LTC3555 Datasheet, PDF (12/32 Pages) Linear Technology – High Effi ciency USB Power Manager + Triple Step-Down DC/DC

LTC3555/LTC3555-X
PIN FUNCTIONS
SW1 (Pin 17): Power Transmission Pin for Switching
Regulator 1.
V_IN1 (Pin 18): Power Input for Switching Regulator 1.
This pin will generally be connected to V_OUT. A 1μF MLCC
capacitor is recommended on this pin.
FB1 (Pin 19): Feedback Input for Switching Regulator 1.
When regulator 1’s control loop is complete, this pin servos
to a fixed voltage of 0.8V.
PROG (Pin 20): Charge Current Program and Charge
Current Monitor Pin. Connecting a resistor from PROG
to ground programs the charge current. If sufficient in-
put power is available in constant-current mode, this pin
servos to 1V. The voltage on this pin always represents
the actual charge current.
CHRG (Pin 21): Open-Drain Charge Status Output. The
CHRG pin indicates the status of the battery charger. Four
possible states are represented by CHRG: charging, not
charging, unresponsive battery and battery temperature
out of range. CHRG is modulated at 35kHz and switches
between a low and a high duty cycle for easy recogni-
tion by either humans or microprocessors. See Table 1.
CHRG requires a pull-up resistor and/or LED to provide
indication.
GATE (Pin 22): Analog Output. This pin controls the gate
of an optional external P-channel MOSFET transistor used
to supplement the ideal diode between V_OUT and BAT. The
external ideal diode operates in parallel with the internal
ideal diode. The source of the P-channel MOSFET should
be connected to V_OUT and the drain should be connected
to BAT. If the external ideal diode FET is not used, GATE
should be left floating.
BAT (Pin 23): Single Cell Li-Ion Battery Pin. Depending on
available V_BUS power, a Li-Ion battery on BAT will either
deliver power to V_OUT through the ideal diode or be charged
from V_OUT via the battery charger.
V_OUT (Pin 24): Output voltage of the Switching PowerPath
Controller and Input Voltage of the Battery Charger. The
majority of the portable product should be powered from
V_OUT. The LTC3555 family will partition the available power
between the external load on V_OUT and the internal battery
charger. Priority is given to the external load and any extra
power is used to charge the battery. An ideal diode from
BAT to V_OUT ensures that V_OUT is powered even if the load
exceeds the allotted power from V_BUS or if the V_BUS power
source is removed. V_OUT should be bypassed with a low
impedance ceramic capacitor.
V_BUS (Pin 25): Primary Input Power Pin. This pin delivers
power to V_OUT via the SW pin by drawing controlled current
from a DC source such as a USB port or wall adapter.
SW (Pin 26): Power Transmission Pin for the USB Power
Path. The SW pin delivers power from V_BUS to V_OUT via the
step-down switching regulator. A 3.3μH inductor should
be connected from SW to V_OUT.
I_LIM0, I_LIM1 (Pins 27, 28): Logic Inputs. I_LIM0 and I_LIM1
control the current limit of the PowerPath switching
regulator. See Table 3. Both of the I_LIM0 and I_LIM1 pins are
logically OR-ed with their corresponding bits in the I²C
serial port. See Table 2.
Exposed Pad (Pin 29): Ground. The Exposed Pad should
be connected to a continuous ground plane on the second
layer of the printed circuit board by several vias directly
under the part.
3555fd
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