LTC4069EDC-TRPBF(RevB) データシートの表示（PDF） - Linear Technology

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LTC4069EDC-TRPBF
(Rev.:RevB)

Standalone 750mA Li-Ion Battery Charger in 2 × 2 DFN with NTC Thermistor Input

Linear Technology 

LTC4069

APPLICATIONS INFORMATION

LTC4069 will demand a charge current higher than the

current limit of the voltage supply, the supply voltage will

drop to the battery voltage plus 600mA times the “on”

resistance of the internal PFET. The “on” resistance of the

LTC4069 power device is approximately 450mΩ with a 5V

supply. The actual “on” resistance will be slightly higher

due to the fact that the input supply will drop to less than

5V. The power dissipated during this phase of charging

is less than 240mW. That is a 76% improvement over the

non-current limited supply power dissipation.

USB and Wall Adapter Power

Although the LTC4069 allows charging from a USB port,

a wall adapter can also be used to charge Li-Ion batteries.

Figure 4 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 Schottky diode, D1, is used to prevent

USB power loss through the 1k pull-down resistor.

Typically a wall adapter can supply significantly more

current than the 500mA-limited USB port. Therefore, an

N-channel MOSFET, MN1, and an extra program resistor

are used to increase the charge current to 750mA when

the wall adapter is present.

Stability Considerations

The LTC4069 contains two control loops: constant-voltage

and constant-current. The constant-voltage loop is stable

without any compensation when a battery is connected

with low impedance leads. Excessive lead length, however,

may add enough series inductance to require a bypass

5V WALL

ADAPTER

750mA

ICHG

USB

POWER

500mA

ICHG

ICHG

BAT

D1 LTC4069

SYSTEM

LOAD

VCC

MP1

PROG

+ Li-Ion

BATTERY

MN1 4.02k

1k

2k

4069 F04

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.

High value capacitors with very low ESR (especially

ceramic) 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, not the battery. Because of the additional pole created

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

resistor. The pole frequency at the PROG pin should be

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

with a capacitance, CPROG, the following equation should

be used to calculate the maximum resistance value for

RPROG:

RPROG

≤

2π

•

1

105 •

CPROG

Average, rather than instantaneous, battery current may

be 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

LTC4069

PROG

GND

10k

RPROG

CFILTER

CHARGE

CURRENT

MONITOR

CIRCUITRY

4069 F05

Figure 4. Combining Wall Adapter and USB Power

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

4069fb

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