LTC4098-1 datasheet(22/32 Pages) LINER

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LTC4098/LTC4098-1

40981fb

CLPROG Resistor and Capacitor

As described in the Bat-Track Input Current Limited Step

Down Switching Regulator section, the resistor on the

CLPROG pin determines the average input current limit

in each of the six current limit modes. The input cur-

rent will be comprised of two components, the current

that is used to drive VOUT and the quiescent current of

the switching regulator. To ensure that the total average

input current remains below the USB speciﬁcation, both

components of input current should be considered. The

Electrical Characteristics table gives the typical values for

quiescent currents in all settings as well as current limit

programming accuracy. To get as close to the 500mA or

100mA speciﬁcations as possible, a precision resistor

should be used.

An averaging capacitor is required in parallel with the

resistor so that the switching regulator can determine

the average input current. This capacitor also provides

the dominant pole for the feedback loop when current

limit is reached. To ensure stability, the capacitor on

CLPROG should be 0.1μF or larger.

Choosing the Inductor

Because the input voltage range and output voltage range

of the PowerPath switching regulator are both fairly nar-

row, the LTC4098/LTC4098-1 were designed for a speciﬁc

inductance value of 3.3μH. Some inductors which may be

suitable for this application are listed in Table 3.

VBUS and VOUT Bypass Capacitors

The style and value of capacitors used with the LTC4098/

LTC4098-1 determine several important parameters such

as regulator control loop stability and input voltage

ripple. Because the LTC4098/LTC4098-1 use 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 con-

nected directly across the USB power bus. If additional

capacitance is required for noise performance, it may be

connected directly to the VBUS pin when using the OVP

feature of the LTC4098/LTC4098-1. This extra capacitance

will be soft-connected over several milliseconds to limit

inrush current and avoid excessive transient voltage

drops on the bus.

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 10μF with low ESR are required on VOUT. Additional

capacitance will improve load transient performance

and stability.

Table 3. Recommended Inductors for the LTC4098/LTC4098-1

INDUCTOR TYPE

(μH)

MAX IDC

(A)

MAX DCR

(

Ω)

SIZE IN mm

× W × H)

MANUFACTURER

LPS4018

3.3

2.2

0.08

3.9

× 3.9 × 1.7

Coilcraft

www.coilcraft.com

D53LC

DB318C

3.3

2.26

1.55

0.034

0.070

× 5 × 3

3.8

× 3.8 × 1.8

Toko

www.toko.com

WE-TPC Type M1

3.3

1.95

0.065

4.8

× 4.8 × 1.8

Würth Elektronik

www.we-online.com

CDRH6D12

CDRH6D38

3.3

2.2

3.5

0.0625

0.020

6.7

× 6.7 × 1.5

× 7 × 4

Sumida

www.sumida.com

APPLICATIONS INFORMATION

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