LTC3555 datasheet(21/32 Pages) LINER

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LTC3555/LTC3555-X

3555fd

the LTC3555 family will remember the last input of valid

data that it received. Once all chips on the bus have been

addressed and sent valid data, a global STOP condition can

be sent and the LTC3555 family will update its command

latch with the data that it had received.

In certain circumstances the data on the I2C bus may

become corrupted. In these cases the LTC3555 family

responds appropriately by preserving only the last set of

complete data that it has received. For example, assume

the LTC3555 family has been successfully addressed and is

receiving data when a STOP condition mistakenly occurs.

The LTC3555 family will ignore this STOP condition and will

not respond until a new START condition, correct address,

new set of data and STOP condition are transmitted.

Likewise, with only one exception, if the LTC3555 family was

previously addressed and sent valid data but not updated

with a STOP, it will respond to any STOP that appears on

the bus, independent of the number of REPEAT-STARTS

that have occurred. If a REPEAT-START is given and the

LTC3555 family successfully acknowledges its address and

ﬁrst byte, it will not respond to a STOP until both bytes of

the new data have been received and acknowledged.

Disabling the I2C Port

The I2C serial port can be disabled by grounding the DVCC

pin. In this mode, control automatically passes to the in-

dividual logic input pins EN1, EN2, EN3, ILIM0, ILIM1, SDA

and SCL. Some functionality is not available in this mode

such as the programmability of switching regulators 2

and 3’s output voltage and the battery charger disable

feature. In this mode, both of the programmable switching

regulators have a ﬁxed servo voltage of 0.8V.

Because the SDA and SCL pins have no other context when

DVCC is grounded, these pins are re-mapped to control

the switching regulator mode bits B5 and B6. SCL maps

to B5 and SDA maps to B6.

RST3 Pin

The RST3 pin is an open-drain output used to indicate that

switching regulator 3 has reached its ﬁnal voltage. RST3

remains low impedance until regulator 3 reaches 92% of

its regulation value. A 230ms delay is included to allow a

system microcontroller ample time to reset itself. RST3

may be used as a power-on reset to the microprocessor

1 and/or 2 for supply sequencing. RST3 is an open-drain

output and requires a pull-up resistor to the output voltage

of regulator 3 or another appropriate power source.

General Purpose Step-Down Switching Regulators

The LTC3555 family contains three general purpose

2.25MHz step-down constant-frequency current mode

switching regulators. Two regulators provide up to 400mA

and a third switching regulator can produce up to 1A.

All three switching regulators can be programmed for a

minimum output voltage of 0.8V and can be used to power

a microcontroller core, microcontroller I/O, memory, disk

drive or other logic circuitry. Two of the switching regulators

have I2C programmable set-points for on-the-ﬂy power

savings. All three converters support 100% duty cycle

operation (low dropout mode) when their input voltage

drops very close to their output voltage. To suit a variety

of applications, selectable mode functions can be used

to trade-off noise for efﬁciency. Four modes are available

to control the operation of the LTC3555 family’s general

purpose switching regulators. At moderate to heavy loads,

the pulse skip mode provides the least noise switching

solution. At lighter loads, either Burst Mode operation,

forced Burst Mode operation or LDO mode may be selected.

The switching regulators include soft-start to limit inrush

current when powering on, short-circuit current protection

and switch node slew limiting circuitry to reduce radiated

EMI. No external compensation components are required.

The operating mode of the regulators may be set by either

I2C control or by manual control of the SDA and SCL pins

if the I2C port is not used. Each converter may be individu-

ally enabled by either their external control pins EN1, EN2,

EN3 or by the I2C port. Switching regulators 2 and 3 have

individual programmable feedback servo voltages via I2C

control. The switching regulator input supplies VIN1, VIN2

and VIN3 will generally be connected to the system load

pin VOUT.

OPERATION

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