Garmin MC34676B - Manual (Page 4)

Using the Dual 28V Input Voltage Charger with Linear Regulator, Rev. 1.0
4 Freescale Semiconductor
Component Selection
5 Component Selection
5.1 Input capacitors C1 and C3
The input capacitor is used to minimize the input voltage transient that may cause instability. A ceramic capacitor of
1.0
μF or above is required for most applications. X5R and X7R dielectrics have better temperature stability. The
evaluation board uses 1.0 μF X5R ceramic capacitors. Considering the maximum input voltage rating of the MC34676B
is 28V, the input capacitor must have 16V DC rated voltage.
5.2 Output capacitors C4 and C5
The charger output capacitor is used for stable operation. An X5R ceramic capacitor minimum of a 1.0 μF 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 1.0 μF 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.
5.3 AC CC-mode charge current setting resistors R1, R2, and R3
The resistor between the ISET pin and GND sets the AC CC-mode charge current by the following equation:
Eqn. 1
where R
ISET
is in units of Ω, I
AC
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.
5.4 USB CC-mode charge current setting resistors R8 and R9
The resistor between the IUSB pin and GND sets the USB CC-mode charge current by the following equation:
Eqn. 2
where R
USB
is in units of Ω, I
USB
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 1. The AC CC-mode Charge Current Settings
J6 J7 Charge Current
Open Open 150mA
Short Open 450mA
Open Short 750mA
Short Short 1050mA
Table 2. The USB CC-mode Charge Current Settings
J10 J11 Charge Current
I
AC
3950
R
ISET
-------------- =
I
USB
1975
R
IUSB
-------------- =