LTC3455 Datasheet, PDF (7/24 Pages) Linear Technology – Dual DC/DC Converter with USB Power Manager and Li-Ion Battery Charger

LTC3455
PI FU CTIO S
FB1 (Pin 1): Feedback Pin for Switcher 1. Set the output
voltage by connecting feedback resistors to this pin.
PROG (Pin 2): Charge Current Program and Charge Cur-
rent Monitor Pin. Connect a resistor, R_PROG, from this pin
to ground to program battery charge current.
I_BAT = 1000 • 1.23V / R_PROG
In all modes the voltage on the PROG pin can be used to
measure charge current. PROG has a weak pull-up current
source to turn the charger off if the pin is left open.
TIMER (Pin 3): Timer Capacitor Pin. Connect a capacitor,
C_TIMER, between this pin and ground to set the charge
cycle termination time. The timer starts when USB or wall
power is first present. The timer period is:
T_TIMER (hours) = C_TIMER • (3 hours) / (0.1µF)
Tie TIMER to ground to disable just the internal timer
function. Tie TIMER to V_MAX to use the charger in a
constant-current-only mode (which disables the timer,
voltage amplifier and trickle charge function).
CHRG (Pin 4): Open-Drain Charge Status Pin. This pin is
pulled low with an internal N-channel MOSFET whenever
the battery charger is enabled, and is forced into a high
impedance state whenever it is disabled.
USBHP (Pin 5): USB High Power Mode Pin. This pin is
used to select the appropriate USB current limit (either
500mA or 100mA). Pull high to select 500mA (high power
mode); low to select 100mA (low power mode).
SUSPEND (Pin 6): USB Suspend Pin. When this pin is
pulled high, the internal USB power controller is disabled
and the USB pin current reduces to less than 20µA.
SW1 (Pin 7): Switch Pin for Switcher 1. Minimize the
length of the metal trace connected to this pin. Place the
inductor for Switcher 1 as close to this pin as possible.
USB (Pin 8): USB Supply Pin. Input current into this pin is
limited to either 100mA or 500mA based on the state of the
USBHP pin. The charger and Switcher 1 will remain alive
whenever USB power is present (when USB pin is above
3.9V and SUSPEND is low).
V_BAT (Pin 9): Battery Input Pin. Bypass this pin with a
capacitor as close to the device as possible.
V_MAX (Pin 10): Max Voltage Pin. This pin is used to power
the two internal step-down DC/DC converters and is
provided externally to power other devices (i.e. LDOs or
Switchers for LCD bias, white LED backlight drive, etc).
When the LTC3455 is on and neither USB or wall power are
available, an internal PMOS switch connects this pin to the
V_BAT pin. When either USB or wall power is present, they
provide power to this pin, and the battery charger draws
power from this pin. In shutdown, this pin is discharged to
ground to provide output disconnect.
WALLFB (Pin 11): Wall Power Detection Pin. This pin is
the input to a comparator used to signal the presence of a
5V wall adapter. A resistor divider taken from the wall
adapter input is connected to this pin to tell the LTC3455
when the adapter voltage is high enough to provide power
to the LTC3455. When this pin is higher than 1.23V, the
battery charger is enabled. The 5V wall adapter is con-
nected to the V_MAX pin through a Schottky diode. Tie
WALLFB to ground if a wall adapter is not used.
SW2 (Pin 12): Switch Pin for Switcher 2. Minimize the
length of the metal trace connected to this pin. Place the
inductor for Switcher 2 as close to this pin as possible.
HSI (Pin 13): Hot Swap Input Pin. This pin is connected to
the HSO pin through a current-limited PMOS switch.
HSO (Pin 14): Hot Swap Output Pin. This output is used for
memory cards or other devices that would appear as a
short if they were hot-plugged directly to one of the
outputs (typically the 3.3V output). The current out of this
pin is limited to 160mA.
HSON (Pin 15): Hot Swap Enable Pin. This pin turns on the
PMOS that connects the HSI and HSO pins.
AI (Pin 16): Gain Block Input Pin. This pin is the inverting
input to an amplifier that can be used as a low-battery
detector or as an LDO with the addition of an external PNP
or PMOS. The non-inverting input of the gain block is
connected to the 0.8V internal reference.
AO (Pin 17): Gain Block Output Pin. This pin is an open-
drain output, and is pulled low when the AI pin is less than
800mV. This output can be used as a low-battery detector,
or as an LDO with the addition of an external PNP or PMOS.
This pin can sink up to 1mA.
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