ISL6292C Datasheet, PDF (7/11 Pages) Intersil Corporation – Li-ion/Li Polymer Battery Charger

ISL6292C
When using a current-limited adapter, the thermal situation in
the ISL6292C is totally different. Figure 4 shows the typical
charge curves when a current-limited adapter is employed.
The operation requires the I_REF to be programmed higher
than the limited current I_LIM of the adapter, as shown in
Figure 4. The key difference of the charger operating under
such conditions occurs during the CC mode.
The Block Diagram, Figure 1, aids in understanding the
operation. The current loop consists of the current amplifier
CA and the sense MOSFET Q_SEN. The current reference I_R
is programmed by the IREF pin. The current amplifier CA
regulates the gate of the sense MOSFET Q_SEN so that the
sensed current I_SEN matches the reference current I_R. The
main MOSFET Q_MAIN and the sense MOSFET Q_SEN form a
current mirror with a ratio of 100,000:1, that is, the output
charge current is 100,000 times I_R. In the CC mode, the
current loop tries to increase the charge current by
enhancing the sense MOSFET Q_SEN, so that 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
Q_SEN as well as the main MOSFET Q_MAIN, 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:
P_CH = R_DS(ON) ⋅ I_CHARGE2
(EQ. 2)
where R_DS(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 4. The 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 by the dotted lines in the charge
current and power curves. Appropriate design of the adapter
can further reduce the peak power dissipation of the
ISL6292C. See the Application Information section of the
ISL6292 data sheet (www.intersil.com) for more information.
Figure 5 illustrates the typical signal waveforms for the linear
charger from the power-up to a recharge cycle. More
detailed Applications Information is given below.
Applications Information
Power on Reset (POR)
The ISL6292C 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 5
illustrates the start up of the charger between t₀ to t₂.
The ISL6292C 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 V_MIN (2.84V typical) for
15 consecutive cycles of the internal oscillator. If the battery
voltage drops below V_MIN 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 V_CH, the CV mode begins. The
terminal voltage is regulated at the constant V_CH in the CV
mode and the charge current is expected to decline. After
the charge current drops below I_MIN (1/10 of I_REF, see End-
of-Charge Current for more detail), the ISL6292C indicates
the end-of-charge (EOC) with the STATUS pin. The charging
actually does not terminate until the internal timer completes
its length of TIMEOUT in order to bring the battery to its full
capacity. Signals in a charge cycle are illustrated in Figure 5
between points t₂ to t₅.
The following events initiate a new charge cycle:
• POR,
• the battery voltage drops below a recharge threshold after
completing a charge cycle,
• or, the EN pin is toggled from GND to floating.
Further description of these events are given later in this
data sheet.
VIN
V2P8
POR Threshold
Charge Cycle
Charge Cycle
STATUS
FAULT
15 Cycles to
1/8 TIMEOUT
VBAT
V_RECHRG
2.8V V_MIN
15 Cycles
I_CHARGE
I_MIN
t₀ t₁ t₂ t₃
t₄
t₅
t₆ t₇
t₈
FIGURE 5. OPERATION WAVEFORMS
7
FN9133.2
July 22, 2005