LTC4155 datasheet(45/52 Pages) LINER

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LTC4155

4155fc

Table 38. Recommended P-Channel Battery Charger MOSFETs

MANUFACTURER

PART

NUMBER

RDS(ON)

(mΩ)

VT (V)

BVDSS (V)

Fairchild

FDMC510P

7.6

–0.5

–20

Vishay

Si7123DN

11.2

–1

–20

Vishay

Si5481DU

–1

–20

VBUS and VOUT Bypass Capacitors

The style and value of the capacitors used with the LTC4155

determine several important parameters such as regulator

control loop stability and input voltage ripple. Because

the LTC4155 uses a step-down switching power supply

from VBUS to VOUT, its input current waveform contains

high frequency components. It is strongly recommended

that a low equivalent series resistance (ESR) multilayer

ceramic capacitor be used to bypass VBUS. Tantalum and

aluminum capacitors are not recommended because of

their high ESR. The value of the capacitor on VBUS directly

controls the amount of input ripple for a given load current.

Increasing the size of this capacitor will reduce the input

ripple. The USB specification allows a maximum of 10μF

to be connected directly across the USB power bus. If the

overvoltage protection circuit is used to protect VBUS, then

its soft-starting nature can be exploited and a larger VBUS

capacitor can be used if desired. If one or both of the input

channels are never used for USB, additional capacitance

placed upstream of the overvoltage protection NMOS de-

vices can absorb significant high frequency current ripple.

To prevent large VOUT voltage steps during transient load

conditions, it is also recommended that a ceramic capacitor

be used to bypass VOUT. The output capacitor is used in

the compensation of the switching regulator. At least 22μF

with low ESR are required on VOUT. Additional capacitance

will improve load transient performance and stability.

Multilayer ceramic chip capacitors typically have excep-

tional ESR performance. MLCCs combined with a tight

board layout and an unbroken ground plane will yield very

good performance and low EMI emissions.

The actual in-circuit capacitance of a ceramic capacitor

should be measured with a small AC signal and DC bias,

as is expected in-circuit. Many vendors specify the capaci-

tance versus voltage with a 1VRMS AC test signal and, as

a result, overstate the capacitance that the capacitor will

present in the application. Using similar operating condi-

tions as the application, the user must measure, or request

from the vendor, the actual capacitance to determine if the

selected capacitor meets the minimum capacitance that

the application requires.

Programming the Input and Battery Charge

Current Limits

The LTC4155 features independent resistor programma-

bility of the input current limit and battery charge current

limit to facilitate optimal charging from a wide variety of

input power sources. The battery charge current should

be programmed based on the size of the battery and its

associated safe charging rate. Typically this rate is close

to “1C”, or equal to the current which would discharge

the battery in one hour. For example, a 2000mAH battery

would be charged with no more than 2A. With the full-

scale (default) charge current programmed with a resistor

between PROG and GND, all other I2C selectable charge

current settings are lower and may be appropriate for cus-

tom charge algorithms at extreme temperature or battery

voltage. If the battery charge current limit requires more

power than is available from the selected input current limit,

the input current limit will be enforced and the battery will

charge with less than the programmed current. Thus, the

battery charger should be programmed optimally for the

battery without concern for the input source.

Resistive Inputs and Test Equipment

Care must be exercised in the laboratory while evaluat-

ing the LTC4155 with inline ammeters. The combined

resistance of the internal current sense resistor and fuse

of many meters can be 0.5Ω or more. At currents of 3A

to 4A, it is possible to drop several volts across the meter,

possibly resulting in unusual voltage readings or artificially

high switch duty cycles.

A resistive connection to the source of input power can

be particularly troublesome. With the undervoltage cur-

rent limit feature enabled, the switching regulator output

power will be automatically reduced to prevent VBUS from

falling below 4.3V. This feature greatly improves tolerance

of resistive input power sources (from either undersized

wiring and connectors or test equipment) and facilitates

APPLICATIONS INFORMATION

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