LTC3556_15 datasheet(21/36 Pages) LINER

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LTC3556

3556f

Disabling the I2C Port

The I2C serial port can be disabled by grounding the

DVCC pin. In this mode, control automatically passes to

the individual logic input pins ENALL and SEQ. However,

considerable functionality is not available in this mode such

as the ability to independently enable the three switching

regulators and disable the battery charger. In addition,

with the I2C port disabled, both programmable switching

regulators default to a ﬁxed servo voltage of 0.8V, both

400mA bucks default to pulse skip mode, the 1A buck-

boost defaults to PWM mode, and the USB input current

limit defaults to 1x mode (100mA Limit).

PGOODALL Pin

The PGOODALL pin is an open-drain output used to in-

dicate that all enabled switching regulators have reached

their ﬁnal voltage. PGOODALL remains low impedance

until the last enabled regulator in the sequence reaches

92% of its regulation value. A 230ms delay is included

to allow a system microcontroller ample time to reset

itself. PGOODALL may be used as a power-on reset to the

microprocessor powered by one (or more) of the three

regulated outputs. PGOODALL is an open-drain output

and requires a pull-up resistor to the input voltage of

the monitoring microprocessor or another appropriate

power source.

400mA Step-Down Switching Regulators

The LTC3556 contains two 2.25MHz step-down (buck)

constant-frequency current mode switching regulators.

Each buck regulator can provide up to 400mA of output

current. Both buck 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. One of the buck regulators

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

savings. Both buck 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 LTC3556’s buck 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 buck 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 buck

regulators can be set by I2C control and defaults to pulse

skip mode if the I2C port is not used. Both buck converters

are enabled (along with the buck-boost) when the ENALL

pin is asserted or each may be individually enabled by the

I2C port. Buck regulator 1 has a programmable feedback

servo voltage via I2C control (which defaults to 800mV if

the I2C port is not used) whereas buck regulator 2 has a

ﬁxed feedback servo voltage of 800mV. The buck regulator

input supplies VIN1 and VIN2 will generally be connected

to the system load pin VOUT.

Buck Regulator Output Voltage Programming

Both buck regulators can be programmed for output

voltages greater than 0.8V. The full-scale output voltage

for each buck regulator is programmed using a resistor

divider from the buck regulator output connected to the

feedback pins (FB1 and FB2) such that:

OUTX

FBX

⎛

⎝⎜

⎞

⎠⎟

where VFBX ranges from 0.425V to 0.8V for buck regula-

tor 1 and VFBX is ﬁxed at 0.8V for buck regulator 2. See

Figure 4.

Typical values for R1 are in the range of 40k to 1M. The

capacitor, CFB, cancels the pole created by feedback resis-

tors and the input capacitance of the FBx pin and also helps

to improve transient response for output voltages much

OPERATION

VINx

LTC3556

SWx

COUT

CFB

VOUTx

3556 F04

FBx

GND

Figure 4. Buck Converter Application Circuit

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