LTC4061
17
4061fd
applicaTions inForMaTion
be calculated in degrees centigrade. Consider the Vishay
NTHS0603N01N1003J thermistor, which has a temperature
coefficient of –4%/°C at 40°C. Dividing the tolerance by
the temperature coefficient, ±5%/(4%/°C) = ±1.25°C, gives
the temperature error of the hot trip point.
The cold trip point error depends on the tolerance of the
NTC thermistor and the degree to which the ratio of its
value at 0°C and its value at 40°C varies from 6.14 to 1.
Therefore, the cold trip point error can be calculated us-
ing the tolerance, TOL, the temperature coefficient of the
thermistor at 0°C, TC (in %/°C), the value of the thermistor
at 0°C, R
COLD
, and the value of the thermistor at 40°C,
R
HOT
. The formula is:
Temperature Error C
TOL
R
R
COLD
HOT
( )
.
• –
° =
+
1
6 14
1
• 100
TC
For example, the Vishay NTHS0603N01N1003J thermistor
with a tolerance of ±5%, TC of -5%/°C and R
COLD
/ R
HOT
of 6.13, has a cold trip point error of:
Temperature Error C( )
.
.
• . – •
° =
+
1 0 05
6 14
6 13 1 10
00
5
0 95 1 05
–
– . , .= ° °C C
Thermal Considerations
In order to deliver maximum charge current under all
conditions, it is critical that the exposed metal pad on the
backside of the LTC4061 package is properly soldered to
the PC board ground. Correctly soldered to a 2500mm
2
double sided 1oz copper board, the LTC4061 has a ther-
mal resistance of approximately 40°C/W. Failure to make
thermal contact between the exposed pad on the backside
of the package and the copper board will result in thermal
resistances far greater than 40°C/W. As an example, a
correctly soldered LTC4061 can deliver over 800mA to a
battery from a 5V supply at room temperature. Without
a good backside thermal connection, this number could
drop to less than 500mA.
V
CC
Bypass Capacitor
Many types of capacitors can be used for input bypassing;
however, caution must be exercised when using multilayer
ceramic capacitors. Because of the self-resonant and high
Q characteristics of some types of ceramic capacitors, high
voltage transients can be generated under some start-up
conditions such as connecting the charger input to a live
power source. Adding a 1.5Ω resistor in series with an X5R
ceramic capacitor will minimize start-up voltage transients.
For more information, see Application Note 88.
Charge
Current Soft-Start and Soft-Stop
The LTC4061 includes a soft-start circuit to minimize the
inrush current at the start of a charge cycle. When a charge
cycle is initiated, the charge current ramps from zero to the
full-scale current over a period of approximately 100µs.
Likewise, internal circuitry slowly ramps the charge cur-
rent from full-scale to zero when the charger is shut off
or self terminates. This has the effect of minimizing the
transient current load on the power supply during start-up
and charge termination.
Reverse
Polarity Input Voltage Protection
In some applications, protection from reverse polarity
voltage on V
CC
is desired. If the supply voltage is high
enough, a series blocking diode can be used. In other
cases, where the diode voltage drop must be kept low, a
P-channel MOSFET can be used (as shown in Figure 7).
USB
and Wall Adapter Power
The LTC4061 allows charging from both a wall adapter
and a USB port. Figure 8 shows an example of how to
combine wall adapter and USB power inputs. A P-channel
V
CC
V
IN
4061 F07
LTC4061
DRAIN-BULK
DIODE OF FET
Figure 7. Low Loss Input Reverse Polarity Protection
