MC34674CEPSLASHR2 datasheet(21/26 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34674

TYPICAL APPLICATIONS

INTRODUCTION

When the battery voltage rises above the trickle charge

threshold, the charger enters the CC-mode. The MC34674

tries to raise the charge current to the internally set reference,

such as 1.05 A, by enhancing the power MOSFET. However,

since the current provided by the AC/DC regulator is limited

and can never reach the set reference, the charger will keep

enhancing the MOSFET until it is fully enhanced and is fully

turned on. In this mode, the internal power MOSFET behaves

as a switch instead of a linearly regulating device. The

voltage difference between the input and the output is

determined by the on resistance, RDS(ON), of the power

MOSFET and the limited output current of the ad/dc

regulator.

The power dissipation, PD, in the MOSFET can be calculated

as,

The charge current in CC-mode is not determined by the

MC34674, instead, it is determined by the AC/DC regulator

current limit, ILIM, which is a value lower than the charger

internally set current reference. The internally set current

reference is used as a secondary protection threshold, in

case if an AC/DC regulator with a wrong current limit is

connected to the input.

The key advantage of using the MC34674 with a current-

limited AC/DC regulator is the significant reduction of the

power dissipation during the CC-mode. Figure 26 illustrates

the small voltage difference between the input and the output

of the charger, which is directly proportional to the power

dissipation.

When entering the CC-mode, the charger output I/V

trajectory jumps from point b to c and then moves from c to d

as the battery voltage rises to 4.2 V. The AC/DC regulator

output trajectory moves from B to C, as shown in Figure 25.

When the battery voltage reaches the target 4.2V, the

charger enters the CV-mode. The charge current starts to

decline and the AC/DC regulator output enters its constant-

voltage mode. The charger then operates as a regular linear

charger again until the charging completes. The battery I/V

trajectory moves from d to the EOC moment (point e) while

the AC/DC regulator output trajectory jumps from C to D and

then moves to E at the EOC moment.

BALANCING YELLOW COLOR IN LED

The red and the green colors in the LED are driven by two

matched 6.0 mA current sources. Such design ensured a

consistent brightness of the LED over a large range of the

input voltage. When both colors are turned on, the resulting

color should be yellow. One can adjust the resulting color by

adjusting the brightness of the individual color. A resistor can

be added to reduce the brightness of one color, such as the

R1 shown in Figure 27.

Figure 27. LED Color Balancing Scheme.

INPUT CAPACITOR

The input capacitor is used to reduce the input voltage

transient that may cause instability. A 1.0

μF, X5R, 16 V

rated ceramic capacitor is recommended for most

applications.

OUTPUT CAPACITOR

For stable operation, an X5R ceramic capacitor with a

minimum 1.0

μF nominal value is recommended at the

output. The output capacitance should not be larger than

240

μF to allow the 585 μA current to discharge the capacitor

voltage to the recharge threshold within 82 ms.

NTC INTERFACE DESIGN

The NTC interface is designed to be able to work with most

types of NTC thermistors. This section describes in details

how to select the two resistors RU and RS shown in

Figure 19. In addition, the hysteresis and the tolerance of the

temperature thresholds are discussed. The

NCP15W104F03RC from Murata is used as an example for

the calculations in this section. The partial temperature

characteristics of the NCP15W104F03RC are given in

Table 7.

Table 7. NTC Thermistor Temperature Characteristics.

OUT

–I

LIM

DS ON

()

LIM

DS ON

()

Temp (°C)

R-low (k

Ω)

R-center (k

Ω)

R-high (k

Ω)

-2

389.2453

398.6521

408.2455

-1

368.4960

377.1927

386.0560

348.9722

357.0117

365.1999

313.2543

320.1216

327.1067

296.9408

303.2866

309.7370

...

38.4596

39.2132

39.9778

36.8626

37.6010

28.3503

32.5022

33.1946

33.8983

28.7183

29.3660

30.0253

27.5694

28.2026

28.8474

GRN

VIN

RED

MC34674CEP/R2 Datasheet(PDF) 21 Page - Freescale Semiconductor, Inc

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