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LM3550

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SNVS569C – MAY 2009 – REVISED OCTOBER 2016

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Copyright © 2009–2016, Texas Instruments Incorporated

Device Functional Modes (continued)

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.