MAX8903NETI+T datasheet(22/30 Pages) MAXIM

22 / 30 page

MAX8903A–E/G/H/J/N/Y

2A 1-Cell Li+ DC-DC Chargers

for USB and Adapter Power

Maxim Integrated

where:

RT = The resistance in Ω of the thermistor at tempera-

ture T in Celsius

R25 = The resistance in Ω of the thermistor at +25°C

β = The material constant of the thermistor, which typi-

cally ranges from 3000K to 5000K

T = The temperature of the thermistor in °C

Table 3 shows the MAX8903_ THM temperature limits

for different thermistor material constants.

Some designs might prefer other thermistor temperature

limits. Threshold adjustment can be accommodated by

changing RTB, connecting a resistor in series and/or in

parallel with the thermistor, or using a thermistor with dif-

ferent

β. For example, a +45°C hot threshold and 0°C

cold threshold can be realized by using a thermistor

with a

β of 4250 and connecting 120kΩ in parallel. Since

the thermistor resistance near 0°C is much higher than it

is near +50°C, a large parallel resistance lowers the

cold threshold, while only slightly lowering the hot

threshold. Conversely, a small series resistance raises

the hot threshold, while only slightly raising the cold

threshold. Raising RTB lowers both the cold and hot

thresholds, while lowering RTB raises both thresholds.

Note that since VL is active whenever valid input power

is connected at DC or USB, thermistor bias current

flows at all times, even when charging is disabled (

CEN

= high). When using a 10k

Ω thermistor and a 10kΩ

pullup to VL, this results in an additional 250µA load.

This load can be reduced to 25µA by instead using a

100k

Ω thermistor and 100kΩ pullup resistor.

Power Enable on Battery Detection

The power enabled on battery detection function allows

the MAX8903B/MAX8903E/MAX8903G to automatically

enable/disable the USB and DC power inputs when the

battery is applied/removed. This function utilizes the

battery pack’s integrated thermistor as a sensing mech-

anism to determine when the battery is applied or

removed. With this function, MAX8903B/MAX8903E/

MAX8903G-based systems shut down when the battery

is removed regardless of whether external power is

available at the USB or DC power inputs.

The MAX8903B/MAX8903E/MAX8903G implement the

power enabled on battery detection function with the ther-

mistor detector comparator as shown in Figure 7. If no bat-

tery is present, the absence of the thermistor allows RTB to

pull THM to VL. When the voltage at the THM pin increases

above 87% of VL, it is assumed that the battery has been

removed and the system powers down. However, there is

also the option to bypass this thermistor sensing option

completely, and so retain the ability to remove the battery

and let the system continue to operate with external power.

If the THM pin is tied to GND (voltage at THM is below 3%

of VL), the thermistor option is disabled and the system

does not respond to the thermistor input. In those cases, it

is assumed that the system has its own temperature sens-

ing, and halts changing through

CEN when the tempera-

ture is outside of the safe charging range.

Minimum SYS Output Capacitor

Based on the version of the MAX8903_, the SYS load

regulation is either 25mV/A or 40mV/A. The 25mV/A ver-

sions achieve better load regulation by increasing the

feedback loop gain. To ensure feedback stability with

this higher gain, a larger SYS output capacitor is

required. Devices with 25m/V SYS load regulation

require 22µF SYS output capacitor whereas devices

with 40m/V only require 10µF. See Table 6 for more

information about the various versions of the

MAX8903_.

Inductor Selection for

Step-Down DC-DC Regulator

The MAX8903_'s control scheme requires an external

inductor (LOUT) from 1.0µH to 10µH for proper operation.

This section describes the control scheme and the consid-

erations for inductor selection. Table 5 shows recommend-

ed inductors for typical applications. For assistance with

the calculations needed to select the optimum inductor for

a given application, refer to the spreadsheet at:

www.maximintegrated.com/design/tools/calcula-

tors/files/MAX8903-INDUCTOR-DESIGN.xls.

The MAX8903 step-down DC-DC regulator implements a

control scheme that typically results in a constant switch-

ing frequency (fSW). When the input voltage decreases to

a value near the output voltage, high duty cycle operation

occurs and the device can operate at less than fSW due

to minimum off-time (tOFFMIN) constraints. In high duty

cycle operation, the regulator operates with tOFFMIN and

a peak current regulation. Similarly, when the input

voltage is too high to allow fSW operation due to minimum

28-PIN 4mm x 4mm THIN QFN

SINGLE-LAYER PCB

MULTILAYER PCB

Continuous

Power

Dissipation

1666.7mW

Derate 20.8mW/°C

above +70°C

2286mW

Derate 28.6mW/°C

above +70°C

θJA

48°C/W

35°C/W

θJC

3°C/W

Table 4. Package Thermal Characteristics

Power Dissipation

MAX8903NETI+T 数据表(PDF) 22 Page - Maxim Integrated Products

MAX8903NETI+T 数据表(HTML) 22 Page - Maxim Integrated Products

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

数据表下载