LTC4080X datasheet(15/20 Pages) LINER

15 / 20 page

LTC4080X

4080Xf

APPLICATIO S I FOR ATIO

BATTERY CHARGER

Programming Charge Current

The battery charge current is programmed using a single

resistor from the PROG pin to ground. The charge current

is 400 times the current out of the PROG pin. The program

resistor and the charge current are calculated using the

following equations:

PROG

BAT

PROG

400

•,

•

The charge current out of the BAT pin can be determined

at any time by monitoring the PROG pin voltage and using

the following equation:

BAT

PROG

• 400

Stability Considerations

The LTC4080X battery charger 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 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.

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

the feedback loop, not the battery voltage. Because of

the additional pole created by PROG pin capacitance,

capacitance on this pin must be kept to a minimum. With

no additional capacitance on the PROG pin, the battery

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:

kHz C

PROG

≤

100

••

Average, rather than instantaneous, battery current may be

of interest to the user. For example, when the switching

regulator 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 2. A 10k resistor has

been added between the PROG pin and the ﬁlter capacitor

to ensure stability.

4080X F02

CFILTER

CHARGE

CURRENT

MONITOR

CIRCUITRY

RPROG

LTC4080X

PROG

GND

10k

Figure 2. Isolating Capacitive Load

on PROG Pin and Filtering

Dropout Operation

When the BAT pin voltage approaches VOUT, the duty cycle

of the switching regulator approaches 100%. When VBAT

is approximately equal to VOUT, the regulator is said to be

in dropout. In dropout, the main switch (MP2) stays on

continuously with the output voltage being equal to the

battery voltage minus the voltage drops across the main

switch and the inductor.

OPERATIO

Global Thermal Shutdown

The LTC4080X includes a global thermal shutdown which

shuts off the entire device (battery charger and switch-

ing regulator) if the die temperature exceeds 160°C. The

LTC4080X resumes normal operation once the temperature

drops approximately 14°C.

LTC4080X Datasheet(PDF) 15 Page - Linear Technology

LTC4080X Datasheet(HTML) 15 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL