LM3550_13 Datasheet, PDF (16/37 Pages) Texas Instruments – 5A Flash LED Driver with Automatic VLED and ESR Detection for Mobile Camera Systems

LM3550
SNVS569B – MAY 2009 – REVISED MAY 2013
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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 V_LED + V_TREG (V_TREG ≈ 300 mV).
When in the Torch state, the LM3550 will regulate the proper output voltage (either 3.0V or V_LED + V_REG) 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.
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 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 will deliver different charge currents
to the super-capacitor. Charge current is dependent on the charge-pump gain.
525 mA
I_CHARGE
(Typ.)
Gain
Of
1x
333 mA
262 mA
Gain
Of
3/2x
Gain
Of
2x
V_CAP
Figure 46. 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 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 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 Mode description in the Application Information section of this datasheet.
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