LM3554 datasheet(18/43 Pages) TI1

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IN x

(

)

OUT - V

OUT

(

)

PEAK

- '

LOAD =

OUT

LM3554

SNVS549A – JUNE 2009 – REVISED APRIL 2011

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VOLTAGE OUTPUT MODE

Bit 2 (VM) of the Torch Brightness Register, bit 2 (VM) of the Flash Brightness Register, or the ENVM input

enables or disables the Voltage Output mode. In Voltage Output mode the device operates as a simple boost

converter with two selectable voltage levels (4.5V and 5V). Write a (1) to bit 1 (OV) of Configuration Register 1 to

set VOUT to 5V. Write a (0) to this bit to set VOUT to 4.5V. In Voltage Output mode the LED current sources can

continue to operate; however, the difference between VOUT and VLED will be dropped across the current sources.

(See MAXIMUM OUTPUT POWER section.) In Voltage Output mode when VIN is greater than VOUT the LM3554

operates in Pass Mode (see PASS MODE section).

At light loads the LM3554 switches over to a pulsed frequency mode operation (light load comparator enabled).

In this mode the device will only switch as necessary to maintain VOUT within regulation. This mode provides a

better efficiency due to the reduction in switching losses which become a larger portion of the total power loss at

light loads.

OVER-VOLTAGE PROTECTION

The output voltage is limited to typically 5.6V (5.7V max). In situations such as the current source open, the

LM3554 will raise the output voltage in order to try and keep the LED current at its target value. When VOUT

reaches 5.6V the over-voltage comparator will trip and turn off both the internal NFET and PFET. When VOUT

falls below 5.4V (typical), the LM3554 will begin switching again.

CURRENT LIMIT

The LM3554 features 4 selectable current limits: 1A, 1.5A, 2A, and 2.5A. These are selectable through the I2C-

compatible interface via bits 5 (CL0) and 6 (CL1) of the Flash Duration Register. When the current limit is

reached, the LM3554 stops switching for the remainder of the switching cycle.

Since the current limit is sensed in the NMOS switch there is no mechanism to limit the current when the device

operates in Pass Mode. In situations where there could potentially be large load currents at OUT, and the

LM3554 is operating in Pass mode, the load current must be limited to 2.5A. In Boost mode or Pass mode if

VOUT falls below approximately 2.3V, the part stops switching, and the PFET operates as a current source

limiting the current to typically 350mA. This prevents damage to the LM3554 and excessive current draw from

the battery during output short circuit conditions.

MAXIMUM LOAD CURRENT (VOLTAGE MODE)

Assuming the power dissipation in the LM3554 and the ambient temperature are such that the device will not hit

thermal shutdown, the maximum load current as a function of IPEAK is:

(1)

Where

η is efficiency and is found in the efficiency curves in the Typical Performance Characteristics and

(2)

Figure 3 shows the theoretical maximum Output current vs theoretical Efficiency at different input and output

voltages using the previous two equations for

ΔIL and ILOAD with a peak current of 2.5A. This plot represents the

theoretical maximum output current (for the LM3554 in Voltage Output mode) that the device can deliver just

before hitting current limit.

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