Datasheet

LTC3677-3
20
36773f
operaTion
support 100% duty cycle operation as well as operating
in Burst Mode operation for high efficiency at light load.
No external compensation components are required for
the switching regulators. The two low dropout regulators
can output up to 150mA.
All regulators can be programmed for a minimum output
voltage of 0.8V and can be used to power a microcon-
troller core, microcontroller I/O, memory or other logic
circuitry.
USB PowerPath Controller
The input current limit and charge control circuits of the
LTC3677-3 is designed to limit input current as well as
control battery charge current as a function of I
VOUT
.
V
OUT
drives the combination of the external load, the
three step-down switching regulators, two LDOs and the
battery charger.
If the combined load does not exceed the programmed
input current limit, V
OUT
will be connected to V
BUS
through
an internal 200mΩ P-channel MOSFET. If the combined
load at V
OUT
exceeds 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
while maintaining the programmed input current. Even if
the battery charge current is set to exceed the allowable
USB current, the average input current USB specification
will not be violated. Furthermore, load current at V
OUT
will always be prioritized and only excess available cur-
rent will be used to charge the battery. The current out
of the CLPROG pin is a fraction (1/h
CLPROG
) of the V
BUS
current. When a programming resistor is connected from
CLPROG to GND, the voltage on CLPROG represents the
input current:
I I
V
R
h
VBUS BUSQ
CLPROG
CLPROG
CLPROG
= + •
where I
BUSQ
and h
CLPROG
are given in the Electrical
Characteristics table.
The input current limit is programmed by the I
LIM0
and
I
LIM1
pins. The LTC3677-3 can be configured to limit input
current to one of several possible settings as well as be
deactivated (USB suspend). The input current limit will be
set by the appropriate servo voltage and the resistor on
CLPROG according to the following expression:
I I
V
R
h x Mode
I
VBUS BUSQ
CLPROG
CLPROG
V
= +
( )
0 2
1
.
•  
BBUS BUSQ
CLPROG
CLPROG
VBUS
I
V
R
h x Mode
I
= +
( )
1
5•  
== +
( )
I
V
R
h x Mode
BUSQ
CLPROG
CLPROG
2
10•  
Under worst-case conditions, the USB specification for
average input current will not be violated with an R
CLPROG
resistor of 2.1k or greater. Table 1 shows the available
settings for the I
LIM0
and I
LIM1
pins:
Table 1. Controlled Input Current Limit
I
LIM1
I
LIM0
I
BUS(LIM)
1 1 100mA (1x)
1 0 1A (10x)
0 1 Suspend
0 0 500mA (5x)
Notice that when I
LIM0
is low and I
LIM1
is high, the input
current limit is set to a higher current limit for increased
charging and current availability at V
OUT
. This mode is
typically used when there is a higher power, non-USB
source available at the V
BUS
pin.