LTC4095
13
4095fa
APPLICATIONS INFORMATION
Figure 6. USB Soft Connect Circuit
charge currents, the LTC4095 power dissipation is
approximately:
P
D
= (IN – BAT) • I
BAT
where P
D
is the power dissipated, IN is the input supply
voltage, BAT is the battery voltage and I
BAT
is the charge
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:
T
A
= 115°C – P
D
θ
JA
T
A
= 115°C – (IN – BAT) • I
BAT
• θ
JA
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:
T
A
= 115°C – (5V – 3.5V) • (500mA) • 60°C/W
T
A
= 115°C – 0.75W • 60°C/W = 115°C – 45°C
T
A
= 70°C
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 CW
C
CA
BAT
=
°°
()
°
=
°
°
=
115 88
535 60
27
90
–
–. • / /
3300mA
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 coeffi cient 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 fi rst 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
10µF
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.
