LM3554 datasheet(34/43 Pages) TI1

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Bit 0

Bit 1

Bit 2

Bit 3

Bit 4

Bit 5

N/A

Bit 7

Bit 6

MSB

LSB

VIN Monitor Register

N/A

Enable

N/A

Threshold

VIN

Monitor

LM3554

SNVS549A – JUNE 2009 – REVISED APRIL 2011

www.ti.com

Table 8. GPIO Register Bit Settings

Bit 7 (Not

Bit 6 (NTC

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Used)

External Flag)

(ENVM/TX2/GP (ENVM/TX2/GP (ENVM/TX2/GP (TX1/TORCH/G (TX1/TORCH/G (TX1/TORCH/G

IO2 data)

IO2 data

IO2 Control)

PIO1 data)

PIO1 data

PIO1 Control)

direction)

Reads Back '1'

0 = NTC

This bit is the

0 =

This bit is the

0 =

External Flag

read or write

ENVM/TX2/GPI ENVM/TX2/GPI

read or write

TX1/TORCH/G

mode is

data for the

O2 is a GPIO

O2 is

data for the

PIO1 is a GPIO

PIO1 pin is

disabled

ENVM/TX2/GPI

Input (default)

configured

TX1/TORCH/G

input (default)

configured as

(default)

O2 pin in GPIO

according to

PIO1 pin in

an active low

mode (default

the

GPIO mode

reset input

is 0)

Configuration

(default is 0)

(default)

1 = When

1 =

ENVM/TX2/GPI

ENVM/TX2/GPI ENVM/TX2/GPI

TX!/TORCH/GP

TX1/TORCH/G

O2 is

O2 is a GPIO

O2 is

IO1 is an

PIO1 pin is

configured as a

Output

configured as a

output

configured as a

GPIO output

GPIO

the

ENVM/TX2/GPI

O2 pin will pull

low when the

LED Thermal

Fault Flag is set

VIN MONITOR REGISTER

The VIN Monitor Register controls the on/off state of the VIN Monitor comparator as well as selects the 4

programmable thresholds. Figure 26 and Table 9 describe the bit settings of the VIN Monitor feature.

Figure 26. VIN Monitor Register Description

Table 9. VIN Monitor Register Bit Settings

Bit 7 (Not

Bit 6 (Not

Bit 5 (Not

Bit 4 (Not

Bit 3 (Not used)

Bit 2 (VIN

Bit 1 (VIN

Bit 0 (VIN

used)

Threshold)

Monitor Enable)

Reads Back '1'

Reads Back '0'

00 = 3.1V threshold (VIN falling)

0 = VIN

Default

Monitoring

01=3.2V threshold (VIN falling)

Comparator is

10 = 3.3V threshold (VIN falling)

disabled

11 = 3.4V threshold (VIN falling)

(default)

1 = VIN

Monitoring

Comparator is

enabled.

Applications Information

OUTPUT CAPACITOR SELECTION

The LM3554 is designed to operate with a at least a 4.7µF ceramic output capacitor in LED mode and a 10µF

output capacitor in Voltage Output Mode. When the boost converter is running the output capacitor supplies the

load current during the boost converters on-time. When the NMOS switch turns off the inductor energy is

discharged through the internal PMOS switch supplying power to the load and restoring charge to the output

capacitor. This causes a sag in the output voltage during the on-time and a rise in the output voltage during the

off-time. The output capacitor is therefore chosen to limit the output ripple to an acceptable level depending on

load current and input/output voltage differentials and also to ensure the converter remains stable.

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