NCP1800
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12
Selecting External Components
External Adaptor Power Supply Voltage Selection
Since the NCP1800 is using a linear, charging algorithm,
the efficiency is lower. Adapter voltage selection must be
done carefully in order to minimize the heat dissipation. In
general, the power supply input voltage should be around
5.0 to 6.0 V. The minimum input voltage should be chosen
to minimize the heat dissipation in the system. Excessively
high input voltages can cause too much heat dissipation and
will complicate the thermal design in applications like
cellular phones. With the overvoltage protection feature of
the NCP1800, input voltages higher than 7.0 V will activate
the overvoltage protection circuit and disconnect the power
supply input to the battery and other circuitry.
For the application shown in Figure 18 (assuming
NTGS3441 and MBRM130L):
V
IN(min)
Li−ion regulated voltage,
V
REG
(0.6 A)(R
DS(ON)
)
4.2 V (0.6 A) (100 m) 0.38 V
V
F
of Schottky Diode voltage drop of R
SNS
(0.6 A) (0.4 ) 4.88 V 4.9 V
Therefore, for the application shown in Figure 17
(assuming NTHD4P01FT1):
V
IN(min)
Li−ion regulated voltage
4.2 V (0.12 A)(130m) 0.43
(0.12 A)(2.0 ) 4.89 V 4.9 V
If the output voltage accuracy is 5%, then a typ. 5.2 V
5% output voltage adaptor must be used.
And for a very good regulated adaptor of accuracy 1%,
5.0 V ±1% output voltage adaptor can then be used. It is
obvious that if tighter tolerance adaptors are used, heat
dissipation can be minimized by using lower nominal
voltage adaptors.
Pass Element Selection
The type and size of the pass transistor is determined by
input−output differential voltage, charging current, current
sense resistor and the type of blocking diode used.
The selected pass element must satisfy the following
criteria:
Drop across pass element =
V
IN(min)
Li−ion regulated voltage V
F
I
REG
R
SNS
With:
V
IN(min)
5.0 V
V
REG
4.2 V
R
SNS
0.4
I
REG
0.6 A
Dropout across pass element =
5.0 V 4.2 V 0.38 V (0.6 A) (0.4 ) 0.18 V
Maximum R
DS(on)
should be less than (0.18 V)/(0.6 A) =
0.3 at 0.6 A.
V
IN(min)
5.0 V
V
REG
4.2 V
R
SNS
2.0
I
REG
0.12 A
Dropout across pass element = 5.0 V − 4.2 V − 0.43 V −
(0.12)(2.0 V
Therefore, maximum R
DS(on)
should be less than
(0.13 V)/(0.12 A) = 1.08 at 0.12 A.
External Output Capacitor
Any good quality output filter can be used, independent of
the capacitor’s minimum ESR. However, a 10 F tantalum
capacitor or electrolytic capacitor is recommended at the
output to suppress fast ramping spikes at the V
SNS
input and
to ensure stability for 1.0 A at full range. The capacitor
should be mounted with the shortest possible lead or track
length to the VSNS and GND pins.
Current Sense Resistor
The charging current can be set by the value of the current
sense resistor as in the previous formula. Proper de−rating
is advised when selecting the power dissipation rating of the
resistor. If necessary, R
ISEL
can also be changed for proper
selection of the R
SNS
values. Take note of the recommended
full−charge current ranges specified in the electrical
characteristics section. Also notice the effect of RISEL on
the accuracy of pre−charge current and end−of−charge
detection as noted in Figures 10 and 12, respectively.
