reverse voltage rating must be greater than the maximum input voltage
and a current rating greater than the peak inductor current
• 48 V VIN requires a rating of 60 V or above Schottky diode
• The maximum duty cycle during high-side MOSFET off is 1 – 10/48 =
79.2% or 0.792, average current flow through is 1.5 x 0.792 = 1.19A
• Assume a 0.7V diode, the power dissipation will be at 0.833W
• A 1W/60V/1.5A diode could be used as the free wheeling diode
In partnership with:
Calculating inductance value
• The value of the buck inductor impacts the peak-to-peak ripple-current
(IR) amplitude, a minimum of 75 mA IR is recommended
• Inductance value L can be calculated according to:
• Sometimes, the calculated inductance might not be available in
standard value, next lowest standard value inductance should be
chosen in such case
• Peak-to-peak ripple current IR can then be calculated:
• In this example, calculated L is 39 mH
• Standard value of 33 mH is being chosen for this example
IR
In partnership with:
Calculating input capacitance value
• Given a 2 mF input capacitance is required for every 1A of LED current,
a 1.5A design would require a minimum of 3 mF
• This capacitance should be at least doubled to account for any ceramic
capacitor tolerances and variances
• Higher capacitance value will provide better overall performance
• Hence, 50V / 10 mF input capacitor is chosen
In partnership with:
Calculating output capacitance value
• During start-up, the TPS92512 uses the discharged output capacitor as
a charging path for the BOOT capacitor
• To ensure BOOT capacitor charges and the converter begins switching
immediately, the value of the output capacitor should be 10 times larger
than the BOOT capacitor, the output capacitor also reduces the high
frequency ripple through the LED string
• To calculate minimum effective output capacitance required, use the
equations:
• In this particular example, calculated COUT is 3.34 mF
• A 4.7 mF with voltage rating of 16V is chosen
In partnership with:
Design example 2
Specification:
• VIN range of 12 V to 48 V
• UVLO set to 12 V with 0.8 V hysteresis
• 3 LED output, 9.7 V stack, VOUT = 10 V
• 1.5 A LED current (at VISENSE = 300 mV for best accuracy)
• Switching frequency of 1.5 MHz
• LED current ripple of 10 mA or less
RRT
69.8 kW
L
10 mH
COUT
4.7 mF
In partnership with:
Parametric difference between 2 examples
Design example 1 Design example 2
Switching frequency 570 kHz 1.5 MHz
LED current ripple < 10 mA < 10 mA
RT resistance value 200 kW 69.8 kW
Inductance suggested 33 mH 10 mH
Output capacitance suggested 4.7 mF 4.7 mF
In partnership with:
1st DESIGN TIP
- Switching frequency -
In partnership with:
Core Material – power conversion application
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0,01 0,1 1 10 100 1000
f/MHz
XL(NiZn) XL(MnZn) XL(Fe)
Imp
ed
ance
„0“-400kHz „0“-10MHz „0“-40MHz
In partnership with:
Core material – filtering application
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0,01 0,1 1 10 100 1000
R (NiZn) R (MnZn) R (Fe)
200kHz-4MHz
3-60MHz 20-2000MHz
Impe
dance
f/MHz
In partnership with:
Inductance value
Design Tip 1
switching frequency < 100 kHz: suitable core material: iron powder, ferrite
switching frequency > 100 kHz: suitable core material: ferrite
frequency < 100 kHz
core material: ironpowder; MnZn; Superflux,
NiZn, WE-Perm
frequency > 100 kHz….. 1000 kHz
core material: MnZn; Superflux, NiZn, WE-Perm
frequency > 1000 kHz
core material: NiZn, WE-Perm
• switching frequency of typical ICs in the market
In partnership with:
2nd DESIGN TIP
- Inductance value -
In partnership with:
Inductance value
rippleswitch
outin
If
VVDL
)(
• calculation of inductance value, if no software
BUCK
Design Tip 2
Inductance value
higher inductance - smaller ripple current
lower inductance - higher ripple current
The ripple current is essential in determining the core losses.
Besides the switching frequency, it is therefore an important parameter
for minimising the power loss of the power inductor.
outripple II %40%...20
rippleswitch
out
If
DVDL
)²1( BOOST
In partnership with:
Comparing different inductor values
1.25A
1,5A
1,75A
0µs 1,75µs 3,5µs 5,25µs 7,0µs
ripple range
20-50%
10µH
33µH AI peak 03.0
higher ripple current higher losses (AC)
Inductor – ripple current
In partnership with:
3rd DESIGN TIP
- Inductor current -
In partnership with:
General
• current load for power inductor can be calculated by
software (WEBENCH, WE Component Selector …)
calculation step-by-step
use following approach as a rough calculation
Saturation current > Imax
outL II *5.1max outL II *2max
BUCK BOOST
In partnership with:
Saturation current
definition?
which standard ?
There is no standard....
• saturation current ISAT
In partnership with:
Saturation current
In partnership with:
=> Print in the data sheet and in the catalogue, drop of the inductance value
Saturation Current
In partnership with:
Saturation Current
0
10
20
30
40
50
60
70
80
90
100
110
0 5 10 15 20 25 30 35
ind
ucta
nce L
/Lo
[ %
]
∆L= -
10%
Saturation Current
I [A]
Definition
Wurth Elektronik:
e.g. WE-PD
• the saturation current always refers to a certain inductance drop
and differs in Inductor construction and varies by manufacturers !
check the specification !
Isat = 25 A Isat = 30 A
In partnership with:
nominal (rated) current
definition?
which standard ?
There is no standard....
• rated current IDC
In partnership with:
In partnership with:
In partnership with:
inductor currents
Design Tip 3
Please observe the definitions for the data sheet specifications.
nominal current
The nominal current for power inductors is usually linked to the specified
self-heating with DC current – here self-heating of 40°C is common at
the nominal current.
saturation current
According to semiconductor manufacturers‚ recommendations, the
saturation current is the point at which the inductance value has fallen by
10%. Unfortunately, this is not a standard value for power inductor data
sheet specifications and often leads to misinterpretation among users.
In partnership with:
4th DESIGN TIP
- DC/AC losses -
In partnership with:
DC resistance
Design Tip 4
DC resistance with the same physical inductor size
higher inductance - higher DC resistance
lower inductance - lower DC resistance
same inductance for a shielded inductor - lower DC resistance
The DC resistance is essential in determining the wire heating losses;
this is another important parameter for minimising the power loss of the
power inductor.
• Hysteresis losses
• Eddy current losses
• DC losses – depending on DCR
• AC-losses – dep. on winding structure
Skin-Effect
Proximity-Effect
totalP COREP CuP
In partnership with:
10mm
10mm
10mm
SIZE DCR IRMS
Same material, size, inductance and DCR Irated must be the same
Copper losses
In partnership with:
REDEXPERT
• Another easy way to calculate and to select the right inductor
Design Tip 8
Use REDEXPERT for inductor selection and calculations
BUCK-converter
BOOST-converter
to calculate the right inductance value in mind of nominal / saturation current
and DC resistance as well.