LM3655 datasheet(11/21 Pages) NSC

11 / 21 page

11.0 Charge Control Functions (Continued)

Unless otherwise noted, the following specifications apply over the Normal Temperature Range.

Specification

Test Conditions

Min

Typ

Max

Units

“V

TERMX =VTERMH” −20˚C to +70˚C

Chemistry Pin HIGH

3.90

4.25

Q1 Unsaturated Threshold “Q

1UNSAT”

@ 25˚C

550

795

Over Normal Temperature Range

550

795

“V

PHONE_ON” at CELL Pin

Cell voltage at which phone is operational

2.88

3.00

3.12

Maximum Full Rate Current “I

CHRG-MAX”

@ 25˚C

1.0

1.2

1.4

Over Normal Temperature Range

1.0

1.2

1.4

Maximum Power Dissipation of Pass

Transistor

For R

4A +R4B =2k

Ω

“P

PASS_MAX”VPS –VCELL =5Vto22V

@ 25˚C

530

650

800

@ −20˚C

530

650

880

@ +70˚C

475

650

800

11.2.6 Trickle/Top-Off Mode

After the full-rate charge is completed and the cell is charged to V

TERMX, the current will be reduced to the trickle charge current.

The pass transistor Q1 will no longer be in saturation and will now operate in linear mode. The maximum trickle current is

TRICKLE_MAX unless the power dissipation of Q1 exceeds PPASS_MAX. If the power dissipation of Q1 exceeds PPASS_MAX, the

trickle current will be further reduced until the power dissipation of Q1 is less than or equal to P

PASS_MAX.

During the transition from full-rate to trickle charge, the voltage of the cell will relax (IR drop due to cell internal impedance), but

the cell will again eventually be charged up again to V

TERMX. While the LM3655 allows the trickle current continue to flow to the

cell, it will constantly monitor the cell voltage not to exceed V

TERMX. The IC will maintain the cell voltage at VTERMX by gradually

reducing the trickle current, and eventually no charge current will flow into the cell.

The charge rate between V

SUV and VCHARGE_LOW is limited to the lowest setting of the Trickle charge range (ITRICKLEMIN). Once

CHARGE_LOW is reached, the trickle charger will remain charging but can use the full range of trickle charge, from ITRICKLEMIN to

TRICKLEMAX.

11.2.7 Peak Detector Function

The LM3655 device uses two independent peak detector circuits. One is used for filtering the ripple from the input power supply,

and the other for minimizing the IR drop of the cell voltage during transmit pulses (if the phone is operated while connected to a

charger). The peak detector circuits allow the device to be less sensitive to the ripple-induced noise that could unintentionally

trigger the internal circuit thresholds of the IC.

The peak detector circuit for smoothing the power supply ripple has its input connected across to the emitter of Q1 to CELL for

monitoring Q1 V

CE voltage and voltage across M5. The filtered voltage from the peak detector is used by the circuit that regulates

the power of the external PNP pass transistor Q1. The power limiting circuit determines the maximum charge current such that

the maximum power dissipation of Q1 [(Q1 V

CE +Ma5)* Ic] is lways at PPASS_MAX.

The peak detector for the cell voltage is used to smooth the sensed values for cell voltage drop during the transmit pulses. The

peak detector is in essence a digital filter with transient response characteristics as outlined below. Two parameters are

associated with the peak detector, decay rate and attack rate. Figure 4 a) and b) below show the decay and attack rates of the

power supply ripples and for the transmit pulses on the cell (output) voltage respectively. The typical input voltage filter decay and

attack rates are respectively 5.0 ms and 15.6 µs. The typical output voltage filter decay and attack rates are respectively 4 mV/ms

and 1.4V/ms.

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