LTC3556
15
3556f
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 I
2
C 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:
II
V
R
h
VBUS VBUSQ
CLPROG
CLPROG
CLPROG
=+ +
()
•1
Figure 1 shows the range of possible voltages at V
OUT
as
a function of battery voltage.
BAT (V)
2.4
4.5
4.2
3.9
3.6
3.3
3.0
2.7
2.4
3.3 3.9
3556 F01
2.7 3.0
3.6 4.2
V
OUT
(V)
NO LOAD
300mV
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
FORWARD VOLTAGE (mV) (BAT – V
OUT
)
0
CURRENT (mA)
600
1800
2000
2200
120
240
300
3556 F02
200
1400
1000
400
1600
0
1200
800
60
180
360
480420
VISHAY Si2333
OPTIONAL EXTERNAL
IDEAL DIODE
LTC3556
IDEAL DIODE
ON
SEMICONDUCTOR
MBRM120LT3
Figure 2. Ideal Diode Operation
