(Typ.)

525 mA

333 mA

262 mA

Gain

Of

1x

Gain

Of

3/2x

Gain

Of

2x

LM3550

SNVS569B – MAY 2009 – REVISED MAY 2013

www.ti.com

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-

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,

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.

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.

16

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