LTC3566/LTC3566-2
12
3566fb
OPERATION
Introduction
The LTC3566 family are highly integrated power manage-
ment ICs which includes a high effi ciency 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 specifi cally 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 family allows the load current on V
OUT
to exceed
the current drawn by the USB port, making maximum
use of the allowable USB power for battery charging.
For USB compatibility, the switching regulator includes
a precision average input current limit. The PowerPath
switching regulator and battery charger communicate to
ensure that the average input current never exceeds the
USB specifi cations.
The PowerPath switching regulator and battery charger
communicate to ensure that the input current never violates
the USB specifi cations.
The ideal diode from BAT to V
OUT
guarantees that ample
power is always available to V
OUT
even if there is insuf-
fi cient 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 family 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 signifi cantly reduced quiescent
current under light load conditions.
High Effi ciency 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, effi ciency 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 satisfi ed. Even if the battery charge current is
programmed to exceed the allowable USB current, the
USB specifi cation for average input current will not be
violated. The battery charger will reduce its current as
needed. 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, and the load require-
ment does not cause the switching regulator to exceed its
input current limit set point, V
OUT
will regulate at a fi xed
3.6V as shown in Figure 1 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. In
the case where the battery is not present, and again, the
load requirement does not cause the switching regulator
to exceed the USB specifi cation, V
OUT
will regulate at a
fi xed 4.5V or 300mV above the 4.2V battery fl oat voltage
also providing Instant-On operation. In this case when
the load exceeds the available USB power, V
OUT
will 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 specifi cation,
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
on CLPROG represents the average input current of the
switching regulator. When the input current approaches
