Shutdown

Charge

Flash

Charge and

Torch

Torch

Optimal

Charge

Mode

Voltage

Mode

Charge

21

LM3550

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SNVS569C – MAY 2009 – REVISED OCTOBER 2016

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Device Functional Modes (continued)

7.4.1.8 State Diagram FGATE = 1

By default, the LM3550 prevents a flash event from occurring if the super-capacitor has not reached the target

voltage (EOC = 0). In the event that this restriction is not desired, the flash gate bit (FGATE in the General

Purpose Register) can be set to a 1 disabling the end-of-charge requirement. Setting FGATE to a 1 allows the

flash state to be entered at anytime. If the super-capacitor is not charged to the proper voltage before the EOC

pin indicates a full charge, the perceived duration and flash level could be lower than desired.

Figure 41. FGATE = 1 State Diagram

7.4.1.9 Optimal Charge Mode vs Fixed Voltage Mode

The LM3550 provides two types of super-capacitor charging modes: fixed voltage and optimal charge.

In fixed voltage mode, the LM3550 charges and regulate the super-capacitor to either 4.5 V, 5 V, or 5.3 V. This

mode is useful if the LM3550 is going to be used for both flash and fixed-rail applications (power supply for audio

or PA sub-systems).

If the LM3550 is only going to be used as a super-capacitor charger and flash controller, the optimal charge

mode provides many advantages over the fixed voltage mode. optimal charge mode charges the super-capacitor

to the minimum voltage that is required to sustain a flash pulse compensating for variations in super-capacitor

ESR and LED forward voltage due to temperature and process. To properly use the optimal charge mode, the

overhead voltage (VOH) must be determined. The overhead voltage is equal to the voltage required to maintain

current source regulation (VHR) plus the voltage droop (VDROOP) on the super-capacitor due to the flash event.

VOH = VDROOP + VHR = (IFLASH × tFLASH / CSC) + VFB + (IFLASH × RDSDON)

(6)

and

VCAP = VOH + VLED + [IFLASH × (RESR+ RBAL / N)]

where

•

N = Number of flash LEDs

(7)