LM3550

SNVS569B – MAY 2009 – REVISED MAY 2013

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CIRCUIT DESCRIPTION

BASIC OPERATION

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 (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

DETAILED PIN DESCRIPTION

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-

duration value, the LM3550 will automatically disable the flash current.

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.

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 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.

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