Preliminary

Datasheet

LP4062

LP4062 –00

Ver. 1.0 Datasheet

Nov.-2007

Page 11 of 13

thermally regulated charge current.:

Using RCC = 0.25Ω, VS = 5V, VBAT = 3.75V,

TA = 25℃ and θJA = 125℃/W we can calculate the

thermally regulated charge current to be:

While this application delivers more energy to the

battery and reduces charge time in thermal mode, it

may actually lengthen charge time in voltage mode if

VCC becomes low enough to put the LP4062 into

dropout. This technique works best when RCC values

are minimized to keep component size small and avoid

dropout. Remember to choose a resistor with adequate

power handling capability.

VCC Bypass Capacitor

Many types of capacitors can be used for input

bypassing, however, caution must be exercised when

using multilayer ceramic capacitors. Because of the

self-resonant and high Q characteristics of some types

of ceramic capacitors, high voltage transients can be

generated under some start-up conditions, such as

connecting the charger input to a live power

source .Adding a 1.5Ω resistor in series with an

X5R ceramic capacitor will minimize start-up

voltage

transients.

Charge Current Soft-Start

The LP4062 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. A weak pull-down current of

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, a micro-processor can

detect all three states from this pin.

To detect when the LP4062 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.)