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Introduction
2
Considerations When Testing and Using bq24070 ICs
Introduction
The bq24070 powers the system while independently charging the battery. This feature reduces the
charge and discharge cycles on the battery, allows for proper charge termination, and allows the system
to run with an absent or defective battery pack. This feature also allows for the system to turn on
instantaneously from an external power source even when using a deeply discharged battery pack.
The IN pin can be programmed to perform like a USB input by pulling the MODE pin low or like an adapter
input if the MODE pin is pulled high. An external resistor, RSET1, sets the magnitude of the charge
current. If the charge current exceeds the available input current, the voltage on the OUT pin drops to the
DPPM
OUT
threshold or the battery voltage, whichever is higher. The charging current is reduced to what
current is available (I
BAT
= I
IN
– I
OUT
).
The bq24070 charges the battery in three phases: conditioning, constant current, and constant voltage.
Charge is terminated based on minimum current. A resistor-programmable charge timer provides a
backup safety for charge termination. The bq24070 automatically restarts the charge if the battery voltage
falls below an internal threshold. The bq24070 automatically enters sleep mode when both supplies are
removed (a drop to the battery voltage).
It is recommended to read the bq24070 data sheet prior to evaluating this EVM. Consider the following
noteworthy items while testing and using the bq24070 IC.
The two potential sources to power the system (V
OUT
) are: IN (adapter or USB source) and the battery.
The IC is designed to power the system continuously. The battery, in most cases, is the last line of
backup. If the adapter or USB input is not available (or disabled), the battery connects to the system.
In thermal regulation condition (T
J
= 125°C—not a first-choice design mode of operation), the charge
current is reduced to the battery, and the system still gets its power from the input. The battery
supplement is still available in thermal regulation if V
OUT
falls to V
BAT
. In thermal cutoff (~155°C), the input
sources are disconnected, but the internal battery FET connects the battery to V
OUT
.
There are two types of OUT-pin short circuit, one associated with the input IN pin (V
OUT
< 1 V) and the
other associated with the BAT pin (V
BAT
– V
OUT
> 200 mV). For the BAT short circuit, the battery FET
opens if a short on V
OUT
pulls more than ~4 A of current (>200-mV drop across the BAT FET) from the
battery. The recovery method is from a 10-mA current source between the BAT and OUT pins, so the
short and any system load must be removed before the OUT pin can recover within 200 mV of V
BAT
. Note
that the current source is ~10 mA with the OUT pin near 0 V, but falls off to ~2 mA as the OUT pin goes
above 1 V. For the IN-to-OUT short-circuit case where the OUT pin is less than 1 V, the recovery method
is by a 500-
Ω
pullup resistor from IN to OUT. The system load must be reduced (>200
Ω
) such that the
pullup can pull V
OUT
above 1 V.
When there is no power to the system and the battery is hot-plugged, the BAT-pin voltage leads the
OUT-pin voltage due to the system capacitance, and the output may go into BAT short-circuit mode. To
avoid this, a feature was added to the DPPM pin. If the voltage on the DPPM pin is held below 1 V, then
the short-circuit feature is disabled. Therefore, placing a small capacitor (~1 nF to 10 nF) across the
DPPM resistor delays the short-circuit protection on input power-up by a few microseconds.
Another feature that protects system integrity is dynamic power path management (DPPM). The voltage
on the DPPM pin (DPPM
IN
) times a scaling factor of ~1.15 is the DPPM
OUT
voltage. The DPPM
OUT
voltage
is the critical voltage, determined by the designer, where battery charging current is reduced to keep the
system voltage (V
OUT
) from further decay. A special feature to keep in mind is that when in DPPM mode,
the internal oscillator timer is slowed in proportion to the reduction in programmed charger current. This
allows the timers (safety and others) to be appropriately adjusted during operation. Therefore, when
performing any test where time is measured, keep in mind this adjustment factor.
The MODE (High/Low) sets which input source is present (adapter or USB). The CE pin (going high)
immediately enables the chip; disabling it (going low) delays handoff for 5 ms.
bq24070 1.5-A Single-Chip Li-Ion and Li-Pol Charge Management IC EVM
2
SLUU248 – May 2006
