LTC3556 Datasheet, PDF (15/36 Pages) Linear Technology – High Effi ciency USB Power Manager with Dual Buck and Buck-Boost DC/DCs

LTC3556
OPERATION
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 cur-
rent by that 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 specification
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
switching 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
on CLPROG represents the average input current of the
switching regulator. When the input current approaches
the programmed limit, CLPROG reaches V_CLPROG, 1.188V
and power out is held constant. The input current limit is
programmed by the B1 and B0 bits of the I²C serial port.
It can be configured to limit average input current to one
of several possible settings as well as be deactivated (USB
Suspend). The input current limit will be set by the V_CLPROG
servo voltage and the resistor on CLPROG according to
the following expression:
( ) ^I_VBUS
= I_VBUSQ
+
V_CLPROG
R_CLPROG
•
h_CLPROG + 1
Figure 1 shows the range of possible voltages at V_OUT as
a function of battery voltage.
4.5
4.2
3.9
NO LOAD
3.6
3.3
300mV
3.0
2.7
2.4
2.4
2.7 3.0 3.3 3.6 3.9 4.2
BAT (V)
3556 F01
Figure 1. V_OUT vs BAT
Ideal Diode from BAT to V_OUT
The LTC3556 has an internal ideal diode as well as a con-
troller for an optional external ideal diode. The ideal diode
controller is always on and will respond quickly whenever
V_OUT drops below BAT.
If the load current increases beyond the power allowed
from the switching regulator, additional power will be
pulled from the battery via the ideal diode. Furthermore,
if power to V_BUS (USB or wall power) is removed, then all
of the application power will be provided by the battery via
the ideal diode. The transition from input power to battery
power at V_OUT will be quick enough to allow only the 10μF
capacitor to keep V_OUT from drooping. The ideal diode
consists of a precision amplifier that enables a large on-
chip P-channel MOSFET transistor whenever the voltage at
2200
2000
1800
VISHAY Si2333
OPTIONAL EXTERNAL
IDEAL DIODE
1600
1400
1200
1000
LTC3556
IDEAL DIODE
800
600
ON
400
SEMICONDUCTOR
MBRM120LT3
200
0
0 60 120 180 240 300 360 420 480
FORWARD VOLTAGE (mV) (BAT – V_OUT)
3556 F02
Figure 2. Ideal Diode Operation
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