LTC4069

13

4069fc

For more information www.linear.com/LTC4069

applicaTions inForMaTion

withlowimpedanceleads.Excessiveleadlength,however,

may add enough series inductance to require a bypass

capacitor of at least 1µF from BAT to GND. Furthermore,

a 4.7µF capacitor with a 0.2Ω to 1Ω series resistor from

BAT to GND is required to keep ripple voltage low when

the battery is disconnected.

HighvaluecapacitorswithverylowESR(especiallyceram-

ic) may reduce the constant-voltage loop phase margin.

Ceramic capacitors up to 22µF may be used in parallel

with a battery, but larger ceramics should be decoupled

with 0.2Ω to 1Ω of series resistance.

In constant-current mode, the PROG pin is in the feedback

loop,notthebattery.Becauseoftheadditionalpolecreated

by the PROG pin capacitance, capacitance on this pin must

be kept to a minimum. With no additional capacitance on

the PROG pin, the charger is stable with program resistor

values as high as 25k. However, additional capacitance on

this node reduces the maximum allowed program resis-

tor. The pole frequency at the PROG pin should be kept

above 100kHz. Therefore, if the PROG pin is loaded with

1

2

Average,ratherthaninstantaneous,batterycurrentmaybe

of interest to the user. For example, if a switching power

supply operating in low current mode is connected in

parallel with the battery, the average current being pulled

out of the BAT pin is typically of more interest than the

instantaneous current pulses. In such a case, a simple RC

filter can be used on the PROG pin to measure the average

battery current as shown in Figure 5. A 10k resistor has

been added between the PROG pin and the filter capacitor

to ensure stability.

Power Dissipation

The conditions that cause the LTC4069 to reduce charge

current through thermal feedback can be approximated

by considering the power dissipated in the IC. For high

charge currents, the LTC4069 power dissipation is ap-

proximately:

current. It is not necessary to perform any worst-case

power dissipation scenarios because the LTC4069 will

automatically reduce the charge current to maintain the

die temperature at approximately 115°C. However, the

MP1

MN1

1k

2k

4.02k

D1

Li-Ion

BATTERY

SYSTEM

LOAD

4069 F04

LTC4069

BAT

USB

POWER

500mA

5V WALL

ADAPTER

750mA

PROG

+

Figure 5. Isolating Capacitive Load on the PROG Pin and Filtering

Figure 4. Combining Wall Adapter and USB Power

4069 F05

CHARGE

CURRENT

MONITOR

CIRCUITRY

LTC4069

PROG

GND

10k