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TYPICAL APPLICATIONS
INTRODUCTION
When the battery voltage rises above the trickle charge
threshold, the charger enters the CC-mode. The MC34674
tries to raise the charge current to the internally set reference,
such as 1.05 A, by enhancing the power MOSFET. However,
since the current provided by the AC/DC regulator is limited
and can never reach the set reference, the charger will keep
enhancing the MOSFET until it is fully enhanced and is fully
VIN
turned on. In this mode, the internal power MOSFET behaves
as a switch instead of a linearly regulating device. The
voltage difference between the input and the output is
determined by the on resistance, R DS(ON) , of the power
MOSFET and the limited output current of the ad/dc
regulator.
R 1
GRN
RED
V IN – V OUT = I LIM × R DS ( ON )
The power dissipation, P D , in the MOSFET can be calculated
Figure 27. LED Color Balancing Scheme.
as,
P D = I LIM × I LIM × R DS ( ON )
INPUT CAPACITOR
The input capacitor is used to reduce the input voltage
The charge current in CC-mode is not determined by the
MC34674, instead, it is determined by the AC/DC regulator
current limit, I LIM , which is a value lower than the charger
internally set current reference. The internally set current
reference is used as a secondary protection threshold, in
case if an AC/DC regulator with a wrong current limit is
connected to the input.
The key advantage of using the MC34674 with a current-
limited AC/DC regulator is the significant reduction of the
power dissipation during the CC-mode. Figure 26 illustrates
the small voltage difference between the input and the output
of the charger, which is directly proportional to the power
dissipation.
When entering the CC-mode, the charger output I/V
trajectory jumps from point b to c and then moves from c to d
as the battery voltage rises to 4.2 V. The AC/DC regulator
When the battery voltage reaches the target 4.2V, the
charger enters the CV-mode. The charge current starts to
decline and the AC/DC regulator output enters its constant-
voltage mode. The charger then operates as a regular linear
charger again until the charging completes. The battery I/V
trajectory moves from d to the EOC moment (point e ) while
transient that may cause instability. A 1.0 μ F, X5R, 16 V
rated ceramic capacitor is recommended for most
applications.
OUTPUT CAPACITOR
For stable operation, an X5R ceramic capacitor with a
minimum 1.0 μ F nominal value is recommended at the
output. The output capacitance should not be larger than
240 μ F to allow the 585 μ A current to discharge the capacitor
voltage to the recharge threshold within 82 ms.
NTC INTERFACE DESIGN
The NTC interface is designed to be able to work with most
types of NTC thermistors. This section describes in details
how to select the two resistors R U and R S shown in
temperature thresholds are discussed. The
NCP15W104F03RC from Murata is used as an example for
the calculations in this section. The partial temperature
characteristics of the NCP15W104F03RC are given in
Table 7. NTC Thermistor Temperature Characteristics.
the AC/DC regulator output trajectory jumps from C to D and
then moves to E at the EOC moment.
BALANCING YELLOW COLOR IN LED
The red and the green colors in the LED are driven by two
matched 6.0 mA current sources. Such design ensured a
consistent brightness of the LED over a large range of the
input voltage. When both colors are turned on, the resulting
color should be yellow. One can adjust the resulting color by
Temp (°C)
-2
-1
0
2
3
R-low (k Ω )
389.2453
368.4960
348.9722
313.2543
296.9408
R-center (k Ω )
398.6521
377.1927
357.0117
320.1216
303.2866
R-high (k Ω )
408.2455
386.0560
365.1999
327.1067
309.7370
adjusting the brightness of the individual color. A resistor can
...
be added to reduce the brightness of one color, such as the
46
47
50
53
54
38.4596
36.8626
32.5022
28.7183
27.5694
39.2132
37.6010
33.1946
29.3660
28.2026
39.9778
28.3503
33.8983
30.0253
28.8474
34674
Analog Integrated Circuit Device Data
Freescale Semiconductor
21
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