Download Driving HBLEDs - C. Jensen

Driving HB LEDs, Issues and
Resolutions
Chris Richardson (Santa Clara)
Clinton Jensen (Longmont)
1
LED Applications
• Old days
– Signal indicator
– Numeric and Alpha-numeric display
• Nowadays
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Automotive
Backlight
Flashlight for portable device
General illumination
Projector Light Source
Signage
Torchlight
Traffic Light
LCD and DLP backlighting
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© 2006 National Semiconductor Corporation
LED Market Segmentation
Automotive
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Headlamps
Forward Lighting
High Beam Lights
Interior Lighting
Dashboard Lighting
CHMStopL
Rear Lights
Turn Signals
Puddle lights
Emergency Vehicle Lighting
Aftermarket Accent Lighting
General
Illumination
• Bulb replacement
– Home
– Office
– Flashlights
• Traffic lights and signs
• Signage
– Billboards
– Community Information
– Security, Exits
• Indicator / Fun lights
– Gaming Machines
– Clavilux (Disco)
• Medical / Dental
– Endoscopes
– MRI / CAT scan
Other Markets
• Backlighting
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Monitors
TVs
Portable devices
Camera/Camcorders
Handsets
Instruments
• Camera Flash
• User experience
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Handset fun lights
Handset indicators
Toys
Instruments
3
© 2006 National Semiconductor Corporation
Topics Covered
• Critical Design Considerations
• Topology Selection
• High Current Challenges and Solutions
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© 2006 National Semiconductor Corporation
Critical Design Considerations
and Topology Selection
5
Device and Topology
Selection Criteria Outline
1.
2.
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3.
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4.
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5.
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Input Voltage
Output Voltage
# of LEDs
LED Type
LED Current
How much?
How accurate?
Light Output Control.
LED ripple current allowed.
Dimming? Analog (average current) or PWM? Two wire?
How fast do the transitions have to be?
Current Sense
High side or low side?
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Input Voltage
• Not as easy as “what is it?”, there are many concerns:
– Is the design of the source still flexible?
– How much will it vary and under what conditions? Automotive cold
crank and load dump are good examples.
– What functionality is required / desired over the range?
• Regulation / Safety Shutdown / Survival
– What does the source schematic look like, a wall wart, etc.
– Can the source handle low frequency, unfiltered PWM dimming?
– Can the source handle 100% load steps? If not, sequencing or
softstart may be required  similar to voltage regulator sequencing.
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Output Voltage
• Always determined by the LEDs, but dependent on many factors:
– Number of LEDs.
– Manufacturer and color which determines forward voltage at operating
current.
– Temperature.
– Age dependent.
– Dynamics of dimming method.
– Dynamic resistance (see next slide).
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© 2006 National Semiconductor Corporation
Dynamic Resistance
as a Load
• Dynamic Resistance, rD, is the
inverse of the IF vs. VF curve
• rD is typically 5x to 10x lower
than the result of simply
dividing VF-TYP by IF-TYP
• The control loop sees rD, so
the load impedance
• RL = rD + RSNS
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LED Current
• How much current can they run effectively?
– Different applications will change the order of importance of the following:
•
•
•
•
Heat sinking of the LED in the application.
Desired dominant wavelength.
LED lifetime.
Peak efficiency requirements.
• Any transient steps? How fast? What is the dynamic range?
• Accuracy – can be important because:
– LEDs are not tightly manufactured / matched
– Binned LEDs are expensive. 2D binning (light per current, dominant
wavelength) is not uncommon and hugely expensive.
– Color (dominant wavelength) variation is very perceptible
• Ripple current allowed.
– Higher ripple tends to broaden the spectrum, which can be pleasing to the eye.
– High ripple usually means smaller components (inductor), especially because
there is often no output capacitor (buck regulators).
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Light Output vs. Current
• Luminus flux output can vary greatly depending on operating
region. Yes, those are Amps on the x-axis (we’ll get to that).
• Observe the difference between continuous wave (CW) operation
and pulsed.
* Information courtesy of Luminus Devices, Inc.
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Dominant Wavelength
vs. Current
• Again, notice the difference between continuous wave (CW)
operation and pulsed.
* Information courtesy of Luminus Devices, Inc.
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CCT vs. Current
• CCT is the Correlated
Color Temperature,
which is effectively
the “white” that the
LED is providing.
• Cx and Cy are
coordinates that
specify that white
point. Observe how
they shift with
changes in current.
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Advanced Topology Selection
Input Current
Output
Current
Buck
Boost
Buck-Boost
Discontinuous
Continuous
Discontinuous
L
J
L
Discontinuous
Discontinuous
L
L
Continuous
J
Perfect for LEDs!
• Discontinuous input current can lead to EMI issues.
• Discontinuous output current requires an output capacitor which makes
PWM dimming more difficult, and usually requires multiple sense points.
• Buck-boost is best for wide input ranges but is most difficult for an LED
driver.
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Why Bucks Are So Great
• Continuous output current allows us to only sense the current in one
place. Such a simple control algorithm requires no compensation!
• Hysteretic or simple constant on-time control.
• Single sense point.
• Constant on-time.
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Multiple Sense Points
• For the boost and buck-boost, output current is a function of duty
cycle, so both inductor current and LED current must be sensed.
VIN
LM3423
Vin
Vcc
GATE
IS
RCT
PGND
FLT
HSP
LED Ready
Enable
LRDY
EN
nDIM
COMP
CSH
LED current sense adds
an element to the control
function to ensure the
average current in the
LEDs in correct.
HSN
TIMR
DDRV
OVP
AGND
BUT! Compensation is
required for the more
complex control loop. L
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COT’s So-so Tolerance of ΔiF
and IF
• LM3402 and LM3404 fix valley of inductor
current
– Peak changes with VIN
– Peak changes with VO:
• LED VF shifts with temp
• Large shifts if one circuit drives different # of LEDs
• COT Control was designed for constant VO
– Basic circuit controls fSW as VIN changes
– On-time, tON, varies with VIN only
– How to make tON proportional to VO as well?
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The Constant On-Time
Buck Architecture (1)
• Control architecture for LM3402 / LM3404 / LM3406. TON is proportional
to current in the RON pin.
• Valley-mode control – the valley is a fixed reference and the on-time is
adjustable.
• How does this provide for constant frequency?
(1)
(2)
tON + tOFF = C (for constant frequency)
(V IN - V OUT) tON = V OUT  tOFF (volt - sec product)
combining (1) and (2) yields...
tON = C 
V OUT
V IN
But, VOUT is constant (first order) because it is an LED driver, so if tON
is proportional to 1/VIN, frequency will be constant.
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The Constant On-Time
Buck Architecture (2)
• Some customers have complained that frequency shifts when they
change the number of LEDs in production, so we fixed that on the
LM3406.
LM3406
VOUT
VIN
Terminate
Cycle
Instead of having a fixed
reference, this timer
references VOUT so that
frequency is truly fixed.
RON
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The Constant On-Time
Buck Architecture (3)
• Why constant frequency? EMC issues are supposedly lessened.
• Why not constant frequency? Because ripple is important in LEDs and
we want ripple to be constant  Hysteretic.
• Can we do constant ripple / hysteretic with the constant on-time
architecture? Sure!
ΔI =
(V IN - V OUT) tON
L
Because the inductor is a constant (first order), if we make tON
proportional to 1/(VIN-VOUT), ripple will be constant with variation in
V
VIN and VOUT.
IN
RON
VOUT
This simple circuit transforms
the LM3402/04/06 from constant
frequency to constant ripple.
to RON pin
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© 2006 National Semiconductor Corporation
Adding a PNP to Get Constant
Ripple
CB
VIN
VIN
BOOT
L1
VO
SW
RON
CIN
V0
IF
D1
LM3402/04
Q1
CS
tON α IRON
RON
RSNS
DIM
GND
VCC
CF
t ON =
1 .34  10 -10  RON
V IN - VO + 0 .6
Fixing ΔiL means that
fSW will vary!
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Test Circuit Results
• VIN = 24V, IF-NOM = 350 mA
• Drives one to five white LEDs
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Test Circuit Results
Sw
IF
Valley is controlled by
comparator
Peak is constant
because ΔiF is constant
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Main Drawback: Parallel FET
Dimming
Sw
DIM
IF
VRON
• VRON drops to 0.2V
+ VSAT of PNP
• VSAT can drop to
~0.2V at hot
• VRON is used for low
power shutdown
Potential for unwanted
shutdown
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The Constant On-Time
Buck Architecture (4)
• Will this trick still work with the LM3406 now that our on-timer
references VOUT instead of a fixed potential?
• Yes, but we need to adjust the circuit a bit.
VCC
LM3406
VOUT
VIN
Terminate
Cycle
Tie the “VOUT” pin to the VCC pin,
the internal reference is once
again a fixed point and the circuit
works the same as before.
RON
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Boost
VIN
LM3423
Vin
Vcc
IS
High side sense
resistor
GATE
RCT
PGND
FLT
HSP
LED Ready
Enable
LRDY
EN
nDIM
COMP
CSH
HSN
TIMR
DDRV
OVP
DIM FET can be
Removed for
Industrial and
Automotive apps.
AGND
• High side current sense and what it allows.
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Flyback / Buck-Boost
VIN
LM3423
Vin
TIMR
RCT
IS
FLT
LED Ready
LRDY
GATE
PGND
HSN
Enable
EN
HSP
• LM3423 in a
buck-boost
configuration
for a wide
input voltage
range.
Vcc
nDIM
OVLO
DDRV
COMP
DPOL
CSH
AGND
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PWM vs. Analog Dimming
• Analog dimming consists of changing the constant
current through the LED by adjusting the sense
voltage.
– Quiet, does not generate additional noise in the system.
– The dominant wavelength varies with LED current however,
so the color will change using this method.
• PWM dimming consists of setting a desired LED
current and turning the LED on and off at speeds
faster than the human eye can detect.
– Noisier. The input supply must be filtered properly to
accommodate the high input current transients.
– The dominant wavelength does not change so color can be
well controlled. This is usually the preferred method of
dimming high current LEDs.
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High Speed Dimming
• High speed PWM dimming can be desirable
in order to avoid certain frequency bands,
such as audio.
• Some big questions…
– How do we do this when LED current is very
high?
– How do we do this with various topologies?
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Buck – Short Out LEDs
CB
VIN = 24V
VIN
CIN
BOOT
L1
Inductor current is
continuous in buck
converters when
dimming
SW
RON
PWM
D1
IF = 1A
RON
LM3404/04HV
CS
RSNS
DIM
GND
VCC
CF
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Parallel FET Results
IF
FET
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Boost – Series Dimming FET
VIN
LM3423
Vin
Vcc
IS
Inductor current is not
continuous in boost
converters when dimming
GATE
RCT
PGND
FLT
HSP
LED Ready
Enable
LRDY
EN
nDIM
COMP
CSH
HSN
TIMR
Series DIM FET
DDRV
OVP
AGND
• High frequency dimming in boost converters
requires a FET in series with the LEDs.
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Boost – Series Shutdown
Switch
• In the event of an
output short
circuit, a boost
converter cannot
provide short
circuit protection
for the source.
The LM3423
includes a series
shutdown switch
which will
disconnect the
circuit from the
source in the
event of an output
short circuit.
Shutdown Switch
VIN
LM3423
Vin
Vcc
IS
GATE
RCT
PGND
FLT
HSP
LED Ready
Enable
LRDY
EN
nDIM
COMP
CSH
HSN
TIMR
DDRV
OVP
AGND
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© 2006 National Semiconductor Corporation
2-Wire Dimming
CB
VIN
L1
D1
VIN
BOOT
SW
RON
CIN
D2
RON
LM3406B
or
LM3406BHV
VINS
VOUT
CS
RSNS
COMP
CC
GND
VCC
CF
• 2 wire dimming is
achieved by sensing
when an input voltage is
present. When the input is
present the LED is
powered normally. When
IF
the input voltage is not
present the regulator
does not supply the LED
any current but is kept
alive using a diode and a
capacitor so that it can
quickly supply current to
the LED when the input
voltage returns.
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High Current
Challenges and Solutions
35
The Challenge
• Drive a 6-36A LED that has it’s Anode tied to chassis
ground for thermal performance.
• Pulse Width Modulate the LED with duty cycles from 0100% at 30 kHz.
• Sense as low a voltage as possible with 5% or better
accuracy.
• 90%+ Efficiency required.
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© 2006 National Semiconductor Corporation
The Standard Buck Regulator
NMOS Switch
12V
Control
and
Gate Drive
Main
FET
Sync
FET
Using a series sense resistor for
average current mode control turns
the inductor into a true current
source ideal for LED driving.
12V
Main
FET
Control
and
Gate Drive VCC
Charge
Discharge
Sync
FET
The NMOS solution uses a charge
pump or bootstrap type of drive.
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© 2006 National Semiconductor Corporation
LEDs Go Megalythic
• It turns out that the Magical Longhairs at Luminus Devices Inc.
determined that the light prefers to exit the Cathode side of the LED
and the heat prefers to exit the Anode side. The LEDs are upside
down!
• This requires entry into the underground… a mirrored Buck
converter. Under the normal light of day it looks a little confusing
with the top and bottom FETs swapped. Boost pump driving the
synchronous FET producing negative output voltages and currents!
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Negative Buck Regulator
Chassis Ground
Sync
FET
Control
and
Gate Drive VCC
12V
Discharge
Charge
Main
FET
Current is flowing through
the LED is both charge
and discharge phases of
the cycle.
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© 2006 National Semiconductor Corporation
The LM3433 Negative Buck
Regulator with Dimming
Chassis Ground
12V
Sync
FET
LM3433
VCC
Main
FET
The shunt NMOS
determines where the
current is steered.
The regulator runs 100% of
the time. Thus, the inductor
is used as a current source
and is indifferent as to
where the current is going.
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Fast Pulse Width Modulating
of Massive LED Currents
PHYSICISTS INSIST THAT E = L * dI/dT
Inductance is the problem.
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The LED
• LEDs have inherent inductance that cannot be ignored at these
current levels.
• In PWM Dimming, the ideal is to go from zero current to full
current in zero time, then return back to zero current just as fast.
This is practice would take infinite voltage, both positive and
negative!
• What we end up doing is compromise… we trade rise and fall
time for output voltage compliance of our driver.
• The two sides of this compromise are:
– (1) the inductance in the DIM FET to LED
circuit
– (2) the allowable output compliance of the
driver (our switcher)
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© 2006 National Semiconductor Corporation
All Components
Have Inductance
• Even the best LEDs.
• If we have a current source driving an
LED closely coupled with a DIM
MOSFET such that the MOSFET shunts
the current away from the LED, our
current fall time in the LED will not be
instantaneous…
36A
• The decay loop contains the inductance of the LED, the DIM FET,
and all interconnections.
• The current fall time is determined by the voltage and the
inductance. For this case, the voltage is VF of the LED, the
inductance is that as described above.
• For a 36A LED with a VF of 6V, 250nH of total inductance,
tdecay=1.5ms. A similar overvoltage (VF + 6V) would be require to
achieve this rise time.
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Fast PWM LED Dimming
Scope Photo of Light Out of Green LED at 6A
Light Output
(LED Current)
DIM Pin
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© 2006 National Semiconductor Corporation
The FETs (1)
• The synchronous FET body diode is the single biggest problem in a
synchronous buck converter.
• A 6A converter can have a 50A commutation current (looks like shoot
through but isn’t).
• They’ll say, “Put a Schottky diode in parallel with the FET, that’ll fix it!”
– but sadly it won’t for the same ‘ol reason – inductance!
The current flowing in the FET would
take 2ms to transfer from the FET to
the diode if total L=100nH (V=0.3V,
the difference between the VF’s of the
body diode and Schottky)!!!!
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© 2006 National Semiconductor Corporation
The FETs (2)
• What’s the real problem? 50A reverse recovery current drops
efficiency and bounces the whole system (120MHz ringing at 30V).
• So, what’s the solution? Add some ferrite beads to the drain of the low
side (main switcher) FET to slow the transition, decrease the reverse
recovery current, and create a softer recovery which effectively snubs
the ringing!
Chassis Ground
People are tempted to put
a resistor in the gate of
the main FET to slow the
transition down, but the
SYNC FET body diode is
a short during this time,
so no Miller effect can be
used!
Sync
FET
12V
6A
Ferrite
Bead
* All switching losses
are in the main FET.
Main
FET
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The Board –
“Star” Grounds
The quasar, the ultimate
magnetic field generator!
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Magnetic Field Lines
• Remember the right hand
rule? Current passing
through a conductor will
always generate a magnetic
field. Current in a loop of wire
will also generate a magnetic
field.
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© 2006 National Semiconductor Corporation
First Pass, Non-ideal Layout
• Our first pass
LM3433 layout. As
you can see, the
current loops are
parallel to the
board plane, which
means magnetic
flux lines will be
coming out of, and
going into the
board planes,
inducing high
frequency
voltages.
Board top view
D
GND
S
SW
C
D
L
S
-12V
Red: Current path with main FET on.
Blue: Current path with synchronous
FET on.
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© 2006 National Semiconductor Corporation
Solution? Ideal Layout!
• Placing the two high
current FETs on
opposite sides of the
board allowed us to
have a current path
orthogonal to the
board planes. The
result? Magnetic flux
lines parallel to the
board planes,
meaning they don’t
cross the planes and
induce voltage!
Board side view
S
SW
via D
D
C
S
GND
via
-12V
Red: Current path with main FET on.
Blue: Current path with synchronous
FET on.
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Almost Ideal Current Source
• Error amplifier loop provides better than 80dB of
common mode rejection.
• Output impedance of current source is 1KΩ
1000Ω
6000V
Equivalent Circuit
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© 2006 National Semiconductor Corporation
52
Driving HB LEDs,
Issues and Resolutions
What advantage does fast PWM
dimming provide over analog dimming?
A. Lower input current ripple
B. Low color variation over
dim range.
C. Dim time allows
components to cool.
D. Allows color shift to
account for LED aging.
25%
25%
25%
25%
10
A.
B.
C.
53 D.
© 2006 National Semiconductor Corporation
Driving HB LEDs,
Issues and Resolutions
Which of the following factors is not
important concerning the input supply?
A. Range and required
functionality over range.
B. Startup / UVLO /
sequencing requirements.
C. Load regulation.
D. Ability to handle the
unfiltered dimming
frequency ripple.
25%
25%
25%
25%
10
A.
B.
C.
54
D.
© 2006 National Semiconductor Corporation
Driving HB LEDs,
Issues and Resolutions
What is the steady state output voltage
not dependent on?
A.
B.
C.
D.
E.
Dimming method.
Type of LED.
Temperature.
DC LED current.
LED dynamic
resistance.
20%
20%
20%
20%
20%
10
A.
B.
C.
D.
55
E.
© 2006 National Semiconductor Corporation
Driving HB LEDs,
Issues and Resolutions
Which of the following fact is true
regarding the importance of LED current
accuracy?
A.
LEDs are tightly manufactured.
B.
As an LED Heats up its apparent
brightness decreases, thus
compensating for the current
value. (Self Ballasts)
The high bulk resistively in LEDs
is designed to compensate for
minor current variations.
C.
D.
25%
25%
25%
25%
Binned LEDs are very expensive.
10
A.
B.
C.
56
D.
© 2006 National Semiconductor Corporation
Driving HB LEDs,
Issues and Resolutions
What statement is true of an adaptive
COT (constant on time) architecture.
A.
It is difficult to stabilize.
B.
The frequency is always
unpredictable.
C.
You can achieve constant ripple
if adapted to Vout.
D.
You can achieve constant
frequency if adapted to Vin.
E.
C & D.
20%
20%
20%
20%
20%
10
A.
B.
C.
D.
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E.
Driving HB LEDs,
Issues and Resolutions
What is the most important factor
determining LED operating current?
A. Heat sinking of the
LED.
B. Dominant wavelength.
C. LED life.
D. LED current to light
efficiency.
E. All of the above.
20%
20%
20%
20%
20%
10
A.
B.
C.
D.
58
E.
© 2006 National Semiconductor Corporation
Driving HB LEDs,
Issues and Resolutions
Why is ripple current sometimes
desirable in LEDs?
A. It isn’t.
B. Broadens the wavelength
which is nicer to look at.
C. Smaller, lighter, cheaper
drive electronics.
D. B and C.
25%
25%
25%
25%
10
A.
B.
C.
59
D.
© 2006 National Semiconductor Corporation
Driving HB LEDs,
Issues and Resolutions
Why do customers want the anode of
the LED tied to the grounded chassis?
A. Better mechanical shock
resistance.
B. Safety concerns (shock
hazard).
C. Safety concerns (fire hazard).
D. Efficient thermal transfer.
E. C and D.
20%
20%
20%
20%
20%
10
A.
B.
C.
D.
60
E.
© 2006 National Semiconductor Corporation
Driving HB LEDs,
Issues and Resolutions
What is the best way to slow down a
rapid body diode commutation in the
synchronous FET of a buck switcher?
A. Slow down the turn on speed of
the main FET with a gate
resistor.
B. Put a schottky diode in parallel
with the synchronous FET.
C. Place ferrite beads in series with
the drain of the main FET.
D. Put a snubber across the
synchronous FET.
25%
25%
25%
25%
10
A.
B.
C.
61
D.
© 2006 National Semiconductor Corporation
Driving HB LEDs,
Issues and Resolutions
According to the LM3402 datasheet, tON=1.34x1010 / I
RON. What RON should be used if we have a
220mH inductor and we want 100mA of constant
ripple? Neglect the Vbe drop of the pnp.
25%
25%
25%
25%
VIN
A. 62 kW
Clues
B. 247 kW
C. 110 kW
D. 164 kW
ΔI =
(VIN - VOUT )  tON
RON
L
VOUT
10
What is the equation for
IRON is this circuit?
to RON pin
62
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