LM3655 datasheet(9/21 Pages) NSC

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11.0 Charge Control Functions (Continued)

In region 1 the cell voltage is below V

TERMX and the charge current is in full-rate, the charging will continue in full-rate after the

test pulse. At the end of region 1 when the cell voltage reaches V

TERMX, the charge current now drops to trickle current and the

cell voltage drops slightly due to the IR drop of the cell internal impedance. The trickle current will continue in region 2 until the

next test pulse arrives in region 3, the internal circuit will delay the test pulse for

TEST_DELAY while resetting the charge current

to zero. After the delay the current will then ramp back to full-rate and the cell voltage will charge back to V

TERMX. The test pulse

stays high for at least

TEST_WIDTH and then reset to off as seen in region 4. In this region the charge current drops to trickle

current as the cell voltage also drops slightly. In regions 5, 6, and 7, the circuit continues charging the cell while cell voltage

gradually tapers to V

TERMX and the height of pulse current is gradually getting smaller. In regions 8 and 9 the cell voltage reaches

TERMX before the next pulse arrives, the trickle charge current will begin to reduce to less than ITRICKLE-MAX. The current will

continue to reduce in region 10 and so on until the current goes to zero. The cell voltage will be at the maximum charge capacity

voltage of V

TERMX. The charge profile will be different for different types of power supplies and discharge currents.

Specification

Test Conditions

Min

Typ

Max

Units

Pulse width of Voltage Regulator Reset

“

TEST_DELAY”

0.7

1.0

1.3

Fast Charging Pulse Width “

TEST_WIDTH”

180

256

332

Fast Charging Pulse Period “

TEST_PERIOD”

Falling edge of previous pulse to rising edge

of next pulse

Sec

11.2.4 Low Cell Voltage Charging (Pre-Charging)

When the cell is very deeply discharged, the IC’s normal cell voltage rail may be too low for proper operation. When connected

to a power supply, if full-rate charge were immediately entered, the external supply voltage would collapse to the (very low) cell

voltage level once the power supply’s current limit has been reached.

To avoid this situation, and also to protect a deeply-discharged cell from potential damage by fast-charging too rapidly, a reduced

current charge is applied to the cell until it reaches a minimum “safe” operating level, V

PHONE_ON.

The current the IC draws from the cell to operate its internal circuitry when the charge power supply is detected is I

Q_CHRG.

When the cell voltage is less than the Phone On Voltage Threshold, 3.00V, and above the Secondary Under-voltage threshold,

1.98V, the cell will be charged at a low charge current. In this mode, the power dissipation management circuit is active thus Q1

will not be over-dissipated. Burp mode is not active in this voltage range.

A special charge circuit is provided that operates for cell voltages between 0V and 1.98V. This circuit is active only if a charger

is applied and the cell is in this voltage range. The charge current is limited to 70 mA maximum in this mode in order to limit the

power dissipation of Q1 and M5 to 800 mW for power supply voltages of 18V or less. Once the cell increases to 1.98V, the sub-2V

charging deactivates and the conventional charger takes over. A diode path from the V

DETECT pin to the CELL + pin allows for a

trickle charge current (limited by R4A and R4B ) for deeply charged cells (<0.7V).

20111506

FIGURE 3. Li-ion Charge Profile

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