FC5754 datasheet(17/19 Pages) FS

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FC5754

2014. 11. 04

17/19

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CHGB Status Output Pin

The CHGB pin can provide an indication that the input

voltage is greater than the undervoltage lockout

threshold level. A weak pull-down current

approximately 24 A indicates that sufficient voltag

applied to VIN to begin charging. When a discharged

battery is connected to the charger, the constant

current portion of the charge cycle begins and the

CHGB pin pulls to ground. The CHGB pin can sink up to

10mA to drive an LED that indicates that a charge cycle

is in progress.

When the battery is nearing full charge, the charger

enters the constant-voltage portion of the charge cycle

and the charge current begins to drop. When the

charge current drops below 1/10 of the programmed

current, the charge cycle ends and the strong

pull-down is replaced by the 24uA pull-down, indicating

that the charge cycle has ended. If the input voltage is

removed or drops below the undervoltage lockout

threshold, the CHGB pin becomes high impedance.

Figure 3 shows that by using two different value pull-up

resistors, a microprocessor can detect all three states

from this pin.

To detect when the FC5754 is in charge mode, force

the digital output pin (OUT) high and measure the

voltage at the CHGB pin. The internal 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 24υA current

source is connected to the CHGB 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 CHGB 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 VIN 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 4).

USB and Wall Adapter Power

The FC5754 allows charging from both a wall adapter

and a USB port. Figure 5 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 580mA when the

wall adapter is present.

Figure 5. Combining Wall Adapter and USB Power

Figure 4. Low Loss Input Reverse Polarity Protection

Figure 3. Using a Microprocessor to Determine CHGB State

FC5754 Datasheet(PDF) 17 Page - First Silicon Co., Ltd

FC5754 Datasheet(HTML) 17 Page - First Silicon Co., Ltd

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