APPLICATIONS INFORMATION
USB and Wall Adapter Power
The SGM4054 allows charging from both a wall adapter
and a USB port. Figure 7 shows an example of how to
combine wall adapter and USB power inputs. A P-channel
MOSFET, MP1, is used to prevent back conducting into the
USB port when a wall adapter is present and a Schottky
diode, D1, is used to prevent USB power loss through the
1k pull-down resistor.
Typically a wall adapter can supply more current than the
500mA-limited USB port. Therefore, an N-channel
MOSFET, MN1, and an extra 10k program resistor are used
to increase the charge current to 600mA when the wall
adapter is present.
Figure 5. Using a Microprocessor to Determine
CHGR
State
To detect when the SGM4054 is in charge mode, force the
digital output pin (OUT) high and measure the voltage at
the
CHGR
pin. The N-channel MOSFET will pull the pin
voltage low even with the 2k pull-up resistor. Once the
charge cycle terminates, the N-channel MOSFET is turned
off and a 20µA current source is connected to the
CHGR
pin. The IN pin will then be pulled high by the 2k pull-up
resistor. To determine if there is a weak pull-down current,
the OUT pin should be forced to a high impedance state.
The weak current source will pull the IN pin low through
the 800k resistor; if
CHGR
is high impedance, the IN pin
will be pulled high, indicating that the part is in a UVLO
state.
Reverse Polarity Input Voltage Protection
In some applications, protection from reverse polarity
voltage on V+ is desired. If the supply voltage is high
enough, a series blocking diode can be used. In other cases,
where the voltage drop must be kept low a P-channel
MOSFET can be used (as shown in Figure 6).
Figure 7. Combining Wall Adapter and USB Power
Figure 6. Low Loss Input Reverse Polarity Protection
12
SGM4054
TOREX [ TOREX SEMICONDUCTOR ]
