LTC3566 Datasheet, PDF (12/28 Pages) Linear Technology – High Effi ciency USB Power Manager Plus 1A Buck-Boost Converter

LTC3566
OPERATION
Introduction
The LTC3566 is a highly integrated power management IC
which includes a high efficiency switch mode PowerPath
controller, a battery charger, an ideal diode, an always-on
LDO, and a 1A buck-boost switching regulator. The entire
chip is controlled via direct digital inputs.
Designed specifically for USB applications, the PowerPath
controller incorporates a precision average input current
step-down switching regulator to make maximum use of
the allowable USB power. Because power is conserved,
the LTC3566 allows the load current on V_OUT to exceed
the current drawn by the USB port without exceeding the
USB load specifications.
The PowerPath switching regulator and battery charger
communicate to ensure that the 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 insuf-
ficient or absent power at V_BUS.
An “always-on” LDO provides a regulated 3.3V from avail-
able power at V_OUT. Drawing very little quiescent current,
this LDO will be on at all times and can be used to supply
up to 25mA.
The LTC3566 also has a general purpose buck-boost
switching regulator, which can be independently enabled
via direct digital control. Along with constant frequency
PWM mode, the buck-boost regulator has a low power
burst-only mode setting for significantly reduced quiescent
current under light load conditions.
High Efficiency Switching PowerPath Controller
Whenever V_BUS is available and the PowerPath switching
regulator is enabled, power is delivered from V_BUS to V_OUT
via SW. V_OUT drives both the external load (including the
buck-boost regulator) and the battery charger.
If the combined load does not exceed the PowerPath
switching regulator’s programmed input current limit, V_OUT
will track 0.3V above the battery (Bat-Track). By keeping
the voltage across the battery charger low, efficiency is
optimized because power lost to the linear battery char-
ger is minimized. Power available to the external load is
therefore optimized.
If the combined load at V_OUT is large enough to cause the
switching power supply to reach 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. Even if the battery charge current is
set to exceed the allowable USB current, the USB specifica-
tion will not be violated. The switching regulator will limit
the average input current so that the USB specification
is never violated. Furthermore, load current at V_OUT will
always be prioritized and only remaining available power
will be used to charge the battery.
If the voltage at BAT is below 3.3V, or the battery is not
present and the load requirement does not cause the switch-
ing regulator to exceed the USB specification, V_OUT will
regulate at 3.6V, thereby providing “Instant-On” operation.
If the load exceeds the available power, V_OUT will drop to
a voltage between 3.6V and the battery voltage. If there
is no battery present when the load exceeds the available
USB power, V_OUT can drop toward ground.
The power delivered from V_BUS to V_OUT is controlled
by a 2.25MHz constant-frequency step-down switching
regulator. To meet the USB maximum load specification,
the switching regulator includes a control loop which
ensures that the average input current is below the level
programmed at CLPROG.
The current at CLPROG is a fraction (h_{CLPROG–1}) of the V_BUS
current. When a programming resistor and an averaging
capacitor are connected from CLPROG to GND, the voltage
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