ISL6292_14 Datasheet, PDF (11/20 Pages) Intersil Corporation – Li-ion/Li Polymer Battery Charger

ISL6292
When using a current-limited adapter, the thermal situation
in the ISL6292 is totally different. Figure 18 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 18. The key difference of the charger operating under
such conditions occurs during the CC mode.
The Block Diagram (Figure 16) 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 Equation 2:
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 18. 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 ISL6292.
See“Applications Information” on page 11 for more
information.
Figure 19 illustrates the typical signal waveforms for the
linear charger from the power-up to a recharge cycle. More
detailed Applications Information is given in the following.
Applications Information
Power on Reset (POR)
The ISL6292 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 19
illustrates the start-up of the charger between t₀ to t₂.
The ISL6292 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 (2.8V 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 (programmable for the 4x4 and 5x5
package and programmed to 1/10 of I_REF for the 3x3
package; see “End-of-Charge (EOC) Current” on page 13 for
more detail), the ISL6292 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 19 between
points t₂ to t₅.
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 19. OPERATION WAVEFORMS
11
FN9105.9
December 17, 2007