LTC3577_15 Datasheet, PDF (21/54 Pages) Linear Technology – Highly Integrated 6-Channel Portable PMIC

LTC3577/LTC3577-1
OPERATION
a small voltage above the BAT pin voltage. This control
method provides a high input voltage, high efficiency
battery charger and PowerPath function.
The LTC3577 also includes a pushbutton input to control
the three synchronous step-down switching regulators
and system reset. The three 2.25MHz constant frequency
current mode step-down switching regulators provide
500mA, 500mA and 800mA each and support 100% duty
cycle operation as well as Burst Mode operation for high
efficiency at light load. No external compensation compo-
nents are required for the switching regulators.
The onboard LED backlight boost circuitry can drive up
to 10 series LEDs and includes versatile digital dimming
via the I²C input. The I²C input also controls two 150mA
low dropout (LDO) linear regulators.
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
LTC3577 are 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, LED backlight
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_VBUS
= I_BUSQ
+
V_CLPROG
R_CLPROG
• h_CLPROG
where I_BUSQ and h_CLPROG are given in the Electrical Char-
acteristics table.
The input current limit is programmed by the I_LIM0 and
I_LIM1 pins. The LTC3577 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_VBUS
= I_BUSQ
+
0.2V
R_CLPROG
• h_CLPROG
1x Mode
( ) I_VBUS
= I_BUSQ
+
1V
R_CLPROG
• h_CLPROG
5x Mode
( ) I_VBUS
= I_BUSQ
+
2V
R_CLPROG
• h_CLPROG
10x Mode
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
0
I_LIM0
0
0
1
1
0
1
1
I_BUS(LIM)
100mA (1x)
1A (10x)
Suspend
500mA (5x)
Notice that when I_LIM0 is high and I_LIM1 is low, 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.
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