LTC4099 datasheet(20/52 Pages) LINER

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LTC4155

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discharged. This feature allows instant-on operation

when the low state of charge would otherwise prevent

operation of the system. If the system load plus battery

charger load exceeds the available input power, battery

charge current will be sacrificed to prioritize the system

load and maintain the switching regulator output voltage

while continuing to observe the input current limit. If the

system load alone exceeds the power available from the

input, the output voltage must fall to deliver the additional

current, with supplemental current eventually being sup-

plied by the battery.

Input Current Regulation

To meet the maximum load specification of the available

supply (USB/wall adapter), the switching regulator contains

a measurement and control system which ensures that

the average input current is below the level programmed

at the CLPROG1 or CLPROG2 pin and the I2C port. Con-

necting a single 1% tolerance resistor of the recommended

value to both the CLPROG1 and CLPROG2 pins guarantees

compliance with the 2.5mA, 100mA, 500mA, and 900mA

USB 2.0/3.0 current specifications, while also permitting

other I2C selectable current limits up to 3A. The input

current limit is independently selectable for each of the

two inputs, with the USBILIM[4:0] and WALLILIM[4:0]

bits in the I2C port. The USB input current limit setting

resets to 100mA when power is removed from its respec-

tive input. The WALL input current limit setting resets to

100mA when power is removed from both the USB and

WALL inputs. Refer to Alternate Default Input Current Limit

in the Operation section to program a non USB compli-

ant default input current limit for use with wall adapters

or other power sources. Refer to Table 8 for a complete

listing of I2C programmable input current limit settings.

If the combined external load plus battery charge current is

large enough to cause the switching power supply to reach

the programmed input current limit, the battery charger

will reduce its charge current by precisely the amount

necessary to enable the external load to be satisfied. Even

if the battery charge current is programmed to exceed

the allowable input power, the specification for average

input current will not be violated; the battery charger will

reduce its current as needed. Furthermore, if the load at

VOUT exceeds the programmed power level from VBUS, the

extra load current will be drawn from the battery via the

ideal diode independent of whether the battery charger is

enabled or disabled.

Input Undervoltage Current Limit

The LTC4155 can tolerate a resistive connection to the

input power source by automatically reducing power

transmission as the VBUS pin drops to 4.3V preventing a

possible UVLO oscillation. The undervoltage current limit

feature can be disabled by I2C using the DISABLE_INPUT_

UVCL bit. Refer to Table 5.

USB On-The-Go 5V Boost Converter

The LTC4155 switching regulator can be operated in

reverse to deliver power from the battery to VBUS while

boosting the VBUS voltage to 5V. This mode can be used

to implement features such as USB On-The-Go without

any additional magnetics or other external components.

The step-up switching regulator may be enabled in one of

two ways. The LTC4155 can optionally monitor the ID pin of

a USB cable and autonomously start the step-up regulator

when a host-side A/B connector is detected with a grounded

ID pin. The switching regulator may also be enabled

directly with the REQUEST_OTG I2C command. Note that

the step-up regulator will not operate if input power is

present on either the USB or WALL input, or if the battery

voltage is below VLOWBAT,typically2.8V.TheI2Cstatusbits

OTG_ENABLED, LOWBAT and OTG_FAULT can be used to

determine if the step-up converter is running. Refer to ID

Pin Detection in the Operation section for more information

about the autonomous step-up regulator start-up.

The step-up regulator only provides power to the USB input.

It is not possible to provide power to the WALL input. The

I2C PRIORITY setting has no effect on step-up regulator

operation. Refer to Dual-Input Overvoltage Protection and

Undervoltage Lockout in the Operation section for more

information about multiple input operation.

The switching regulator is guaranteed to deliver 500mA

to VBUS and will limit its output current to approximately

1.4A while allowing VBUS to fall when overloaded. If a

short-circuit fault is detected, the channel will be shut

down after approximately 8ms and the problem will be

reported with the I2C status bit OTG_FAULT.

OPERATION

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