ISL78692

10

FN8692.0

September 10, 2014

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When using a current-limited adapter, the thermal situation in

the ISL78692 is totally different. Figures 19 shows the typical

charge curves when a current-limited adapter is employed. The

charger operating under such conditions occurs during the CC

mode.

The “Block Diagram” on page 2 aids in understanding the

operation. The current loop consists of the current amplifier CA

programmed by the IREF pin. The current amplifier CA regulates

of 100,000:1, which the output charge current is 100,000 times

sensed current matches the reference current. On the other

hand, the adapter current is limited, the actual output current will

never meet what is required by the current reference. As a result,

Therefore, the main MOSFET becomes a power switch instead of

a linear regulation device. The power dissipation in the CC mode

becomes Equation 2:

turned on. This power is typically much less than the peak power

in the traditional linear mode.

The worst power dissipation when using a current-limited adapter

typically occurs at the beginning of the CV mode, as shown in

Figure 19.

Equation 1 applies during the CV mode. When using a very small

PCB whose thermal impedance is relatively large, it is possible

that the internal temperature can still reach the thermal

foldback threshold. In that case, the IC is thermally protected by

lowering the charge current, as shown with the dotted lines in the

charge current and power curves. Appropriate design of the

adapter can further reduce the peak power dissipation of the

ISL78692. See “Applications Information” for more information.

Figure 20 illustrates the typical signal waveforms for the linear

charger from the power-up to a recharge cycle. More detailed

information is given in the following.

Applications Information

Power on Reset (POR)

The ISL78692 resets itself as the input voltage rises above the

POR rising threshold. The V2P8 pin outputs a 2.8V voltage, the

internal oscillator starts to oscillate, the internal timer is reset,

and the charger begins to charge the battery. The two indication

pins, STATUS and FAULT, indicate a LOW and a HIGH logic signal

respectively. Figure 20 illustrates the start-up of the charger

The ISL78692 has a typical rising POR threshold of 3.4V and a

falling POR threshold of 2.4V. The 2.4V falling threshold

guarantees charger operation with a current-limited adapter to

minimize the thermal dissipation.

Charge Cycle

A charge cycle consists of three charge modes: trickle mode,

constant current (CC) mode and constant voltage (CV) mode. The

charge cycle always starts with the trickle mode until the battery

cycles of the internal oscillator. If the battery voltage drops below

the charger stays in the trickle mode. The charger moves to the CC

mode after verifying the battery voltage. As the battery pack

CV mode and the charge current starts to reduce towards zero.

FIGURE 18. TYPICAL CHARGE CURVES USING A CONSTANT VOLTAGE

ADAPTER

FIGURE 19. TYPICAL CHARGE CURVES USING A CURRENT-LIMITED

ADAPTER

TRICKLE

MODE

CONSTANT CURRENT

MODE

CONSTANT VOLTAGE

MODE

INHIBIT

TIMEOUT

INPUT VOLTAGE

BATTERY VOLTAGE

CHARGE CURRENT

POWER DISSIPATION

TRICKLE

MODE

CONSTANT CURRENT

MODE

CONSTANT VOLTAGE

MODE

INHIBIT

TIMEOUT

INPUT VOLTAGE

BATTERY VOLTAGE

CHARGE CURRENT

POWER DISSIPATION

2



=

(EQ. 2)