Datasheet

LTC3555/LTC3555-X
15
3555fd
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 excess 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. 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 I
LIM0
and I
LIM1
pins or by 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:
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.
The LTC3555 vs the LTC3555-1 and LTC3555-3
For very low battery voltages, the battery charger acts
like a load and, due to limited input power, its current will
tend to pull V
OUT
below the 3.6V “instant-on” voltage. To
prevent V
OUT
from falling below this level, the LTC3555-1
and LTC3555-3 include an undervoltage circuit that auto-
matic detects that V
OUT
is falling and reduces the battery
charge current as needed. This reduction ensures that load
current and output voltage are always prioritized and yet
delivers as much battery charge current as possible. The
standard LTC3555 does not include this circuit and thus
favors maximum charge current at all times over output
voltage preservation.
If instant-on operation under low battery conditions is a
requirement then the LTC3555-1 or LTC3555-3 should be
used. If maximum charge efficiency at low battery voltages
is preferred, and instant-on operation is not a requirement,
then the standard LTC3555 should be selected. All versions
of the LTC3555 family will start up with a removed battery.
The LTC3555-3 has a battery charger float voltage of 4.100V
rather than the 4.200V float voltage of the LTC3555 and
LTC3555-1.
Ideal Diode from BAT to V
OUT
The LTC3555 family has an internal ideal diode as well as
a controller 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
OPERATION
Figure 1. V
OUT
vs BAT
BAT (V)
2.4
4.5
4.2
3.9
3.6
3.3
3.0
2.7
2.4
3.3 3.9
3555 F01
2.7 3.0
3.6 4.2
V
OUT
(V)
NO LOAD
300mV