LM3647 datasheet(7/17 Pages) NSC

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Resistor network selection Quick Guide:

4.3.2 Charge Timeout

The LM3647 uses the charge timeout value as a backup ter-

mination method if the normal termination methods fail. The

charge timeout also controls the length of some of the phas-

es, (e.g., the Topping Charge phase). The timeout is select-

able between a charge rate of 3.2C down to 0.4C. The table

below shows R-C values and their resulting timeout.

4.3.3 Charge Current

The charge-current is selected by setting the current sensing

resistor and the gain of the differential ampliﬁcation stage.

The current sensing resistor (R5) should be dimensioned

such that a voltage drop over it is not too small, because the

signal will be more susceptible to noise and offsets in the am-

pliﬁcation-stage. The resistance should not be too large ei-

ther (especially in high-current applications), because this

will only generate more heat from the component. A suitable

value is one that develops about 50mV across the resistor

when maximum current ﬂows through it.

The current-sensing signal is ampliﬁed, inverted and cen-

tered on the 2.5V reference by the operational ampliﬁer and

fed into the CS pin on the LM3647. The ampliﬁcation stage

must be dimensioned by setting the appropriate ratio be-

tween R1 (R3) and R2 (R4). The ﬁgure below is dimensioned

for a maximum current of about 1.1A. This was dimensioned

using the following formula:

4.3.4 Dimensioning the reset circuitry

The reset-circuitry is designed to hold the RESET-pin until

the power supply to the LM3647 has stabilized. The RC-net-

work (R21 and C4) should be dimensioned in the following

way:

The diode D2 discharges the capacitance C4 when power

loss occurs. The resistor R20 is used to protect the SYSOK-

pin, and its value is not critical (typical value is 2k

Ω). The con-

No. of Cells

Li-Ion (3.6V cell)

Normal

Max

3.6V

3.675V

16k

30k

7.2V

7.35V

62k

30k

10.8V

11.025V

27k

7.5k

14.4V

14.7V

22k

3.9k

No. of Cells

Li-Ion (3.7V cell)

Normal

Max

3.7V

3.74V

16k

30k

7.4V

7.48V

62k

30k

11.1V

11.22V

27k

7.5k

14.8V

14.96V

22k

3.9k

R Value

C Value

Appropriate Charge Rates

100 k

Ω

internal

3.2C

100 k

Ω

10 nF

2.4C

100 k

Ω

15 nF

1.4C

100 k

Ω

22 nF

1.2C

100 k

Ω

33 nF

0.9C

100 k

Ω

47 nF

0.7C

100 k

Ω

68 nF

0.5C

100 k

Ω

100 nF

0.4C

MaxCurrent

() R1

()

⁄

---------------------------

5.1k

Ω

100k

Ω

0.047

Ω

MaxCurrent

1.09 Ampere

≈

R21

() 5xPowerSupplyRiseTime

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