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LTC3577-4_15 Datasheet, PDF (21/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC
LTC3577-3/LTC3577-4
OPERATION
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
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.
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 provides additional
regulator controls as well as status read-back.
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
LTC3577-3/LTC3577-4 are designed to limit input current
as well as control battery charge current as a function of
IVOUT. VOUT 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, VOUT will be connected to VBUS through
an internal 200mΩ P-channel MOSFET. If the combined
load at VOUT 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 VOUT
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/hCLPROG) of the VBUS
current. When a programming resistor is connected from
CLPROG to GND, the voltage on CLPROG represents the
input current:
IVBUS
=
IBUSQ
+
VCLPROG
RCLPROG
• hCLPROG
where IBUSQ and hCLPROG are given in the Electrical
Characteristics table.
The input current limit is programmed by the ILIM0 and ILIM1
pins. The LTC3577-3/LTC3577-4 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:
( ) IVBUS
=
IBUSQ
+
0.2V
RCLPROG
• hCLPROG
1x Mode
( ) IVBUS
=
IBUSQ
+
1V
RCLPROG
• hCLPROG
5x Mode
( ) IVBUS
=
IBUSQ
+
2V
RCLPROG
• hCLPROG
10x Mode
Under worst-case conditions, the USB specification for
average input current will not be violated with an RCLPROG
resistor of 2.1k or greater. Table 1 shows the available
settings for the ILIM0 and ILIM1 pins:
Table 1. Controlled Input Current Limit
ILIM1
ILIM0
1
1
1
0
0
1
0
0
IBUS(LIM)
100mA (1x)
1A (10x)
Suspend
500mA (5x)
Notice that when ILIM0 is low and ILIM1 is high, the input
current limit is set to a higher current limit for increased
charging and current availability at VOUT. This mode is
typically used when there is a higher power, non-USB
source available at the VBUS pin.
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