LTC3577-3 datasheet(21/52 Pages) LINER

21 / 52 page

LTC3577-3/LTC3577-4

357734fa

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:

VBUS

BUSQ

CLPROG

•

where IBUSQ and hCLPROG are given in the Electrical

Characteristics table.

The input current limit is programmed by the ILIM0andILIM1

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:

x Mode

VBUS

BUSQ

CLPROG

()

•

BUS

BUSQ

CLPROG

VBUS

x Mode

()

•

==+

()

x Mode

BUSQ

CLPROG

•

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

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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