LM3550
5A Flash LED Driver with Automatic VLED and ESR
Detection for Mobile Camera Systems
General Description
Features
LM3550 is a low-noise, switched-capacitor DC/DC converter
designed to operate as a current-limited and adjustable (up
to 5.3V) super-capacitor charger. LM3550 features user-selectable super-capacitor charge-termination voltages and an
optimal charge-termination mode that maximizes flash-energy efficiency by accounting for flash element losses. Additionally, the device provides one adjustable constant current
output (up to 200 mA) and one NFET controller ideal for driving one or more high-current LEDs either in a high-power flash
mode or a low-power torch mode.
The LM3550 can be configured to utilize a proprietary supercapacitor charging scheme (Optimal Charge Mode), allowing
faster charging times (0 to Target Voltage) and lower currentsource power dissipation. Optimal Charge Mode adapts to
changes in the flash LEDs forward voltage as well as the super-capacitor's ESR ensuring that the super-capacitor is
charged to the ideal voltage required to sustain constant current-flash operation.
The LED current and Flash pulse duration of the LM3550 can
be programmed via an I2C-compatible interface. The
STROBE pin allows the Flash to be toggled via a flash-enable
signal from a camera module. The EOC pin sinks current
when the output voltage reaches 95% of the final value.
The ALD/TEMP input pin allows either a light sensor to adjust
the flash-current level based on the ambient light conditions,
or it allows for over-temperature detection and protection of
the LED during high-power operation or high ambient-temperature conditions by connecting an NTC thermistor temperature monitoring circuit to the pin.
■ Up to 5A Flash Current
■ 4 Selectable Super-Capacitor Charge Voltage Levels
(4.5V, 5.0V, 5.3V, Optimized)
■ Flash Optimized Charge Mode for Optimal Efficiency
■
■
■
■
■
■
■
■
■
■
■
■
−33% Faster Charge Time Using Optimal Mode
−49% Less Power Dissipated in Current Source using
Optimal Charge Mode
Fast Super-Capacitor Charger with 500 mA Input Current
Limit
Adjustable Torch Current (60 mA to 200 mA)
Ambient Light or LED Thermal Sensing with Current
Scaleback
End-of-Charge Output (EOC)
Dedicated Indicator LED Current Source
No Inductor Required
Manual Flash Enable via Strobe Pin Input
Programmable Flash Pulse Duration, and Torch and Flash
Currents via I2C-compatible Interface
True Shutdown (LED Disconnect)
Flash Time-Out Protection
LED Temperature Protection or Ambient Light Sensing Pin
Low Profile 20-Pin LLP Package (3.0 mm × 2.5 mm × 0.8
mm)
Applications
■ Camera Phones
■ Digital Still Camera
■ Voltage Rail Management
Typical Application Circuit
30059459
Solution Size
30059401
© 2012 Texas Instruments Incorporated
300594 SNVS569A
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LM35505A Flash LED Driver with Automatic VLED and ESR Detection for Mobile Camera Systems
February 29, 2012
LM3550
Connection Diagram
30059402
Pin Descriptions
Pin #
Name
14
VIN
1
VOUT
20
C1+
18
C1-
15
C2+
16
C2-
Description
Input voltage connection. A 1 µF ceramic capacitor is required from VIN to GND.
Charge pump output. A 1 µF ceramic capacitor is required from VOUT to GND. Connect the Flash LED
anodes and Super-Capacitor to this pin.
Flying capacitor pins. 1 µF ceramic capacitor should be connected from C1+ to C1− and C2+ to C2−.
3
LED-
4
FET_CON
Regulated current sink input, for Torch Mode.
5
FB
10
SCL
I2C Serial Clock pin.
8
SDA
I2C Serial Data I/O pin.
13
IND
Indicator LED Current Source. Drives one red LED with a 5 mA current.
6
EOC
End-of-charge output/ flash ready. The EOC pin will transition from high to low when an end of charge
state has been reached
11
STROBE
Manual Flash enable pin. The Strobe pin can be configured to be rising edge sensitive with the flash
timing controlled internally, or level sensitive with the flash timing being controlled externally.
2
BAL
12
ALD/TEMP
7,9,17,19,
DAP
GND
External FET controller. Connect gate of flash NFET to this pin.
Programmable Feedback Voltage pin.
Super-capacitor active Balance pin.
Ambient Light Sensor or Temperature Monitoring pin. For ambient light sensing, connect a light sensor /
photo-diode and a resistor to this pin. For temperature monitoring, connect a NTC thermistor from VCC
to the NTC pin and a resistor from the NTC pin to ground.
Ground pins. These pins should be connected directly to a low impedance ground plane.
Ordering Information
Package Marking
Supplied As
LM3550SP
Order Number
3550
1000 units, Tape-and-Reel
LM3550SPX
3550
4500 units, Tape-and-Reel
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2
(Note 1, Note 2)
Input Voltage Range
Junction Temperature Range
(TJ) (Note 4)
Ambient Temperature Range
(TA) (Note 5)
2)
If Military/Aerospace specified devices are required,
please contact the Texas Instruments Sales Office/
Distributors for availability and specifications.
VIN to GND
−0.3V to 6V
VOUT, LED−, FB to GND
−0.3V to 6V
SDA, SCL, STROBE, FET_CON, EOC,
ALD/TEMP, IND to GND
−0.3V to 6V
Continuous Power Dissipation (Note 3) Internally Limited
Junction Temperature (TJ-MAX )
150°C
Storage Temperature Range
−65°C to +150
Maximum Lead Temperature
(Soldering, 10s)
(Note 4)
ESD Rating(Note 5)
Human Body Model
2 kV
Machine Model (Note 6)
200V
2.7V to 5.5V
−30°C to 125°C
−30°C to +85°C
Thermal Properties
Junction-to-Ambient Thermal
Resistance (θJA) (Note 7)
57°C/W
Electrical Characteristics
(Note 2, Note 8)
Limits in standard typeface are for TJ = +25°C. Limits in boldface type apply over the full ambient junction temperature range
(−30°C ≤ TA ≤ +85°C). Unless otherwise noted, specifications apply to the LM3550 Typical Application Circuit with: : VIN = 3.6V,
CIN = 4.7µF, COUT = 2.2µF, C1 = C2 = 1 µF.
Symbol
ILED−
VOVP
Parameter
Current Sink Accuracy
Conditions
Typ
Max
54
60
66
90
100
110
180
200
220
Going into
OVP
5.3
5.479
Hysteresis
0.2
2.7V ≤ VIN ≤ 5.5V
3.0V ≤ VOUT ≤ 5.5V
Output Over Voltage Protection 2.7V ≤ VIN ≤ 5.5V
Min
4.275
4.5
4.666
4.75
5
5.169
5.3
5.479
Units
mA
V
VOUT
Output Voltage Regulation
2.7V ≤ VIN ≤ 5.5V
IOUT = 0 mA
VBAL
BAL Pin Voltage Regulation
2.7V ≤ VIN ≤ 5.5V
IIND
IND Pin Current Regulation
2.7V ≤ VIN ≤ 5.5V
VIND = 2.0V
3.3
4.8
6.3
mA
fSW
Switching Frequency
2.7V ≤ VIN ≤ 5.5V
0.882
1
1.153
MHz
VFB
Feedback Pin Regulation
Voltage
2.7V ≤ VIN ≤ 5.5V
VOUT = 4.6V
94
100
106
mV
VALD/TEMP
ALD/TEMP Pin Reference
Voltage
2.7V ≤ VIN ≤ 5.5V
0.95
1
1.05
V
VEOC
EOC Pin Output Logic Low
ILOAD = 3 mA
IIN-CL
Input Current Limit
VOUT = 0V
ISD
Shutdown Supply Current
5.035
VOUT / 2
Device Disabled
2.7V ≤ VIN ≤ 5.5V
IQ
Quiescent Supply Current
2.7V ≤ VIN ≤ 5.5V
IOUT = 0 mA
5V Charge Mode
Non-Switching
VSTROBE
Strobe Logic Thresholds
2.7V ≤ VIN ≤ 5.5V
V
V
400
mV
534
610
mA
1.8
4
µA
168
240
µA
High
1.23
VIN
Low
0
0.7
V
I2C-Compatible Voltage Specifications (SCL, SDA)
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LM3550
Operating Ratings
Absolute Maximum Ratings (Note 1, Note
LM3550
Symbol
Parameter
Conditions
Min
Typ
Max
Units
VIL
Input Logic Low
2.7V ≤ VIN ≤ 5.5V
0
0.7
V
VIH
Input Logic High
2.7V ≤ VIN ≤ 5.5V
1.23
VIN
V
Output Logic Low
ILOAD = 3 mA
400
mV
VOL
I2C-Compatible Timing Specifications (SCL, SDA)
t1
SCL (Clock Period)
294
ns
t2
Data In Setup Time to SCL High fSCL = 400 kHz.
100
ns
t3
Data Out Stable After SCL Low fSCL = 400 kHz.
0
ns
t4
SDA Low Setup Time to SCL
Low (Start)
fSCL = 400 kHz.
100
ns
t5
SDA High Hold Time After SCL
fSCL = 400 kHz.
High (Stop)
100
ns
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation
of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions,
see the Electrical Characteristics tables.
Note 2: All voltages are with respect to the potential at the GND pin.
Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ = 145ºC (typ.) and disengages at TJ
= 125ºC (typ.). The thermal shutdown is guaranteed by design.
Note 4: For detailed soldering specifications and information, please refer to Texas Instruments Application Note: AN-1187 for Recommended Soldering Profiles.
Note 5: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. The machine model is a 200 pF capacitor discharged
through a 0Ω (nominal) resistor into each pin.(MIL-STD-883 3015.7). Texas Instruments recommends that all integrated circuits be handled with appropriate ESD
precautions. Failure to observe proper ESD handling techniques can result in damage to the device.
Note 6: The LED− pin has a machine model ESD rating of 150V.
Note 7: In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may have to be
derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX-OP = 125ºC), the maximum power
dissipation of the device in the application (PD-MAX), and the junction-to-ambient thermal resistance of the part/package in the application (θJA), as given by the
following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX).
Note 8: Junction-to-ambient thermal resistance (θJA) is taken from a thermal modeling result, performed under the conditions and guidelines set forth in the
JEDEC standard JESD51-7. The test board is a 4-layer FR-4 board measuring 102 mm x 76 mm x 1.6 mm with a 2x1 array of thermal vias. The ground plane
on the board is 50 mm x 50 mm. Thickness of copper layers are 36 µm/18 µm/18 µm/36 µm (1.5oz/1oz/1oz/1.5oz). Ambient temperature in simulation is 22°C,
still air. Power dissipation is 1W.
Note 9: Min and Max limits are guaranteed by design, test, or statistical analysis. Typical (typ.) numbers are not guaranteed, but do represent the most likely
norm. Unless otherwise specified, conditions for Typ specifications are: VIN = 3.6V and TA = 25ºC.
30059411
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LM3550
Typical Performance Characteristics
Unless otherwise specified: TA = 25°C; VIN = 3.6V; CIN = 4. 7 µF, COUT = 2.2 µF, C1 = C2 = 1 µF. Super-Capacitor = 0.5F TDK
EDLC272020-501-2F-50,
Output Voltage vs. Output Current
5.3V Mode, Tri-Temp
Output Voltage vs. Output Current
5.3V Mode
30059436
30059439
Output Voltage vs. Output Current
5.0V Mode
Output Voltage vs. Output Current
5.0V Mode, Tri-Temp
30059435
30059438
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LM3550
Output Voltage vs. Output Current
4.5V Mode
Output Voltage vs. Output Current
4.5V Mode, Tri-Temp
30059434
30059437
Input Current Limit vs. Output Voltage
Input Current Limit vs. Output Voltage
Tri-Temp
30059422
30059423
Converter Efficiency vs. Input Voltage
5.3V Mode
Converter Efficiency vs. Input Voltage
5.0V Mode
30059469
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30059470
6
LM3550
Converter Efficiency vs. Input Voltage
4.5V Mode
Input Current vs. Input Voltage
5.3V Mode
30059471
30059472
Input Current vs. Input Voltage
5.0V Mode
Input Current vs. Input Voltage
4.5V Mode
30059473
30059474
LED Efficiency vs. Input Voltage
Torch Mode, 100 mA
LED Efficiency vs. Input Voltage
Torch Mode, 200 mA
30059467
30059468
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LM3550
Torch Current vs. Brightness Code
Torch Current vs. Input Voltage
Code = 0
30059433
30059431
Torch Current vs. Input Voltage
Code = 1
Torch Current vs. Input Voltage
Code = 2
30059430
30059429
Torch Current vs. Input Voltage
Code = 3
Torch Current vs. Input Voltage
Code = 4
30059428
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30059427
8
LM3550
Torch Current vs. Input Voltage
Code = 5
Torch Current vs. Input Voltage
Code = 6
30059426
30059425
Torch Current vs. Input Voltage
Code = 7
Torch Current vs. Input Voltage
Different VLED
30059424
30059432
Feedback Voltage vs. Input Voltage
Oscillator Frequency vs. Input Voltage
30059441
30059440
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LM3550
Shutdown Current vs. Input Voltage
Indicator Current vs. Input Voltage
Different VLED
30059453
30059454
Indicator Current vs. Input Voltage
Tri-Temp
5.3V Mode Super Capacitor Charge
30059442
30059455
2 LED, 3A Flash
(1.5A each)
5.3V Mode Super Capacitor Recharge
30059444
30059443
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LM3550
Strobe to Flash Delay
Edge-Sensitive Strobe
30059445
30059449
Level-Sensitive Strobe
Flash with FGATE = 0
30059450
30059451
Flash with FGATE = 1
ALS DETECT
Zone 0, VALS = 100 mV
30059452
30059446
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LM3550
ALS DETECT
Zone 1, VALS = 500 mV
ALS DETECT
Zone 2, VALS = 1V
30059447
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30059448
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LM3550
Block Diagram
30059403
Circuit Description
duration value, the LM3550 will automatically disable the flash
current.
BASIC OPERATION
The LM3550 is a super-capacitor charger and high currentflash controller based upon a switched capacitor boost converter. On the charging end of the application, the LM3550
has a 534 mA (typ.) input current limit that prevents the part
from drawing an excessive current when the super-capacitor
voltage is below the target charge voltage. During the charge
phase the LM3550 will run in current limit and adaptively
change gains (1X, 1.5X, 2X) until the super-capacitor reaches
its target charge voltage. Integrated into the LM3550 is an
external NFET controller that allows the flash current drawn
from the super-capacitor to remain regulated throughout the
flash cycle. Flash timing and current level can be changed
through the I2C-compatible interface.
End of Charge Pin (EOC)
The EOC pin provides an external flag alerting the micro-controller/micro-processor that the super-capacitor has reached
the end of charging. When the super-capacitor has reached
the desired end-of-charge level, the EOC pin will transition
from its default state (logic ‘1’) to the EOC state (logic ‘0’). The
EOC pin utilizes an open-drain driver that allows the EOC
logic levels to be compatible with many of the common controller input/output (I/O) levels. Connecting a resistor between
the system I/O supply and the EOC pin on the LM3550 ensures the proper voltage levels are utilized.
The state of the EOC pin can change during a flash event, or
any other event whenever the super-capacitor voltage drops
below 95% of the target charge voltage.
DETAILED PIN DESCRIPTION
ALD/TEMP Pin
The ALD/TEMP pin allows the LM3550 to monitor the ambient
light or ambient temperature and adjust the flash current
through the LED/LEDs without requiring the µC/µP to issue
commands through the control interface.
For ambient light detection, a reverse-biased photosensor/
diode and a resistor are required. For ambient temperature
sensing, a negative temperature coefficient (NTC) thermistor
and a resistor are required. Internal to the LM3550 are two
comparators (based on a 1V reference) connected to the
ALD/TEMP pin that provide three user-selectable regions of
flash current adjustment. The trip-point thresholds are selectable in the ALD/TEMP Sense High and Low Registers.
If the ambient light or ambient temperature are sufficiently low
(LM3550 in low region) the full-scale flash current will be allowed. As the lighting conditions or temperature increase, the
LM3550 ALD/TEMP detection circuit transitions to the second
Strobe Pin
The STROBE pin on the LM3550 provides an external
method of flash triggering. This allows a direct connection
between a camera/imager and the LM3550 to be made avoiding any latency added due to communication delays in the
micro-controller/micro-processor (µC/µP). The STROBE pin
can be configured to be rising-edge sensitive (default) or level
sensitive. In the rising-edge sensitive mode, the flash duration
is controlled internally and will use the value stored in the
FLASH duration bits (Options Control Register bits 3:0) to
determine the pulse length. If Level Sensitive Mode is selected (Options Control Register bit 7 = ‘1’), the flash-pulse duration can be controlled externally. In this mode, when the
STROBE pin is high, the flash will remain on as long as the
duration does not exceed the value stored in the FLASH duration control bits. If the timing does exceed the internal flash-
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LM3550
level that limits the flash current to 70% or the full-scale value.
For conditions where a flash is not required (Ambient Detection) or if the ambient temperature is too high to flash safely,
placing the ALD/TEMP circuit in the high-detection level, the
LM3550 will prevent a flash event from occurring. The functionality of the ALD/TEMP pin can be enabled or disabled
through General Purpose Register (bit 6). These macro-functions, when enabled, off-load the µC/µP and provide significant system-power savings.
To help filter out the 50 to 60 Hz noise caused by indoor lighting, a 1 µF ceramic capacitor tied between the ALD/TEMP pin
and GND is recommended.
IND Pin
The Indicator pin (IND) consists of a current source that is
capable of driving a red indicator LED with 5 mA of drive current. This indicator LED can be turned on and off by toggling
bit 7 in the General Purpose Register.
BAL Pin
The Balance pin (BAL), when connected to a super-capacitor
(if needed), regulates the two sections of the super-capacitor
so that voltage on either cell is equal to ½ the output voltage.
This ensures that an over-voltage condition on either cap
section does not occur.
State Machine Description
30059418
Default State Diagram
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30059415
Charge Current vs. Output Voltage
Fixed Voltage Charge Mode
During the Charge state, the LM3550 will operate in current
limit until the target voltage is reached. For the 4.5V, 5.0V and
5.3V charge modes, the LM3550 will operate in a constantfrequency mode once the target voltage is reached for load
currents greater than 60 mA. This allows the LM3550 to draw
only the required current from the power source when the load
current is less than the maximum. When the average output
current exceeds the maximum of the LM3550, the part will
return to the current limited operation until the target voltage
is reached. If the output current is less than 60 mA, the
LM3550 will operate in a PFM-burst mode.
Optimal Charge Mode
For the Optimal Charge Mode, the current-limited, pulsed
regulation scheme (PFM) is used to maintain the target voltage. In Optimal Charge Mode, the LM3550 charges the super-capacitor to a level that is required to sustain a flash for
a given period of time. Optimal Charge Mode compensates
for variations in LED forward voltage and super-capacitor
ESR by charging the capacitor to an optimal voltage that minimizes the power dissipated in the external current source
during the flash. The user must calculate the required overhead voltage and select this value in the Options Control
Register. For more information regarding the optimal charge
mode, please see the Optimal vs. Fixed Charge Modedescription in the Application Information section of this
datasheet.
SHUTDOWN STATE
The Shutdown state is the default power-up state. The
LM3550 will enter the shutdown state when the STROBE pin
is held low without a flash event occurring, and when the
FLASH, TORCH and CHARGE bits in the General Purpose
Register are equal to '0'.
TORCH STATE
The Torch state of the LM3550 provides the flash LED / LEDs
with a constant current level that is safe for continuous operation. This state is useful in low light conditions when an
imager is placed in movie / video mode. The Torch state is
enabled when the Torch bit in the General Purpose Register
is set to a '1' and the Flash and Charge bits are set to '0'. The
desired torch current level (8 total levels between 60 mA and
200 mA) is set in the Current Control Register.
Enabling the torch bit will start up the LM3550 and begin
charging the capacitor. Before a torch event can occur, the
super-capacitor must be charged to a voltage greater than
3.0V. Once the super-capacitor reaches a voltage of 3.0V, the
LED− pin will begin sinking current. In order for the torch current to be properly regulated, the super-capacitor must be
charged up to a value that is greater than VLED + VTREG
(VTREG ≈ 300 mV).
When in the Torch state, the LM3550 will regulate the proper
output voltage (either 3.0V or VLED + VREG) utilizing a pulsed
regulation scheme (PFM). During this mode, the part will operate in current limit until the output voltage reaches the target
level. At that point, the charge-pump will turn off, and the super-capacitor will supply the load. Once the super-capacitor
voltage drops below the turn-on threshold due to the loading
caused by the torch current, the charge-pump will turn on
again and re-charge the super-capacitor.
Note: When the LM3550 is placed into Optimal Charge Mode, the flash
LEDs will begin to glow once the super-capacitor voltage exceeds
3.0V. The LEDs will continue to glow until the part is placed into shutdown, into the flash state, or into one of the fixed voltage charge
modes.
TORCH AND CHARGE STATE
The Torch and Charge state provides the ability to utilize the
torch functionality while charging to the selected target voltage. The Torch and Charge state is entered by setting the
Torch bit and Charge bit to a '1' and by setting the Flash bit
to a '0' in the General Purpose Register. Additionally, the CM1
and CM0 bits can be configured to define the target charge
voltage.
CHARGE STATE
The Charge state of the LM3550 provides the fastest charge
time when compared to the other states of operation. In this
state, the user has the option of charging the super-capacitor
to a voltage equal to 4.5V, 5.0V, 5.3V or to an optimal voltage.
The Charge state is enabled through the I2C interface by set15
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LM3550
ting the Charge Bit to a '1' and setting the Flash and Torch
Bits to a '0' in the General Purpose Register. The charge voltage is selectable by setting the two charge-mode bits (CM1
and CM0) also found in the General Purpose Register.
Depending on the input voltage and output voltage conditions,
the LM3550 will deliver different charge currents to the supercapacitor. Charge current is dependent on the charge-pump
gain.
BASIC DESCRIPTION
The state machine for the LM3550 involves five different
states: Shutdown, Torch, Charge, Charge and Torch, and
Flash.
The Shutdown state, or standby state, places the LM3550 in
a low-power mode that will typically draw 1.8 µA of current
from the power supply.
The Torch state charges the super-capacitor up to VLED +
VTREG (VTREG ≈ 300 mV) and utilizes the internal current sink
to drive the flash LEDs with a current up to 200 mA.
The Charge state places the LM3550 into a dedicated charge
mode that provides the fastest means of charging the supercapacitor up to the target level (4.5V, 5.0V, 5.3V or Optimal).
The Charge and Torch State combines the functionality of
both the Torch state and Charge state. This state allows the
flash LEDs to be on during the charging of the super-capacitor. During the initial charging, the torch current is limited to
60 mA to allow the majority of the output current to be utilized
in the super-capacitor charging. Once the target capacitor
voltage is reached, the torch-current levels become fully adjustable.
The Flash state is responsible for driving the flash LEDs at
the desired flash current. This state can be entered either
through I2C-controlled event or through an external Strobe
event.
LM3550
During the initial charging of the super-capacitor, the Torch
functionality will not be enabled until the capacitor voltage
reaches 3.0V. Additionally, the Torch current is limited to 60
mA until the target voltage is reached. Once the output reaches the target, the current level specified in the Current Control
Register is allowed.
In the event that the total output current exceeds the capacitor
charge current (ICHARGE = IMAX − ITORCH − IEXTERNAL), causing
the super-capacitor to drop below the target voltage, the
LM3550 will automatically set the T2 bit in the Current Control
Register to a '0', decreasing the torch current.
30059420
Torch Current Diagram
FLASH STATE
EOC FUNCTIONALITY
When entered, the LM3550's Flash state delivers a high-curThe LM3550's EOC provides an indicator alerting the controller that the super-capacitor has reached its target voltage.
rent burst of current to the Flash LEDs. To enter the Flash
The EOC pin will transition low once the capacitor reaches
state, the Flash bit in the General Purpose Register must be
95% of the target voltage for the 4.5V, 5.0V and 5.3V modes
set to a '1' or the STROBE pin must be pulled high (edge or
or once the capacitor has reached the optimal charge voltage
level sensitive). The flash duration and current level are user
in Optimal Charge Mode.
adjustable via the I2C interface (F2-F0 in Current Control and
FD3-FD0 in Options).
During operation, the LM3550 will continue to monitor the
voltage on the super-capacitor and will update the EOC pin
By default, a flash will not occur if the super-capacitor is not
when needed. Any time a mode transition occurs during
fully charged (i.e., the end-of-charge flag (EOC pin) must
transition low). If the Flash state was entered via the I2C inCharge mode or Charge and Torch mode, the EOC state will
terface (Flash bit = '1'), the LM3550 will automatically reset
be re-evaluated.
the Flash bit and the Torch bit to '0' upon completion of the
During Torch Mode, the EOC will always indicate a charging
flash. Additionally, after the flash event has occurred, the
state (EOC = '1') .
LM3550 will return to the charge state/mode that was in operation before the flash event with the exception of Optimal
Charge Mode. (If Optimal Charge Mode was used before a
flash, all charging is halted after the flash.)
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LM3550
30059421
be set to a '1' disabling the end-of-charge requirement. Setting FGATE to a '1' allows the Flash state to be entered at
anytime. If the super-capacitor is not charged to the proper
voltage before the EOC pin indicates a full charge, the perceived duration and flash level could be lower than desired.
STATE DIAGRAM FGATE = '1'
By default, the LM3550 will prevent a flash event from occurring if the super-capacitor has not reached the target voltage
(EOC = '0'). In the event that this restriction is not desired, the
flash gate bit (FGATE in the General Purpose Register) can
30059419
FGATE = '1' State Diagram
17
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LM3550
can generate repeated START conditions. First START and
repeated START conditions are equivalent, function-wise.
The data on SDA line must be stable during the HIGH period
of the clock signal (SCL). In other words, the state of the data
line can only be changed when SCL is LOW.
I2C-Compatible Interface
DATA VALIDITY
The data on SDA line must be stable during the HIGH period
of the clock signal (SCL). In other words, the state of the data
line can only be changed when SCL is LOW.
30059412
Start and Stop Conditions
TRANSFERRING DATA
Every byte put on the SDA line must be eight bits long, with
the most significant bit (MSB) being transferred first. Each
byte of data has to be followed by an acknowledge bit. The
acknowledge-related clock pulse is generated by the master.
The master releases the SDA line (HIGH) during the acknowledge clock pulse. The LM3550 pulls down the SDA line during
the 9th clock pulse, signifying an acknowledge. The LM3550
generates an acknowledge after each byte has been received.
After the START condition, the I2C master sends a chip address. This address is seven bits long followed by an eighth
bit which is a data direction bit (R/W). The LM3550 address
is 53h. For the eighth bit, a '0' indicates a WRITE and a '1'
indicates a READ. The second byte selects the register to
which the data will be written. The third byte contains data to
write to the selected register.
30059404
Data Validity Diagram
A pull-up resistor between VIO (Logic Power Supply) and
SDA must be greater than [ (VIO − VOL) / 3.0 mA] to meet the
VOL requirement on SDA. Using a larger pull-up resistor results in lower switching current with slower edges, while using
a smaller pull-up results in higher switching currents with
faster edges.
START AND STOP CONDITIONS
START and STOP conditions classify the beginning and the
end of the I2C session. A START condition is defined as SDA
signal transitioning from HIGH to LOW while SCL line is
HIGH. A STOP condition is defined as the SDA transitioning
from LOW to HIGH while SCL is HIGH. The I2C master always
generates START and STOP conditions. The I2C bus is considered to be busy after a START condition and free after a
STOP condition. During data transmission, the I2C master
30059413
Write Cycle
w = write (SDA = "0")
ack = acknowledge (SDA pulled down by the slave)
id = chip address, 53h for LM3550
I2C-COMPATIBLE CHID ADDRESS: 0x53
INTERNAL REGISTERS
Internal Hex
Address
Power On
Value
General Purpose
0x10
0000 0000
Current Control
0xA0
1111 1000
Options
0xB0
1000 0000
ALD/TEMP Sense High
0xC0
1111 1001
ALD/TEMP Sense Low
0xD0
1100 0110
Register
30059410
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18
Torch Level Table
30059405
FLASH, CHARGE, and TORCH: Mode Bits (see Control
Modes table).
CM0–CM1: Capacitor Charge Mode (see Capacitor Charge
Level table).
FGATE: Flash Gate Bit. If FGATE is a ‘0’, then an end-ofcharge condition must occur before a flash can take place. If
FGATE is a ‘1’, then an end-of-charge condition does not
have to occur before a flash can take place.
A/T EN: ALD/TEMP Enable Bit
IND EN: Enable Indicator Current Source ('0' = Indicator Off,
'1' = Indicator On)
T2
T1
T0
Level
0
0
0
60 mA
0
0
1
80 mA
0
1
0
100 mA
0
1
1
120 mA
1
0
0
140 mA
1
0
1
160 mA
1
1
0
180 mA
1
1
1
200 mA
F2
F1
F0
FB Voltage Level
0
0
0
30 mV
0
0
1
40 mV
0
1
0
50 mV
0
1
1
60 mV
Flash Level Table
Control Modes
Flash
Charge
Torch
1
0
0
70 mV
0
0
0
Disabled
1
0
1
80 mV
0
0
1
Torch
1
1
0
90 mV
1
1
1
100 mV
Mode
0
1
0
Charge
0
1
1
Charge and Torch
1
x
x
Flash
IFLASH(full-scale) = VFB/RSENSE
Indicator Current Level = 5 mA
Options Control Register Description
Capacitor Charge Level
CM1
CM0
0
0
Optimal Charge Mode
0
1
4.5
1
0
5.0V
1
1
5.3V
Level
30059407
SLE: Strobe Level or Edge Sensitivity. '0' = Edge Sensitive,
'1' = Level Sensitive
FD0-FD3: Flash Duration control bits (see Time-out Duration
Table).
OH0-OH2: Overhead Charge Voltage control bits (see Overhead Charge Voltage Table).
Gated Flash Control
FGATE Bit
LM3550
General Purpose Register Description
Result
0
Flash only allowed after EOC
reached
1
Flash allowed without EOC
reached
ALD/TEMP Control
A/T EN Bit
Result
0
ALD MODE DISABLED
1
ALD MODE ENABLED
Current Control Register Description
30059406
19
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LM3550
Time-out Duration Table
OH2
OH1
OH0
Level
1
0
0
700 mV
1
0
1
800 mV
32
1
1
0
900 mV
0
48
1
1
1
1V
1
1
64
1
0
0
80
0
1
0
1
96
0
1
1
0
112
0
1
1
1
128
1
0
0
0
144
1
0
0
1
160
1
0
1
0
176
1
0
1
1
192
1
1
0
0
208
1
1
0
1
224
1
1
1
0
240
1
1
1
1
512
OH2
OH1
OH0
Level
For ALD/TEMP Sense High and ALD/TEMP Sense Low, the
trip levels are set by the following equation:
0
0
0
300 mV
Sense High/Low = 1V × N/(26 − 1)
0
0
1
400 mV
0
1
0
500 mV
0
1
1
600 mV
where N is the decimal equivalent of the value stored in the
ALD/TEMP Sense High/Low registers. NSENSEHIGH must be
greater than NSENSELOW.
FD3
FD2
FD1
FD0
Time (msec)
0
0
0
0
16
0
0
0
1
0
0
1
0
0
0
ALD/TEMP Sense High/Low Registers
30059408
ALD/TEMP Sense High Register
30059409
ALD/TEMP Sense Low Register
Overhead Charge Voltage Table
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20
Super-Capacitor Flash Variable Definitions:
VBATT
Voltage supplying charger circuit.
VCAP
Super-capacitor voltage at the end of the charge cycle and before a flash.
ICL
Maximum current allowed to be drawn from the battery.
IFLASH LED current during the flash event.
tFLASH Desired flash duration.
CSC
Super capacitor value.
VLED
Flash diode forward voltage at IFLASH.
VHR
The headroom required across the FET and the
Sense resistor to maintain current sink regulation.
VFB
The degeneration resistor RSENSE regulation voltage
that in part sets IFLASH.
RDSON On-Resistance of NFET.
VRDSON The voltage drop across the current source FET.
VPUMP The initial SC voltage required for the Flash.
RSENSE Current set resistor.
VDROOP Voltage droop on the super-capacitor during a flash
of duration tFLASH.
= IFLASH×tFLASH / CSC
RESR
Super-capacitor ESR value.
VESR
Voltage drop due to SC ESR.
VBAL
Voltage drop due to LED ballast resistors
VOH
Overhead charge voltage required for constant current regulation during the entire flash duration.
VPUMP VOH + VLED+ VESR = VFB + VRDSON + VESR + VLED +
VDROOP + VBAL
VHR
VFB + VRDSON
30059416
At the beginning of the flash (tSTART), the super-capacitor voltage will drop due to the super-capacitor's ESR. The magnitude of the drop is equal to the flash current (IFLASH) multiplied
by the ESR (RESR).
VESR = IFLASH × R ESR
Once the initial voltage drop occurs (VESR) the super-capacitor voltage will decay at a constant rate until the flash ends
(tFINISH). The voltage droop (VDROOP) during the flash event is
equal to flash current (IFLASH) multiplied by the flash duration
(tFLASH) divided by the capacitance value of the super-capacitor (CSC) .
VDROOP = (IFLASH × tFLASH) / CSC
After the flash event has finished, the voltage on the supercapacitor will increase due to the absence of current flowing
through the ESR of the super-capacitor. This step-up is equal
to
SUPER-CAPACITOR CHARGING TIME
The time it takes the LM3550 to charge a super-capacitor from
0V to the target voltage is highly dependent on the input voltage, output-voltage target, and super-capacitor capacitance
value.
• The LM3550 will charge up a capacitor faster with higher
input voltage and slower with lower input voltages. This is
due to the LM3550 staying in the lower gains for longer
periods of time.
• The LM3550 will charge up a capacitor faster if the target
output voltage is lower and slower if the target output
voltage is higher. For a given charge profile, a lower
capacitor voltage will be reached faster than a higher
voltage level.
• The LM3550 will charge up a capacitor having a lower
capacitance value faster than a capacitor having a higher
capacitance level.
VESR = IFLASH × RESR
PEAK FLASH CURRENT
To set the peak flash current controlled by the LM3550, a
current setting resistor must be placed between the source of
the current source and ground (FB to GND). The LM3550 will
regulate the voltage across the resistor to a value between
100 mV and 30 mV depending on the setting in the Current
Control Register. Using the 100 mV setting, the peak flash
current can be found using the following equation:
IFLASH = VFB / RSENSE
The LM3550 provides eight feedback voltage levels allowing
eight different current settings. The current ranges from 100%
of Full-Scale (100 mV setting) down to 30% of Full-Scale (30
mV setting) in 10% steps.
Super-Capacitor Charging Times
0.5F Capacitor, 0V to Target
Opt.
MODE
MAXIMUM FLASH DURATION
Several factors determine the maximum achievable flash
pulse duration. The flash current magnitude, feedback voltage, RDSON of the current source FET, super-capacitor capacitance (CSC), super-capacitor ESR (RESR) and supercapacitor charge voltage (VCAP) determine the LM3550's
ability to regulate the flash current for a given amount of time.
FIXED VOLTAGE MODE
VIN
4.38V
4.5V
5.0V
5.3V
4.2V
4.565s
5.087s
6.314s
7.014s
3.6V
5.207s
5.765s
6.978s
7.832s
3.0V
6.090s
6.446s
7.870s
8.904s
tFLASH (max.) = (CSC × VDROOP) / IFLASH
where
Note: Optimal Mode Flash = 2 LEDs @ 3A (1.5A Each) for 48 ms. SuperCapacitor Part#: TDK EDLC272020-501-2F-50
21
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LM3550
SUPER-CAPACITOR VOLTAGE PROFILE
When a constant load current is drawn from the charged super-capacitor, the voltage on the capacitor will change. The
capacitor ESR and capacitance both affect the discharge profile.
Application Information
LM3550
VDROOP = VCAP − VLED − [IFLASH × (RESR + RDSON + {RBAL/N)}]
− VFB where N = # of Flash LEDs
OPTIMAL CHARGE MODE VS. FIXED VOLTAGE MODE
The LM3550 provides two types of super-capacitor charging
modes: Fixed Voltage and Optimal Charge.
In Fixed Voltage Mode, the LM3550 will charge and regulate
the super-capacitor to either 4.5V, 5V or 5.3V. This mode is
useful if the LM3550 is going to be used for both flash and
fixed-rail applications (power supply for audio or PA sub-systems).
If the LM3550 is only going to be used as a super-capacitor
charger and flash controller, the Optimal Charge Mode provides many advantages over the Fixed Voltage Mode. Optimal Charge Mode will charge the super-capacitor to the
minimum voltage that is required to sustain a flash pulse compensating for variations in super-capacitor ESR and LED
forward voltage due to temperature and process. To properly
use the Optimal Charge Mode, the Overhead Voltage (VOH)
must be determined. The Overhead Voltage is equal to the
voltage required to maintain current source regulation (VHR)
plus the voltage droop (VDROOP) on the super-capacitor due
to the flash event.
Example:
If VCAP = 5.3V, VLED = 4V (@1.5A), IFLASH (total) = 3A, CSC =
0.5F, RESR = 50 mΩ,RDSON = 40mΩ, VFB = 100 mV, RBAL =
75 mΩ
then
VDROOP = 0.82V and tFLASH (max.) = 136 ms
30059414
VOH = VDROOP + VHR = (IFLASH × tFLASH / CSC) + VFB + (IFLASH
× RDSDON)
and
VCAP = VOH + VLED + [IFLASH × (RESR+RBAL/ N)] where N =
Number of Flash LEDs
Example:
If VLED (peak)= 4.1V (@1.5A), IFLASH (total) = 3A, CSC = 0.5F,
RESR = 50 mΩ, RDSON = 40 mΩ, VFB = 100 mV, RBAL = 75
mΩ, and tFLASH = 64 ms,
then
VOH = 0.604V and VCAP = 4.97V
Note: VLED (peak) is equal to the LED voltage before self-heating occurs.
Once current flows through the LED, the LED will heat up, and the
forward voltage will decrease until it reaches a steady-state level. This
voltage drop is dependent on the LED and the PCB layout.
Based on this calculation, setting the Overhead Voltage to
600 mV in the Current Control Register should ensure a regulated 3A flash pulse over the entire flash duration.
Unlike Fixed Voltage Mode, Optimal Charge Mode will adjust
the super-capacitor voltage upon changes in LED forward
voltage and variation in super-capacitor ESR, ensuring that
the super-capacitor does not charge to a voltage higher than
needed. By charging optimally, the LM3550 can potentially
charge the super-capacitor to its EOC state faster due to the
target voltage being lower, and it helps ease the thermal loading on the current source FET during the flash.
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22
Boost Capacitors
The LM3550 requires 4 external capacitors for proper operation (C1 = C2 = 1µF; CIN = 4.7 µF; COUT = 2.2 µF). Surfacemount multi-layer ceramic capacitors are recommended.
These capacitors are small, inexpensive and have very low
equivalent series resistance (ESR <20 mΩ typ.). Tantalum
capacitors, OS-CON capacitors, and aluminum electrolytic
capacitors are not recommended for use with the LM3550 due
to their high ESR, as compared to ceramic capacitors.
For most applications, ceramic capacitors with X7R or X5R
temperature characteristic are preferred for use with the
LM3550. These capacitors have tight capacitance tolerance
(as good as ±10%) and hold their value over temperature
(X7R: ±15% over −55°C to 125°C; X5R: ±15% over −55°C to
85°C).
Capacitors with Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM3550. Capacitors with these temperature characteristics typically have
wide capacitance tolerance (+80%, −20%) and vary significantly over temperature (Y5V: +22%, −82% over −30°C to
+85°C range; Z5U: +22%, −56% over +10°C to +85°C range).
Under some conditions, a nominal 1µF Y5V or Z5U capacitor
could have a capacitance of only 0.1 µF. Such detrimental
deviation is likely to cause Y5V and Z5U capacitors to fail to
meet the minimum capacitance requirements of the LM3550.
The recommended voltage rating for the capacitors is
10V to account for DC bias capacitance losses.
30059460
Optimal Charge Mode
Current Source FET
Choosing the proper current source MOSFET is required to
ensure accurate flash current delivery. N-Channel MOSFETs
(NFET) with allowed drain-to-source voltages (VDS) greater
than 5.5V are required. In order to prevent damage to the
current source NFET, special attention must be given to the
pulsed-current rating of the MOSFET. The NFET must be
sized appropriately to handle the desired flash current and
flash duration. Most MOSFET manufacturers provide curves
showing the NFET's pulsed performance in the electrical
characteristics section of their datasheets. A MOSFET's performance rating at temperature, primarily temperatures
greater than 40°C, must also be investigated to ensure NFET
does not become thermally damaged during a flash pulse. An
NFET possessing low RDSON values helps improve the efficiency of the flash pulse.
30059457
5.3V Fixed Voltage Charge Mode
Peak Power Dissipation Across Current Source FET
PNFET (max.) = IFLASH × (VOH − VFB)
Optimal Mode = 1.5W, Fixed Voltage Mode (5.3V) = 2.4W
Average Power Dissipation Across Current Source FET (64
ms Pulse)
PNFET (avg.) = IFLASH × [VOH − (VDROOP÷2) − VFB]
Optimal Mode = 936 mW, Fixed Voltage Mode (5.3V) =
1.824W
COMPONENT SELECTION
ALD/TEMP Components
Super-Capacitor
Super-capacitors, or electrochemical double-layer capacitors
(EDLC's), have a very high energy density compared to other
capacitor types. Most super-capacitors aimed at applications
requiring voltages higher than 3V are three-terminal devices
(two super-capacitor cells stacked in series). Special care
must be taken to ensure that the voltage on each cell of the
super-capacitor does not exceed the maximum rating (typically 2.75V to 2.85V, depending on the manufacturer). The
LM3550 is capable of safely charging super-capacitors of
many different capacitances up to a VOUT(max.) = 5.3V typ.
The capacitor balance pin (BAL) on the LM3550 ensures that
the voltage on each cell is equal to half of the output voltage
to prevent an over-voltage condition on either cell. If either cell
NTC SELECTION
NTC thermistors have a temperature-to-resistance relationship of:
where β is given in the thermistor datasheet and R25°C is the
thermistor's value at +25°C. R1 in is chosen so that it is equal
to:
23
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LM3550
fails as a short, the BAL pin will not prevent the second cell
from being damaged.
NOTE: The LM3550 is not designed to work with low-voltage, single-cell super-capacitors.
Power-Saving Example
Optimal Charge vs. Fixed Voltage Charge
LM3550
where RT(TRIP) is the thermistors value at the temperature trip
point, VBIAS is shown in the Thermistor Resistive Divider Response vs. Temperature graph below, and VTRIP = 800 mV
(typ.). Choosing R1 here gives a more linear response around
the temperature trip voltage. For example, with VBIAS = 1.8V
and a thermistor whose nominal value at +25°C is 100 kΩ and
a β = 4500K, the trip point is chosen to be +85°C. The value
of R(T) at 85°C is:
30059466
Thermistor Voltage Divider and Sensing Circuit
AMBIENT LIGHT SENSOR
If the ALD/TEMP pin is not used for ambient/LED temperature
monitoring, it can be used for ambient light detection. The
LM3550 provides three regions of current control based upon
ambient conditions. The three regions are defined using the
Sense High and Sense Low Registers to set the zone boundaries (user-configurable from 0 to 1V). Most ambient light
sensors are reverse-biased diodes that leak current proportional to the amount of ambient light reaching the sensor. This
current is then translated into a voltage by using a resistor in
series with the light sensor. The voltage-setting resistor will
vary based upon the desired ambient detection range and
manufacturer.
Setting the ALD/TEMP Sense High Register to N = 50 or hex
0x32 will place the upper trip point to approx. 800 mV. Voltages higher than 800 mV will prevent the flash LED from
turning on. Based on the curve, the Sense Low Register can
be set to a lower code to give a second LED current threshold
(70% flash). Voltages lower than the value stored in the Sense
Low Register will allow a full current flash.
30059464
Thermistor Resistive Divider Response vs Temperature
If the temperature changes during a flash event, meaning
VALS/TEMP crosses the Sense High and/or Sense Low values,
the current will scale to the appropriate zone current.
The thermistor should be placed as close to the Flash LEDs
as possible. This will provide the best thermal coupling (lowest thermal resistance).
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30059417
Most ambient light sensors suggest placing a capacitor in
parallel with the voltage-setting resistor in order to help filter
the 50/60 Hz. noise generated by fluorescent overhead lighting. This capacitor can range from no capacitor up to 10 µF.
The key is to filter the noise so that the peak-to-peak voltage
is less than 16 mV (LSB size of the ALD/TEMP Sense High
and Sense Low settings). Please refer to the ambient light
sensor's datasheet for the recommended capacitor value.
24
LAYOUT CONSIDERATIONS
The LLP is a leadless package with very good thermal properties. This package has an exposed DAP (die attach pad) at
the underside center of the package measuring 1.86 mm x
2.2 mm. The main advantage of this exposed DAP is to offer
low thermal resistance when soldered to the thermal ground
pad on the PCB. For good PCB layout a 1:1 ratio between the
package and the PCB thermal land is recommended. To further enhance thermal conductivity, the PCB thermal ground
pad may include vias to a 2nd layer ground plane. For more
detailed instructions on mounting LLP packages, please refer
to Texas Instruments Application Note AN-1187.
The proceeding steps must be followed to ensure stable operation and proper current source regulation.
1. Bypass VIN with at least a 4.7 µF ceramic capacitor.
Connect the positive terminal of this capacitor as close
as possible to VIN.
2. Connect COUT as close to the VOUT pin as possible with
at least a 2.2 µF capacitor.
3. Connect the return terminals of the input capacitor and
the output capacitor as close as possible to the exposed
DAP and GND pins through low impedance traces.
4. Place the two 1 µF flying capacitors (C1 and C2) as close
to the LM3550 C1+/− and C2+/− pins as possible.
5. To minimize losses during the flash pulse, it is
recommended that the flash LEDs, the current source
NFET, and current-setting resistor be placed as close to
the super capacitor as possible.
30059475
The Flash current drops to 70% of the peak once the voltage on the ALD/
TEMP pin exceeds the Sense Low trip point.
Effect of ALD/TEMP Voltage Dropping during a Flash
THERMAL PROTECTION
Internal thermal protection circuitry disables the LM3550
when the junction temperature exceeds 145°C (typ.). This
feature protects the device from being damaged by high die
temperatures that might otherwise result from excessive power dissipation. The device will recover and operate normally
when the junction temperature falls below 125°C (typ.). It is
important that the board layout provide good thermal conduction to keep the junction temperature within the specified
operating ratings.
30059476
The Flash event is not allowed to start if the voltage on ALD/TEMP is higher
that the Sense High Trip point.
25
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LM3550
Effect of ALD/TEMP Voltage Rising during a Flash
LM3550
Physical Dimensions inches (millimeters) unless otherwise noted
NS Package SPF20A
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26
LM3550
Notes
27
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LM35505A Flash LED Driver with Automatic VLED and ESR Detection for Mobile Camera Systems
Notes
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