LTC3553-2_15 Datasheet, PDF (15/36 Pages) Linear Technology – Micropower USB Power Manager with Li-Ion Charger, Always-On LDO and Buck Regulator

LTC3553-2
OPERATION
Introduction
The LTC3553-2 is a highly integrated power management
IC that includes the following features:
PowerPath controller
Battery charger
Ideal diode
Pushbutton controller
200mA buck regulator
Always-on 150mA low dropout (LDO) linear regulator
Designed specifically for USB applications, the PowerPath
controller incorporates a precision input current limit
which communicates with the battery charger to ensure
that input current never violates the USB specifications.
The ideal diode from BAT to V_OUT guarantees that ample
power is always available to V_OUT even if there is insufficient
or absent power at V_BUS. The LTC3553-2 also includes a
pushbutton input to control the two regulators and system
reset. The constant-frequency current mode step-down
switching regulator provides 200mA and supports 100%
duty cycle operation as well as Burst Mode operation for
high efficiency at light load. No external compensation
components are required for the switching regulator. The
LDO can deliver up to 150mA, and is stable with a ceramic
output capacitor of at least 1μF. For application flexibility,
the LDO’s power input pin, V_INLDO, is independent of the
buck’s BVIN pin.
Either regulator can be programmed for a minimum
output voltage of 0.8V and can be used to power a mi-
crocontroller core, microcontroller I/O, memory or other
logic circuitry. The buck regulator operates at 1.125MHz.
Simplified PowerPath Block Diagram
V_BUS 20
100mA/500mA
INPUT CURRENT
LIMIT
CC/CV
CHARGER
18 V_OUT
IDEAL
15mV
17 BAT
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The buck regulator includes a low power standby mode
which can be used to power essential keep-alive circuitry
while draining ultralow current from the battery for ex-
tended battery life.
USB PowerPath Controller
The input current limit and charger control circuits of the
LTC3553-2 are designed to limit input current as well as
control battery charge current as a function of I_VOUT. V_OUT
drives the combination of the external load, the buck and
LDO regulators and the battery charger.
If the combined load does not exceed the programmed
input current limit, V_OUT will be connected to V_BUS through
an internal 350mΩ P-channel MOSFET. If the combined
load at V_OUT exceeds the programmed input current limit,
the battery charger will reduce its charge current by the
amount necessary to enable the external load to be satisfied
while maintaining the programmed input current. Even if
the battery charge current is set to exceed the allowable
USB current, the average input current USB specification
will not be violated. Furthermore, load current at V_OUT will
always be prioritized and only excess available current will
be used to charge the battery.
The input current limit is programmed by the HPWR and
SUSP pins. If SUSP pin set high, the input current limit
is disabled. If SUSP pin is low, the input current limit is
enabled. HPWR pin selects between 100mA input current
limit when it is low and 500mA input current limit when
it is high.
Ideal Diode From BAT to V_OUT
The LTC3553-2 has an internal ideal diode from BAT to
V_OUT designed to respond quickly whenever V_OUT drops
below BAT. If the load increases beyond the input current
limit, additional current will be pulled from the battery via
the ideal diode. Furthermore, if power to V_BUS (USB) is
removed, then all of the application power will be provided
by the battery via the ideal diode. The ideal diode is fast
enough to keep V_OUT from dropping significantly with
just the recommended output capacitor. The ideal diode
consists of a precision amplifier that enables an on-chip
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