LTC3556_15 Datasheet, PDF (21/36 Pages) Linear Technology – High Effi ciency USB Power Manager with Dual Buck and Buck-Boost DC/DCs

LTC3556
OPERATION
Disabling the I²C Port
The I²C serial port can be disabled by grounding the
DV_CC 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 V_IN1 and V_IN2 will generally be connected
to the system load pin V_OUT.
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:
V_OUTX
=
V_FBX
⎛ R1
⎝⎜ R2
+
1^{⎞⎠⎟}
where V_FBX ranges from 0.425V to 0.8V for buck regula-
tor 1 and V_FBX 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, C_FB, 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
V_INx
L
SWx
LTC3556
C_FB
FBx
GND
V_OUTx
R1
C_OUT
R2
3556 F04
Figure 4. Buck Converter Application Circuit
3556f
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