Datasheet

LTC4063
15
4063fc
applicaTions inForMaTion
Regulator Output Noise
Noise measurements on the output should be made with
care to ensure accurate results. Coaxial connections and
proper shielding should be used to maintain measurement
integrity. Figure 7 shows a test setup for taking the mea-
surement. When the output is set to 3V and a 100mA load
is applied, the LTC4063 output noise power in the 10Hz to
100kHz band is typically measured to be 135µV
RMS
. For
more information on obtaining accurate noise measure-
ments for LDOs, see Application Note 83.
Power
Dissipation
When designing the battery charger circuit, it is not
necessary to design for worst-case power dissipation
scenarios because the LTC4063 automatically reduces
the charge current during high power conditions. The
conditions that cause the LTC4063 to reduce charge cur-
rent through thermal feedback can be approximated by
considering the power dissipated in the IC. Most of the
power dissipation is generated from the internal charger
MOSFET (the LDO generates considerably less heat in most
applications). Thus, the power dissipation is calculated to
be approximately:
P
D
= (V
CC
– V
BAT
) • I
BAT
P
D
is the power dissipated, V
CC
is the input supply voltage,
V
BAT
is the battery voltage and I
BAT
is the charge current.
The approximate ambient temperature at which the thermal
feedback begins to protect the IC is:
T
A
= 105°C – P
D
• θ
JA
T
A
= 105°C – (V
CC
– V
BAT
) • I
BAT
• θ
JA
Example: An LTC4063 operating from a 5V wall adapter
is programmed to supply 800mA full-scale current to a
discharged Li-Ion battery with a voltage of 3.3V. Assuming
θ
JA
is 40°C/W (see Thermal Considerations), the ambient
temperature at which the LTC4063 will begin to reduce the
charge current is approximately:
T
A
= 105°C – (5V – 3.3V) • (800mA) • 40°C/W
T
A
= 105°C – 1.36W • 40°C/W = 105°C – 54.4°C
T
A
= 50.6°C
The LTC4063 can be used above 50.6°C ambient, but
the charge current will be reduced from 800mA. The ap-
proximate current at a given ambient temperature can be
approximated by:
I
C T
V V
BAT
A
CC BAT JA
=
°
( )
105 –
– • θ
Using the previous example with an ambient temperature
of 60°C, the charge current will be reduced to approxi-
mately:
I
C C
V V C W
C
C A
I
BAT
BAT
=
° °
( )
°
=
°
°
=
105 60
5 3 3 40
45
68
–
– . • / /
6662mA
It is important to remember that LTC4063 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 ap-
proximately 105°C.
5Hz SINGLE ORDER
HIGHPASS
IN 10Hz TO 100kHz
4063 F07
GAIN = 60dB
10Hz 2nd ORDER
BUTTERWORTH HP
100kHz 4th ORDER
BUTTERWORTH LP
5Hz SINGLE ORDER
HIGHPASS
Figure 7. Filter Structure for Noise Testing LDOs