LM3550SPXSLASHNOPB datasheet(18/43 Pages) TI2

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VCAP

ICHARGE

(Typ.)

525 mA

333 mA

262 mA

Gain

3/2x

Gain

LM3550

SNVS569C – MAY 2009 – REVISED OCTOBER 2016

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Device Functional Modes (continued)

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.

LM3550SPX/NOPB Datasheet(PDF) 18 Page - Texas Instruments

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