MAX8730 Просмотр технического описания (PDF) - Maxim Integrated

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MAX8730

Low-Cost Battery Charger

Maxim Integrated 

Low-Cost Battery Charger

The MAX8730 includes a foldback feature, which

reduces the Schottky requirement at low battery volt-

ages. See the Foldback Current Section.

Setting Input-Current Limit

The total input current, from a wall adapter or other DC

source, is the sum of the system supply current and the

current required by the charger. When the input current

exceeds the set input current limit, the MAX8730

decreases the charge current to provide priority to sys-

tem load current. System current normally fluctuates as

portions of the system are powered up or put to sleep.

The input-current-limit circuit reduces the power

requirement of the AC wall adapter, which reduces

adapter cost. As the system supply rises, the available

charge current drops linearly to zero. Thereafter, the

total input current can increase without limit.

The total input current is the sum of the device supply cur-

rent, the charger input current, and the system load cur-

rent. The total input current can be estimated as follows:

IINPUT

=

ILOAD +

ICHARGE x VBATTERY

VIN x η

where η is the efficiency of the DC-DC converter (typi-

cally 85% to 95%).

CLS sets the maximum voltage across the current-

sense resistor RS1, which determines the input current

limit. The full-scale differential voltage between CSSP

and CSSN is 75mV (5A for RS1 = 15mΩ). Set CLS

according to the following equation:

VCLS

=

ILIMIT

x

RS1

x

VREF

75mV

The input range for CLS is 1.1V to VREF. Choose a cur-

rent-sense resistor (RS1) to have a sufficient power rat-

ing to handle the full system current. The current-sense

resistor may be reduced to improve efficiency, but this

degrades accuracy due to the current-sense amplifier’s

input offset (±3mV). See the Typical Operating Charac-

teristics to estimate the input current-limit accuracy at

various set points. The input current-limit error amplifier

(GMS) is compensated at the CCS pin; see the Com-

pensation section.

Input-Current Measurement

IINP monitors the system-input current sensed across

CSSP and CSSN. The voltage of IINP is proportional to

the input current according to the following equation:

VIINP = IINPUT x RS1 x GIINP x R10

where IINPUT is the DC current supplied by the AC

adapter, GIINP is the transconductance of IINP

(2.8µA/mV typ), and R10 is the resistor connected

between IINP and ground. Connect a 0.1µF filter

capacitor from IINP to GND to reduce ripple. IINP has a

0 to 4.5V output-voltage range. Connect IINP to GND if

it is not used.

The MAX8730 provides a short-circuit latch to protect

against system overload or short. The latch is set when

VIINP rises above 4.2V, and disconnects the adapter

from the system by turning PDS off (PDL does not

change). The latch is reset by bringing SRC below

UVLO (remove and reinsert the adapter). Choose a fil-

ter capacitor that is large enough to provide appropri-

ate debouncing and prevent accidental faults, yet

results in a response time that is fast enough to ther-

mally protect the MOSFETs. See the System Short

Circuit section.

IINP can be used to measure battery-discharge current

(see Figure 1) when the adapter is absent. To disable

IINP and reduce battery consumption to 10µA, drive

INPON to low. Charging is disabled when INPON is

low, even if the adapter is present.

AC-Adapter Detection and

Power-Source Selection

The MAX8730 includes a hysteretic comparator that

detects the presence of an AC power adapter and

automatically selects the appropriate power source.

When the adapter is present (VASNS > VBATT -

-100mV) the battery is disconnected from the system

load with the p-channel (P3) MOSFET. When the

adapter is removed (VASNS < VBATT - -270mV), PDS

turns off and PDL turns on with a 5µs break-before-

make sequence.

The A C O K output can be used to indicate the presence

of the adapter. When VACIN > 2.1V and VASNS > VBATT

- 100mV, A C O K becomes low. Connect a 10kΩ pullup

resistor between LDO and A C O K. Use a resistive volt-

age-divider from the adapter’s output to the ACIN pin to

set the appropriate detection threshold. Since ACIN

has a 6V absolute maximum rating, set the adapter

threshold according to the following equation:

VADAPTER _ THRESHOLD > VADAPTER _MAX

3

Relearn Mode

The MAX8730 can be programmed to perform a relearn

cycle to calibrate the battery’s fuel gauge. This cycle

consists of isolating the battery from the charger and dis-

charging it through the system load. When the battery

18 ______________________________________________________________________________________

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