LED basics, TPS92512 LED driver and passive components ... · In partnership with: LED basics, TPS92512 LED driver and passive components’ selections Issac Hsu, Worldwide Marketing

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.


Contents

• What is an LED

• High current step-down regulators

– Introduction of TPS92512

– Design examples

• Passive components selections (Wuerth)


WHAT IS AN LED?


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.

http://upload.wikimedia.org/wikipedia/commons/6/60/Diod_LED_symbol.png


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


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


TPS92512 HIGH CURRENT LED DRIVERS


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

http://www.ti.com/tool/tps92512evm-001




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


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


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


• 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


LED current setting (IADJ and ISENSE pins) – pins 6 and 7


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


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:


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


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


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


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


Setting the LED current

• Use a 300mV as sense voltage, the RISENSE for 1.5A is calculated as

follow:


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


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


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


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


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


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


1st DESIGN TIP

- Switching frequency -


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


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


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


2nd DESIGN TIP

- Inductance value -


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


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


3rd DESIGN TIP

- Inductor current -


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


Saturation current

definition?

which standard ?

There is no standard....

• saturation current ISAT


Saturation current


=> Print in the data sheet and in the catalogue, drop of the inductance value

Saturation Current

http://www.excursionquebecmotoneige.com/plan_ste_anne.htm


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


nominal (rated) current

definition?

which standard ?

There is no standard....

• rated current IDC




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.


4th DESIGN TIP

- DC/AC losses -


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


10mm

10mm

10mm

SIZE DCR IRMS

Same material, size, inductance and DCR Irated must be the same

Copper losses

http://www.freegifs.de/download.php?image=rauch/rauch00002.gif&host=1&id=8669




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.

Documents

LED basics, TPS92512 LED driver and passive components ... · In partnership with: LED basics, TPS92512 LED driver and passive components’ selections Issac Hsu, Worldwide Marketing