Using the High-Input-Voltage Travel Charger, Rev. 1.0
4 Freescale Semiconductor
Component Selection
5 Component Selection
5.1 Input capacitor C1
The input capacitor is used to minimize the input voltage transient that may cause instability. A
ceramic capacitor of 1.0µF or above is required for most applications. X5R and X7R dielectrics
have better temperature stability. The evaluation board uses 1.0µF X5R ceramic capacitors.
Considering the maximum input voltage rating of the MC34674 is 28V, the input capacitor must
have 16V DC rated voltage.
5.2 Output capacitors C2, C3
The output capacitor is used for stable operation. A ceramic capacitor minimum of 2.2µF is
required for the charger output. For the reference voltage output, a ceramic capacitor minimum
of 0.1µF is required. On the evaluation board, C2 is a 2.2µF and C3 is a 1.0µF. Because the
highest charge output voltage of the MC34674 is 4.2V and the VREF output voltage is 2.5V, a
6.3V DC rated voltage is high enough for the output capacitors.
5.3 Anti-ESD capacitors C4
The capacitor C4 at the TEMP pin is used to enhance the ESD protection at the TEMP pin as
well as to filter the noise on the input voltage. Because R2 has a terminal exposed, the high
voltage ESD at this terminal may damage the TEMP pin. A 0.01µF capacitance or above at the
TEMP will protect the TEMP pin against ESD as high as 8KV HBM.
5.4 NTC interface resistor R1, R2
The MC34674 offers an interface to an external NTC inside the battery pack to monitor the
battery temperature. The high and low temperature thresholds can be set by R1, R2, and the
NTC value. If the temperature is out of the setting window, the charger is disabled to protect the
battery. Meanwhile a temperature fault is issued with a yellow-color LED indication.
After the NTC is selected, the R1 and R2 value are determined by following equations:
R1 = 2(R
COLD
- R
HOT
) / 3
R2 = (R
COLD
- 4R
HOT
) / 3
Where R
COLD
is the resistance of the thermistor at cold temperatures, such as 0°C, and R
HOT
is
the resistance of the thermistor at hot temperatures, such as 45°C.
JP3 HDR102 2-PIN 826629-2 TH 100MIL SP 375H TYCO
JP4 HDR102 2-PIN 826629-2 TH 100MIL SP 375H TYCO
JP5 HDR102 2-PIN 826629-2 TH 100MIL SP 375H TYCO
U1 2x3 uDFN-8 MC34674AEP Single Cell Li+ Battery
Charger IC, Linear
FREESCALE
Table 1. Evaluation Board Components
Designator Footprint Part Value Part Number Description Vendor
