Component Selection
5 Component Selection
5.1Input capacitors C1 and C3
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 MC34676B is 28V, the input capacitor must have 16V DC rated voltage.
5.2Output capacitors C4 and C5
The charger output capacitor is used for stable operation. An X5R ceramic capacitor minimum of a 1.0μF is required for the charger output. Depending on the load transient current, a larger capacitance may be required. Because the highest output voltage of the MC34676B is 4.2V, a 6.3V DC rated voltage is high enough for the output capacitor.
The regulator output capacitor is used for stable operation, too. An X5R ceramic capacitor minimum of a 1.0μF is required for the regulator output. A 6.3V DC rated voltage is high enough for the regulator output capacitor because the highest output voltage of the output regulator is 5V.
5.3AC CC-mode charge current setting resistors R1, R2, and R3
The resistor between the ISET pin and GND sets the AC
I |
| 3950 | Eqn. 1 |
AC | = | ||
| RISET |
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where RISET is in units of Ω, IAC is in units of amps. A metal film with a 1% tolerance resistor should be used for temperature stability. As a result, the charge current will be accurate over the whole temperature range.
On the evaluation board, three resistors with two pin header jumpers are used for the user to conveniently configure different charge current values. Table 1 shows the charge current with the different settings of pin headers J6 and J7.
Table 1. The AC
J6 | J7 | Charge Current |
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Open | Open | 150mA |
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Short | Open | 450mA |
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Open | Short | 750mA |
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Short | Short | 1050mA |
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5.4USB CC-mode charge current setting resistors R8 and R9
The resistor between the IUSB pin and GND sets the USB
I |
| 1975 | Eqn. 2 |
USB | = | ||
| RIUSB |
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where RUSB is in units of Ω, IUSB is in units of amps. A metal film with a 1% tolerance resistor should be used for temperature stability. As a result, the charge current will be accurate over the whole temperature range.
On the evaluation board, two resistors with two pin header jumpers are used for the user to conveniently configure different charge current values. Table 2 shows the charge current with the different settings of pin headers J10 and J11.
Table 2. The USB CC-mode Charge Current Settings
J10
J11
Charge Current
| Using the Dual 28V Input Voltage Charger with Linear Regulator, Rev. 1.0 |
4 | Freescale Semiconductor |