SNVS569C May   2009  – October 2016 LM3550

PRODUCTION DATA.  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Pin Configuration and Functions
  6. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Timing Requirements
    7. 6.7 Typical Characteristics
  7. 7 Detailed Description
    1. 7.1 Overview
    2. 7.2 Functional Block Diagram
    3. 7.3 Feature Description
      1. 7.3.1 STROBE Pin
      2. 7.3.2 End-of-Charge Pin (EOC)
      3. 7.3.3 ALD/TEMP Pin
      4. 7.3.4 IND Pin
      5. 7.3.5 BAL Pin
      6. 7.3.6 Super-Capacitor Charging Time
      7. 7.3.7 Super-Capacitor Voltage Profile
      8. 7.3.8 Peak Flash Current
      9. 7.3.9 Maximum Flash Duration
    4. 7.4 Device Functional Modes
      1. 7.4.1 State Machine Description
        1. 7.4.1.1 Basic Description
        2. 7.4.1.2 Shutdown State
        3. 7.4.1.3 Torch State
        4. 7.4.1.4 Charge State
          1. 7.4.1.4.1 Fixed-Voltage-Charge Mode
          2. 7.4.1.4.2 Optimal Charge Mode
        5. 7.4.1.5 Torch and Charge State
        6. 7.4.1.6 Flash State
        7. 7.4.1.7 EOC Functionality
        8. 7.4.1.8 State Diagram FGATE = 1
        9. 7.4.1.9 Optimal Charge Mode vs Fixed Voltage Mode
          1. 7.4.1.9.1 Optimal Charge Mode vs Fixed Voltage Mode
    5. 7.5 Programming
      1. 7.5.1 I2C-Compatible Interface
        1. 7.5.1.1 Data Validity
        2. 7.5.1.2 Start and Stop Conditions
        3. 7.5.1.3 Transferring Data
        4. 7.5.1.4 I2C-Compatible Child Address: 0x53
    6. 7.6 Register Maps
      1. 7.6.1 Internal Registers
        1. 7.6.1.1 General Purpose Register Description
        2. 7.6.1.2 Current Control Register Description
        3. 7.6.1.3 Options Control Register Description
        4. 7.6.1.4 ALD/TEMP Sense High/Low Registers
  8. 8 Application and Implementation
    1. 8.1 Application Information
    2. 8.2 Typical Application
      1. 8.2.1 Design Requirements
      2. 8.2.2 Detailed Design Procedure
        1. 8.2.2.1 Component Selection
          1. 8.2.2.1.1 Super-Capacitor
          2. 8.2.2.1.2 Boost Capacitors
          3. 8.2.2.1.3 Current Source FET
          4. 8.2.2.1.4 ALD/TEMP Components
            1. 8.2.2.1.4.1 NTC Selection
            2. 8.2.2.1.4.2 Ambient Light Sensor
          5. 8.2.2.1.5 Thermal Protection
      3. 8.2.3 Application Curves
  9. 9 Power Supply Recommendations
  10. 10Layout
    1. 10.1 Layout Guidelines
    2. 10.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Third-Party Products Disclaimer
      2. 11.1.2 Device Nomenclature
    2. 11.2 Related Documentation
    3. 11.3 Receiving Notification of Documentation Updates
    4. 11.4 Community Resources
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

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7 Detailed Description

7.1 Overview

The LM3550 is a super-capacitor charger and high-current-flash controller based upon a switched-capacitor boost converter. On the charging end of the application, the LM3550 has a 534 mA (typical) 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 runs in current limit and adaptively change gains (1×, 1.5×, 2×) 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.

7.2 Functional Block Diagram

LM3550 30059403.gif

7.3 Feature Description

7.3.1 STROBE Pin

The STROBE pin on the LM3550 provides an external method of flash triggering. This means a direct connection between a controller or camera/imager and the LM3550 device can be made, avoiding any latency added due to communication delays. 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 uses 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 remains 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-duration value, the LM3550 automatically disables the flash current.

7.3.2 End-of-Charge Pin (EOC)

The EOC pin provides an external flag alerting the microcontroller/microprocessor 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 transitions 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.

7.3.3 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 microcontroller/microprocessor 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 1-V 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 is allowed. As the lighting conditions or temperature increase, the LM3550 ALD/TEMP detection circuit transitions to the second 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 prevents 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 microcontroller/microprocessor and provide significant system-power savings.

To help filter out the 50 to 60 Hz noise caused by indoor lighting, TI recommends a 1-µF ceramic capacitor tied between the ALD/TEMP pin and GND.

7.3.4 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.

7.3.5 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 overvoltage condition on either capacitor section does not occur.

7.3.6 Super-Capacitor Charging Time

The time it takes the LM3550 to charge a super-capacitor from 0 V to the target voltage is highly dependent on the input voltage, output-voltage target, and super-capacitor capacitance value.

  • The LM3550 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 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 charges 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 is reached faster than a higher voltage level.
  • The LM3550 charges up a capacitor having a lower capacitance value faster than a capacitor having a higher capacitance level.

Table 1. Super-Capacitor Charging Times 0.5-F Capacitor, 0 V To Target

OPTIMAL
MODE(1)		 FIXED VOLTAGE MODE
VIN 4.38 V 4.5 V 5 V 5.3 V
4.2 V 4.565 s 5.087 s 6.314 s 7.014 s
3.6 V 5.207 s 5.765 s 6.978 s 7.832 s
3 V 6.090 s 6.446 s 7.870 s 8.904 s
(1) Optimal Mode Flash = 2 LEDs at 3 A (1.5 A each) for 48 ms. Super-capacitor part number: TDK EDLC272020-501-2F-50.

7.3.7 Super-Capacitor Voltage Profile

When a constant load current is drawn from the charged super-capacitor, the voltage on the capacitor changes. The capacitor ESR and capacitance both affect the discharge profile.

LM3550 30059416.gif

At the beginning of the flash (tSTART), the super-capacitor voltage drops due to the ESR of the super-capacitor. The magnitude of the drop is equal to the flash current (IFLASH) multiplied by the ESR (RESR).

Equation 1. VESR = IFLASH × RESR

Once the initial voltage drop occurs (VESR) the super-capacitor voltage decays 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).

Equation 2. VDROOP = (IFLASH × tFLASH) / CSC

After the flash event has finished, the voltage on the super-capacitor increases due to the absence of current flowing through the ESR of the super-capacitor. This step-up is equal to

Equation 3. VESR = IFLASH × RESR

7.3.8 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 regulates 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 usingEquation 4:

Equation 4. 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.

7.3.9 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 super-capacitor charge voltage (VCAP) determine the ability of the device to regulate the flash current for a given amount of time.

Equation 5. tFLASH (maximum) = (CSC × VDROOP) / IFLASH

where

  • VDROOP = VCAP − VLED − [IFLASH × (RESR + RDSON + {RBAL/N)}] − VFB
  • N = number of flash LEDs

Example:

If VCAP = 5.3 V, VLED = 4 V (at 1.5 A), IFLASH (total) = 3 A, CSC = 0.5 F, RESR = 50 mΩ, RDSON = 40 mΩ,
VFB = 100 mV, and RBAL = 75 mΩ, then VDROOP = 0.82 V and tFLASH (maximum) = 136 ms.

LM3550 30059414.gif Figure 37. Power Loss Model

7.4 Device Functional Modes

7.4.1 State Machine Description

LM3550 30059418.gif Figure 38. Default State Diagram

7.4.1.1 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 typically draws 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 super-capacitor up to the target level (4.5 V, 5 V, 5.3 V 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.

7.4.1.2 Shutdown State

The shutdown state is the default power-up state. The LM3550 enters 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.

7.4.1.3 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 or 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 starts 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 V. Once the super-capacitor reaches a voltage of 3 V, the LED− pin begins 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 regulates the proper output voltage (either 3 V or VLED + VREG) utilizing a pulsed regulation scheme (PFM). During this mode, the device operate in current limit until the output voltage reaches the target level. At that point, the charge-pump turns off, and the super-capacitor supplies the load. Once the super-capacitor voltage drops below the turnon threshold due to the loading caused by the torch current, the charge-pump turns on again and re-charges the super-capacitor.

7.4.1.4 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 4.5 V, 5 V, 5.3 V or to an optimal voltage. The charge state is enabled through the I2C interface by setting 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 delivers different charge currents to the super-capacitor. Charge current is dependent on the charge-pump gain.

LM3550 30059415.gif Figure 39. Charge Current vs Output Voltage

7.4.1.4.1 Fixed-Voltage-Charge Mode

During the charge state, the LM3550 operates in current limit until the target voltage is reached. For the 4.5 V, 5 V and 5.3 V charge modes, the LM3550 operates in a constant-frequency 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 device returns to the current limited operation until the target voltage is reached. If the output current is less than 60 mA, the LM3550 operates in a PFM-burst mode.

7.4.1.4.2 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, see the Application and Implementation description.

NOTE

When the LM3550 is placed into optimal charge mode, the flash LEDs begin to glow once the super-capacitor voltage exceeds 3 V. The LEDs continue to glow until the device is placed into shutdown, into the flash state, or into one of the fixed-voltage charge modes.

7.4.1.5 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.

During the initial charging of the super-capacitor, the torch functionality is enabled until the capacitor voltage reaches 3 V. 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 automatically sets the T2 bit in the Current Control Register to a 0, decreasing the torch current.

LM3550 30059420.gif Figure 40. Torch Current Diagram

7.4.1.6 Flash State

When entered, the flash state of the LM3550 device delivers a high-current burst of current to the flash LEDs. To enter the flash state, the flash bit in the General Purpose Register must be set to a 1 or the STROBE pin must be pulled high (edge or level sensitive). The flash duration and current level are user adjustable via the I2C interface (F2-F0 in current control and FD3-FD0 in options).

By default, a flash does not occur if the super-capacitor is not fully charged (that is, the end-of-charge flag (EOC pin) must transition low). If the flash state was entered via the I2C interface (flash bit = 1 ), the LM3550 automatically resets the flash bit and the torch bit to 0 upon completion of the flash. Additionally, after the flash event has occurred, the LM3550 returns 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.)

7.4.1.7 EOC Functionality

The EOC pin of the device provides an indicator alerting the controller that the super-capacitor has reached its target voltage. The EOC pin transitions low once the capacitor reaches 95% of the target voltage for the 4.5 V, 5 V and 5.3 V modes or once the capacitor has reached the optimal charge voltage in optimal charge mode.

During operation, the LM3550 continues to monitor the voltage on the super-capacitor and updates the EOC pin when needed. Any time a mode transition occurs during charge mode or charge and torch mode, the EOC state is re-evaluated.

During torch mode, the EOC always indicates a charging state (EOC = 1) .

LM3550 30059421.gif

7.4.1.8 State Diagram FGATE = 1

By default, the LM3550 prevents 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 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.

LM3550 30059419.gif Figure 41. FGATE = 1 State Diagram

7.4.1.9 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 charges and regulate the super-capacitor to either 4.5 V, 5 V, or 5.3 V. 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 charges 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.

Equation 6. VOH = VDROOP + VHR = (IFLASH × tFLASH / CSC) + VFB + (IFLASH × RDSDON)

and

Equation 7. VCAP = VOH + VLED + [IFLASH × (RESR+ RBAL / N)]

where

  • N = Number of flash LEDs

Example:

If VLED (peak)= 4.1 V (at 1.5 A), IFLASH (total) = 3 A, CSC = 0.5 F, RESR = 50 mΩ, RDSON = 40 mΩ, VFB = 100 mV, RBAL = 75 mΩ, and tFLASH = 64 ms, then VOH = 0.604 V and VCAP = 4.97 V.

NOTE

VLED (peak) is equal to the LED voltage before self-heating occurs. Once current flows through the LED, the LED heats up, and the forward voltage decreases 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 3-A flash pulse over the entire flash duration.

Unlike fixed voltage mode, optimal charge mode adjusts 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.

7.4.1.9.1 Optimal Charge Mode vs Fixed Voltage Mode

LM3550 30059460.gif Figure 42. Optimal Charge Mode
LM3550 30059457.gif Figure 43. 5.3-V Fixed Voltage Charge Mode

Peak power dissipation across current source FET

Equation 8. PNFET (maximum) = IFLASH × (VOH − VFB)

where

  • Optimal mode = 1.5 W, fixed voltage mode (5.3 V) = 2.4 W

Average power dissipation across current source FET (64 ms pulse)

Equation 9. PNFET (average) = IFLASH × [VOH − (VDROOP ÷ 2) − VFB]

where

  • Optimal mode = 936 mW, fixed voltage mode (5.3 V) = 1.824 W

7.5 Programming

7.5.1 I2C-Compatible Interface

7.5.1.1 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.

LM3550 30059404.gif Figure 44. Data Validity Diagram

A pullup resistor between VIO (logic power supply) and SDA must be greater than [(VIO − VOL) / 3 mA] to meet the VOL requirement on SDA. Using a larger pullup resistor results in lower switching current with slower edges, while using a smaller pullup resistor results in higher switching currents with faster edges.

7.5.1.2 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 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.

LM3550 30059412.gif Figure 45. Start and Stop Conditions

7.5.1.3 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.

LM3550 30059413.gif
w = write (SDA = 0)
ack = acknowledge (SDA pulled down by the slave)
id = chip address, 53h for LM3550
Figure 46. Write Cycle

7.5.1.4 I2C-Compatible Child Address: 0x53

LM3550 30059410.gif

7.6 Register Maps

7.6.1 Internal Registers

Register 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

7.6.1.1 General Purpose Register Description

LM3550 30059405.gif

FLASH, CHARGE, and TORCH: Mode Bits (see Table 2).

CM0–CM1: Capacitor Charge Mode (see Table 3).

FGATE: Flash Gate Bit. If FGATE is a 0, then an end-of-charge 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)

Table 2. Control Modes

Flash Charge Torch Mode
0 0 0 Disabled
0 0 1 Torch
0 1 0 Charge
0 1 1 Charge and Torch
1 x x Flash

Table 3. Capacitor Charge Level

CM1 CM0 Level
0 0 Optimal Charge Mode
0 1 4.5
1 0 5.0V
1 1 5.3V

Table 4. Gated Flash Control

FGATE Bit Result
0 Flash only allowed after EOC reached
1 Flash allowed without EOC reached

Table 5. ALD/TEMP Control

A/T EN Bit Result
0 ALD MODE DISABLED
1 ALD MODE ENABLED

7.6.1.2 Current Control Register Description

LM3550 30059406.gif

Table 6. Torch Level Table

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

Table 7. Flash Level Table

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
1 0 0 70 mV
1 0 1 80 mV
1 1 0 90 mV
1 1 1 100 mV

7.6.1.3 Options Control Register Description

LM3550 30059407.gif

SLE: Strobe Level or Edge Sensitivity. 0 = Edge Sensitive, 1 = Level Sensitive

FD0-FD3: Flash Duration control bits (see Table 8).

OH0-OH2: Overhead Charge Voltage control bits (see Table 9).

Table 8. Time-out Duration Table

FD3 FD2 FD1 FD0 Time (msec)
0 0 0 0 16
0 0 0 1 32
0 0 1 0 48
0 0 1 1 64
0 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

Table 9. Overhead Charge Voltage Table

OH2 OH1 OH0 Level
0 0 0 300 mV
0 0 1 400 mV
0 1 0 500 mV
0 1 1 600 mV
1 0 0 700 mV
1 0 1 800 mV
1 1 0 900 mV
1 1 1 1V

7.6.1.4 ALD/TEMP Sense High/Low Registers

LM3550 30059408.gif Figure 47. ALD/TEMP Sense High Register
LM3550 30059409.gif Figure 48. ALD/TEMP Sense Low Register

For ALD/TEMP Sense High and ALD/TEMP sense low, the trip levels are set by Equation 10:

Equation 10. Sense High/Low = 1 V × N / (26 − 1)

where

  • N is the decimal equivalent of the value stored in the ALD/TEMP Sense High/Low registers

NSENSEHIGH must be greater than NSENSELOW.