LTC4097 查看數據表（PDF） - Linear Technology

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LTC4097

USB/Wall Adapter Standalone Li-Ion/Polymer Battery Charger

Linear Technology 

LTC4097

APPLICATIONS INFORMATION

Using a Single Charge Current Program Resistor

In applications where the programmed wall adapter charge

current and USB charge current are the same, a single

program resistor can be used to set both charge currents.

Figure 2 shows a charger circuit that uses one charge cur-

rent program resistor. In this circuit, one resistor programs

the same charge current for each input supply.

ICHRG(DC)

=

ICHRG(USB)

=

1000V

RSET

The LTC4097 can also program the wall adapter charge

current and USB charge current independently using two

program resistors, RIDC and RIUSB. Figure 3 shows a

charger circuit that sets the wall adapter charge current

to 800mA and the USB charge current to 500mA.

Stability Considerations

The constant-voltage mode feedback loop is stable without

any compensation provided a battery is connected to the

charger output. However, a 4.7µF capacitor with a 1Ω series

resistor is recommended at the BAT pin to keep the ripple

voltage low when the battery is disconnected. When the

charger is in constant-current mode, the charge current

program pin (IDC or IUSB) is in the feedback loop, not the

battery. The constant-current mode stability is affected by

the impedance at the charge current program pin. With no

additional capacitance on this pin, the charger is stable

with program resistor values as high as 20KΩ (ICHRG =

50mA); however, additional capacitance on these nodes

reduces the maximum allowed program resistor.

Power Dissipation

When designing the battery charger circuit, it is not neces-

sary to design for worst-case power dissipation scenarios

because the LTC4097 automatically reduces the charge

current during high power conditions. The conditions

that cause the LTC4097 to reduce charge current through

thermal feedback can be approximated by considering the

power dissipated in the IC. Most of the power dissipation

is generated from the internal MOSFET pass device. Thus,

the power dissipation is calculated to be:

PD = (VCC – VBAT) • IBAT

PD is the power dissipated, VCC is the input supply volt-

age (either DCIN or USBIN), VBAT is the battery voltage

and IBAT is the charge current. The approximate ambient

temperature at which the thermal feedback begins to

protect the IC is:

TA = 115°C – PD • θJA

TA = 115°C – (VCC – VBAT) • IBAT • θJA

Example: An LTC4097 operating from a 5V USB adapter

(on the USBIN input) is programmed to supply 500mA

full-scale current to a discharged Li-Ion battery with a

voltage of 3.3V. Assuming θJA is 60°C/W (see Thermal

Considerations), the ambient temperature at which the

LTC4097 will begin to reduce the charge current is ap-

proximately:

TA = 115°C – (5V – 3.3V) • (500mA) • 60°C/W

TA = 115°C – 0.85W • 60°C/W = 115°C – 51°C

TA = 64°C

WALL

ADAPTER

USB

PORT

1µF

1µF

RISET

2k

1%

LTC4097

DCIN

BAT

USBIN HPWR

IUSB

IDC ITERM

GND

100mA

(USB, HPWR = LOW)

500mA

+

RITERM

2k

1%

4.2V

1-CELL

Li-Ion

BATTERY

4097 F02

Figure 2. Dual Input Charger Circuit. The

Wall Adapter Charge Current and USB Charge

Current are Both Programmed to be 500mA

WALL

ADAPTER

USB

PORT

LTC4097

DCIN

BAT

USBIN VNTC

1µF

1µF

HPWR

NTC

IUSB CHRG

RIUSB

2k

1%

RIDC

1.24k

1%

IDC ITERM

GND

800mA (WALL)

100mA/500mA (USB)

RNTCBIAS

100k

RITERM

2k

1%

1k

+

RNTC

100k

4.2V

1-CELL

Li-Ion

BATTERY

4097 F03

Figure 3. Full Featured Dual Input Charger Circuit

4097f

15

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