LP4054 datasheet(13/14 Pages) ETC

13 / 14 page

Preliminary

LP4054

LP4054-datasheet V1.0

Sep.-2006

13-13

transients.

Charge Current Soft-Start

The LP4054 includes a soft-start circuit to minimize the

inrush current at the start of a charge cycle. When a

charge cycle is initiated, the charge current ramps from

zero to the full-scale current over a period of

approximately 100µs. This has the effect of minimizing

the transient current load on the power supply during

start-up.

CHRG Status Output Pin

The CHRG pin can provide an indication that the input

voltage is greater than the undervoltage lockout

threshold

level.

weak

pull-down

current

approximately 12µA indicates that sufficient voltage is

applied to VCC to begin charging. When a discharged

battery is connected to the charger, the constant current

portion of the charge cycle begins and the CHRG pin

pulls to ground. The CHRG 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 12µA pull-down,

indicating that the charge cycle has ended. If the input

voltage is removed or drops below

the under voltage

lockout threshold, the CHRG pin becomes high

impedance. Figure 10 shows that by using two

different

value

pull-up

resistors,

micro-processor can detect all three states from

this pin.

To detect when the LP4054 is in charge mode, force the

digital output pin (OUT) high and measure the voltage

at the CHRG 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 12µA current source is connected to

the CHRG 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 CHRG 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 VCC 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 Fig 11.)

USB and Wall Adapter Power

The LP4054 allows charging from both a wall adapter

and a USB port. Figure 12 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 extra 10k program resistor are

used to increase the charge current to 600mA when the

wall adapter is present.

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