STANDBY

RESET

INTERNAL

STARTUP

SEQUENCE

Reset Register write

or POR = H or NRST = L

EN_BOOST = H*

TSD = H

~10 ms Delay

BOOST STARTUP

NSTBY = H

NSTBY = L

EN_BOOST = L*

~10 ms Delay

NORMAL MODE

EN_BOOST

rising edge*

* TSD = L

POR = L

LP55281

SNVS458B – JUNE 2007 – REVISED JUNE 2007

www.ti.com

Modes of Operation

RESET: In the RESET mode all the internal registers are reset to the default values and the device goes to

STANDBY mode after reset. NSTBY control bit is low after reset by default. Reset is entered always if

Reset Register is written, internal Power On Reset is active, or NRST pin is pulled down externally. The

LP55281 can be reset by writing any data to the Reset Register (address 60H). Power On Reset (POR)

above 1.5V, POR will inactivate and the device will continue to the STANDBY mode.

STANDBY: The STANDBY mode is entered if the register bit NSTBY is LOW. This is the low power

consumption mode, when all circuit functions are disabled. Registers can be written in this mode and the

control bits are effective immediately after startup.

STARTUP: When NSTBY bit is written high, the INTERNAL STARTUP SEQUENCE powers up all the needed

internal blocks (VREF, Oscillator, etc.). To ensure the correct oscillator initialization, a 10 ms delay is

generated by the internal state-machine. If the device temperature rises too high, the Thermal Shutdown

(TSD) disables the device operation and STARTUP mode is entered until no thermal shutdown is present.

BOOST STARTUP: Soft start for boost output is generated in the BOOST STARTUP mode. The boost output is

raised in PWM mode during the 10 ms delay generated by the state-machine. The Boost startup is

entered from Internal Startup Sequence if EN_BOOST is HIGH or from Normal mode when EN_BOOST is

written HIGH. During the 10 ms Boost Startup time all LED outputs are switched off to ensure smooth

startup.

NORMAL: During NORMAL mode the user controls the device using the Control Registers. The registers can be

written in any sequence and any number of bits can be altered in a register in one write.

8

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