LM3551 datasheet(11/23 Pages) TI1

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LM3551, LM3552

www.ti.com

SNVS371C – AUGUST 2005 – REVISED MARCH 2011

APPLICATION INFORMATION

LM3551 AND LM3552 FUNCTIONALITY TRUTH TABLE

SD (LM3551)

T/F

LM3551 Result

LM3552 Result

EN (LM3552)

Shut-down

Torch Mode

Shut-down

Flash Mode

Torch Mode

Shut-down

Flash Mode

Shut-down

The LM3551 has a 1.2M

Ω pull-up to VIN on SD and the LM3552 has a 1.2MΩ pull-down to GND on EN.

CURRENT SET EQUATIONS

The LM3551/2 utilize inline resistors to set the Torch and Flash LED currents. The Torch-Mode current

(continuous) and the Flash-Mode current (pulsed) are programmed by placing the appropriately selected

resistors between the feedback pin (FB) and FET-T (torch FET) and FET-F (flash FET) pins. Torch-mode is set

by utilizes the current through one resistor while Flash-mode is set by utilizes the currents though both current

set resistors. The following equations are used to set the LED currents.

RTORCH = (1.265V / ITORCH) - RDSON-T

(4)

RFLASH = (1.265V / (IFLASH- ITORCH)) - RDSON-F

(5)

RDSON-T= 0.98Ω and RDSON-F = 0.36Ω

(6)

NOTE

Flash LEDs from different manufacturers can have very different continuous and pulse

current ratings. See the manufacturers datasheets to ensure that the proper current levels

are used to avoid damaging the flash LED.

INDUCTOR SELECTION

Special care must be taken when selecting an inductor for use in LM3551/2 applications. The inductor should

have a current saturation rating that is larger than the worst case peak inductor current of the application to

ensure proper operation. Using an inductor with a lower saturation current rating than is required can cause a

dramatic drop in the inductance and can derate the maximum output current levels severely. It is worth noting

that the output voltage ripple is also affected by the total ripple current in the inductor. The following equations

can help give a good approximation as to what the peak inductor current will be for a given application at room

temperature (TA = +25°C).

IL(average) = [ILED × VOUT-MAX] ÷ [VIN-MIN × Eff.]

(7)

ΔIL = [VIN × D] ÷ [L × FSW]

(8)

IL(peak) = IL(ave) + [ΔIL ÷ 2]

(9)

VOUT-MAX Maximum Output Voltage. Maximum output voltage over temperature with OVP used is 11V (12.4V

typically).

VIN-MIN Minimum Input Voltage. Recommended minimum input voltage is 3.0V. The LM3551/2 will work down to

2.7V however, use at lower input voltages will required an inductor with a higher saturation current rating.

Eff.

Converter Efficiency (approx. 85% over input voltage range).

Duty Cycle = 1 - [VIN / VOUT]

Inductance. Recommended inductance value is 4.7µH.

FSW

Switching Frequency = 1.25MHz

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