ISL78692 Datasheet, PDF (11/18 Pages) Intersil Corporation – Li-ion/Li-Polymer Battery Charger

ISL78692
After the charge current drops below I_(EOC) programmed to 1/10
of I_REF; see “End-of-Charge (EOC) Current” on page 12 for more
detail), the ISL78692 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 20 between points t₂ to t₅.
VIN
V2P8
POR THRESHOLD
CHARGE CYCLE
CHARGE CYCLE
STATUS
FAULT
VBAT
15 CYCLES TO
1/8 TIMEOUT
VCH
VTRICKLE
VRECHRG
15 CYCLES
ICHARGE
IEOC
T0 T1 T2 T3
T4
T5
T6 T7
T8
FIGURE 20. OPERATION WAVEFORMS
The following events initiate a new charge cycle:
• POR
• A new battery being inserted (detected by TEMP pin)
• The battery voltage drops below a recharge threshold after
completing a charge cycle
• Recovery from an battery over-temperature fault
• Or, the EN pin is toggled from GND to floating
• Further description of these events are given later in this
datasheet
Recharge
After a charge cycle completes, charging is prohibited until the
battery voltage drops to a recharge threshold, V_RECHRG of 3.9V
(TYP), (see “Electrical Specifications” on page 5”). Then a new
charge cycle starts at point t₆ and ends at point t₈, as shown in
Figure 20. The safety timer is reset at t₆.
Internal Oscillator
The internal oscillator establishes a timing reference. The
oscillation period is programmable with an external timing
capacitor, C_TIME, as shown in Figure 1. The oscillator charges the
timing capacitor to 1.5V and then discharges it to 0.5V in one
period, both with 10µA current. The period t_OSC is given by
Equation 3:
t_OSC = 0.2  10⁶  C_TIME
 seconds
(EQ. 3)
A 1nF capacitor results in a 0.2ms oscillation period. The
accuracy of the period is mainly dependent on the accuracy of
the capacitance and the internal current source.
Total Charge Time
The total charge time for the CC mode and CV mode is limited to
a length of TIMEOUT. A 22-stage binary counter increments each
oscillation period of the internal oscillator to set the TIMEOUT.
The TIMEOUT can be calculated in Equation 4:
TIMEOUT
=
2²²



t_{--O-----S----C-6----0-S----E----C----}
=
14  -^C---₁-^T---_n-^I--^M-_F---^E-- minutes
(EQ. 4)
A 1nF capacitor leads to 14 minutes of TIMEOUT. For example, a
15nF capacitor sets the TIMEOUT to be 3.5 hours. The charger
has to reach the end-of-charge condition before the TIMEOUT,
otherwise, a TIMEOUT fault is issued. The TIMEOUT fault latches
up the charge and the FAULT pin goes low. There are two ways to
release such a latch-up either to recycle the input power, or
toggle the EN pin to disable the charger and then enable it again.
The trickle charge mode has a time limit of 1/8 TIMEOUT. If the
battery voltage does not reach V_TRICKLE within this limit, a
TIMEOUT fault is issued and the charger latches off. The charger
stays in trickle mode for at least 15 cycles of the internal
oscillator and, at most, 1/8 of TIMEOUT, as shown in Figure 20.
Charge Current Programming
The charge current is programmed by the IREF pin. There are
three ways to program the charge current:
1. Driving the IREF pin above 1.2V
2. Driving the IREF pin below 0.4V,
3. Or using the R_IREF as shown in “TYPICAL APPLICATION” on
page 1.
The voltage of IREF is regulated to a 0.8V reference voltage when
not driven by any external source. The charging current during the
constant current mode is 100,000 times that of the current in
the R_IREF resistor. Hence, depending on how IREF pin is used, the
charge current is given by Equation 5:


500 m A
I_REF=



-_R--⁰--_I-^.-_R-⁸---_E-^V---_F--  10⁵A


80 m A
V_IREF  1.2V
R_IREF
V_IREF  0.4V
(EQ. 5)
The internal reference voltage at the IREF pin is capable of sourcing
less than 100µA current. When pulling down the IREF pin with a
logic circuit, the logic circuit must be able to sink at least 100µA
current. For design purposes, a designer should assume a tolerance
of ±20% when computing the minimum and maximum charge
current from Equation 5.
When the adapter is current limited, it is recommended that the
reference current be programmed to at least 30% higher than the
adapter current limit (which equals the charge current). In addition,
the charge current should be at least 350mA, which the voltage
difference between the VIN and the VBAT pins is higher than 100mV.
The 100mV is the offset voltage of the input/output voltage
comparator shown in “Block Diagram” on page 2.
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FN8692.0
September 10, 2014