MAX2003 datasheet(14/20 Pages) MAXIM

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From solving these simultaneous equations:

RT2 = [(X) (RNTC10) - (RNTC50)] / (1 - X)

RT1 = [(RT2) (RNTC10) (VCC - VTS10)] / [VTS10 (RT2 +

RNTC10)].

[(RNTC50)(VTS10)(VCC - VTS50)]

where X = _____________________________

[(RNTC10) (VTS50) (VCC - VTS10)]

Using RNTC50 = 4.16k

Ω, RNTC10 = 17.96kΩ, VTS50 =

1.72V, and VTS10 = 2.00V, it can be calculated that RT1

= 1.599k

Ω and RT2 = 2.303kΩ.

Select preferred resistor values for RT1 (2.21kΩ) and

RT2 (1.62kΩ). The actual voltages on MCV and TCO

can be verified as follows:

7) Select Maximum Cell Voltage (MCV) and

Temperature Cutoff (TCO). The MCV and TCO can be

selected with a resistor-divider combination (Figure 9).

In our example, TCO has been set to +10°C, which cor-

responds to a voltage of 1.72V at the TS pin. The MCV

for most fast-charge batteries can be set to about 1.9V.

To minimize the current load on VCC, choose R1 in the

range of 20k

Ω to 200kΩ. In this example, choose R1 =

60.4k

Ω, then calculate R3 and R2 as follows:

R3 = (VTCO x R1) / (VCC - VMCV) = 33.5k

Ω (1%)

and

R2 = (VMCV x R1) / (VCC - VMCV) - R3 = 3.51k

Ω (1%)

Select preferred resistor values for R2 (3.48k

Ω) and R3

(33.2k

Ω). The actual voltages on MCV and TCO can be

verified as follows :

VTCO = VCC (R3) / (R1 + R2 + R3) = 1.71V

and

VMCV = VCC (R2 + R3) / (R1 + R2 + R3) = 1.89V.

8) Select Trickle Resistor (MAX2003 only). The trick-

le resistor (RTR) is selected to allow a trickle-charge

rate of C/16 to C/40. The resistor value is given by:

RTR = (VDC - VBAT) / ITR

where ITR is the required trickle current, VDC is the DC

supply voltage, and VBAT is the number of cells times

the cell voltage after fast-charge.

In our example, the 1700mAh NiMH battery needs a

trickle current of C/40; i.e., 42mA (1700mAh/40h).

Therefore, the minimum voltage (from the formula

above) is as follows:

RTR = [13.0V - (6 x 1.2V)] / 42mA

≈ 150Ω

The maximum power dissipated in the resistor can be

calculated by:

Power = (VDC - VBAT(MIN))

2 / RTR

where VBAT(MIN) is the minimum cell voltage, VDC is the

DC supply voltage, and RTR is the trickle resistor value.

Since a shorted battery could have 0V, this must be the

minimum cell voltage possible. Therefore the power

dissipated in the trickle resistor would be:

Power = (13 - 0)2 / 150 = 1.2W

A 2W, 150

Ω resistor should be sufficient for the trickle-

charge resistor. For the MAX2003A, refer to

Trickle-

Charge section.

9) Select Inductor. The inductor value can be calcu-

lated using the formula:

VL = L

δi /δt

where VL is the maximum voltage across the inductor, L

is the minimum inductor value,

δi is the change in induc-

tor current, and t is the minimum on-time of the switch.

II R

5 1.62k

II 17.96k

1.62k

II 17.96k

2.21k

2.01V

II R

5 1.62k

II 4.16k

1.62k

II 4.16k

2.21k

1.72V

TS10

NTC10

TS50

NTC50

(

)

(

) +

[

]

Ω

(

)

Ω

(

) +

Ω

[

]

(

)

(

) +

[

]

Ω

(

)

Ω

(

) +

Ω

[

]

NiCd/NiMH Battery

Fast-Charge Controllers

______________________________________________________________________________________

ILOAD

IMAX = 1.9A

IMIN = 1.5A

TIME

δi = IMAX - IMIN

tOFF

tON

Figure 10. Inductor-Current Waveform in Continuous-

Conduction Mode

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