ISL78692 Datasheet, PDF(10/18 Page) Intersil Corporation

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ISL78692 Datasheet, PDF (10/18 Pages) Intersil Corporation – Li-ion/Li-Polymer Battery Charger

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ISL78692

TRICKLE

MODE

CONSTANT CURRENT

MODE

CONSTANT VOLTAGE

MODE

INHIBIT

VIN

INPUT VOLTAGE

VCH

BATTERY VOLTAGE

VTRICKLE

ICHARGE

ICHARGE/10

CHARGE CURRENT

POWER DISSIPATION

TIMEOUT

FIGURE 18. TYPICAL CHARGE CURVES USING A CONSTANT VOLTAGE

ADAPTER

TRICKLE

MODE

VIN

VCH

CONSTANT CURRENT

MODE

CONSTANT VOLTAGE

MODE

INHIBIT

INPUT VOLTAGE

BATTERY VOLTAGE

VTRICKLE

ICHARGE

ILIM

ICHARGE/10

CHARGE CURRENT

POWER DISSIPATION

TIMEOUT

FIGURE 19. TYPICAL CHARGE CURVES USING A CURRENT-LIMITED

ADAPTER

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

operation requires the IREF to be programmed higher than the

limited current ILIM of the adapter. The key difference of the

charger operating under such conditions occurs during the CC

mode.

The âBlock Diagramâ on page 2 aids in understanding the

operation. The current loop consists of the current amplifier CA

and the sense MOSFET (QSEN). The current reference IR is

programmed by the IREF pin. The current amplifier CA regulates

the gate of the sense MOSFET (QSEN) that the sensed current

ISEN matches the reference current IR. The main MOSFET QMAIN

and the sense MOSFET (QSEN) form a current mirror with a ratio

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

IR. In the CC mode, the current loop tries to increase the charge

current by enhancing the sense MOSFET (QSEN), which the

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,

the current error amplifier CA, keeps enhancing the QSEN as well

as the main MOSFET QMAIN until they are fully turned on.

Therefore, the main MOSFET becomes a power switch instead of

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

becomes Equation 2:

PCH

ï¨ONï©

ICHARG

(EQ. 2)

where rDS(ON) is the resistance when the main MOSFET is fully

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 âApplications Informationâ 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

between t0 to t2.

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

voltage stays above VTRICKLE (2.8V typical) for 15 consecutive

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

VTRICKLE during the 15 cycles, the 15-cycle counter is reset and

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

mode after verifying the battery voltage. As the battery pack

terminal voltage rises to the final charge voltage VCH, the CV mode

begins. The terminal voltage is regulated at the constant VCH in the

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

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FN8692.0

September 10, 2014

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