LTC4095

13

4095fa

APPLICATIONS INFORMATION

Figure 6. USB Soft Connect Circuit

charge currents, the LTC4095 power dissipation is

approximately:

current. It is not necessary to perform any worst-case

power dissipation scenarios because the LTC4095 will

automatically reduce the charge current to maintain the

die temperature at approximately 115°C. However, the

approximate ambient temperature at which the thermal

feedback begins to protect the IC is:

Example: Consider an LTC4095 operating from a USB port

providing 500mA to a 3.5V Li-Ion battery. The ambient

temperature above which the LTC4095 will begin to reduce

the 500mA charge current is approximately:

The LTC4095 can be used above 70°C, but the charge cur-

rent will be reduced from 500mA. The approximate current

at a given ambient temperature can be calculated:

I

CT

IN BAT

BAT

A

JA

=

°

()

115

–

–•

θ

Using the previous example with an ambient temperature of

88°C, the charge current will be reduced to approximately:

I

CC

VV

C W

C

CA

°°

=

°

°

=

115

88

53 5

60

27

90

–

–.

•

/

/

3

300mA

Furthermore, the voltage at the PROG pin will change

proportionally with the charge current as discussed in

the Programming Charge Current section.

It is important to remember that LTC4095 applications do

not need to be designed for worst-case thermal conditions

since the IC will automatically reduce power dissipation

when the junction temperature reaches approximately

115°C.

USB Inrush Limiting

When a USB cable is plugged into a portable product,

the inductance of the cable and the high-Q ceramic input

capacitor form an L-C resonant circuit. If there is not

much impedance in the cable, it is possible for the voltage

at the input of the product to reach as high as twice the

USB voltage (~10V) before it settles out. In fact, due to

the high voltage coefficient of many ceramic capacitors (a

nonlinearity), the voltage may even exceed twice the USB

voltage. To prevent excessive voltage from damaging the

LTC4095 during a hot insertion, the soft connect circuit

in Figure 6 can be employed.

In this circuit, capacitor C2 holds MN1 off when the cable

is first connected. Eventually C2 begins to charge up to

the USB input voltage applying increasing gate support

to MN1. The long time constant of R1 and C1 prevent

the current from building up in the cable too fast thus

dampening out any resonant overshoot.

LTC4095

USB CABLE

IN

GND

C1

10

µF

C2

100nF

4095 F06

MN1

Si2302

R1

40k

8

2

5V

USB

INPUT

Battery Charger Stability Considerations

The LTC4095’s battery charger contains both a constant-

voltage and a constant-current control loop. The constant-

voltage loop is stable without any compensation when a

battery is connected with low impedance leads. Excessive

lead length, however, may add enough series inductance

to require a bypass capacitor of at least 1µF from BAT to

GND. Furthermore, a 4.7µF capacitor in series with a 0.2Ω

to 1Ω resistor from BAT to GND is required to keep ripple

voltage low when the battery is disconnected.

High value, low ESR multilayer ceramic chip capacitors

reduce the constant-voltage loop phase margin, possibly

resulting in instability. Ceramic capacitors up to 22µF may

be used in parallel with a battery, but larger ceramics should

be decoupled with 0.2Ω to 1Ω of series resistance.