BQ24742 Datasheet, PDF (19/35 Pages) Texas Instruments – Li-Ion or Li-Polymer Battery Charger with Low Iq and Accurate Trickle Charge

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bq24741, bq24742
SLUS875B – MARCH 2009 – REVISED OCTOBER 2009
Trickle Charge Current Regulation
The TRICKLE pin is provided to allow accurate current regulation at very low charge current. When CE is set to
HIGH, a logic HIGH level is applied to the TRICKLE pin, the charger will regulate 3 mV from CSP to CSN (150
mA with a 20 mΩ sense resistor), regardless of the voltage applied to the ISET pin. When TRICKLE is LOW,
ISET is used to program the charge current.
Input Adapter Current Regulation
The total input current from an AC adapter or other DC sources is a function of the system supply current and
the battery charging current. System current normally fluctuates as portions of the systems are powered up or
down. Without Dynamic Power Management (DPM), the source must be able to supply the maximum system
current and the maximum charger input current simultaneously. By using DPM, the input current regulator
reduces the charging current when the input current exceeds the input current limit set by ACSET. The current
capacity of the AC adapter can be lowered, reducing system cost.
Similar to setting battery-regulation current, adapter current is sensed by resistor R_AC connected between ACP
and ACN. Its maximum value is set by ACSET, which is ratiometric with respect to VDAC, using Equation 4.
I_ADAPTER
=
V_ACSET
V_VDAC
0.10
´ _RAC
(4)
The input voltage range of ACSET is between 0 and V_DAC, up to 3.6 V.
The ACP and ACN pins are used to sense R_AC with a default value of 10 mΩ. However, resistors of other values
can also be used. A larger sense-resistor value yields a larger sense voltage, and a higher regulation accuracy.
However, this is at the expense of a higher conduction loss.
Adapter Detect and Power Up
An external resistor voltage divider attenuates the adapter voltage before it goes to ACDET. The adapter detect
threshold should typically be programmed to a value greater than the maximum battery voltage and lower than
the minimum allowed adapter voltage. The ACDET divider should be placed before the ACFET in order to sense
the true adapter input voltage whether the ACFET is on or off.
If ACDET is below 0.6 V but PVCC is above 8 V, part of the bias is enabled, including a crude bandgap
reference, IADAPT is disabled and pulled down to GND. The total quiescent current is less than 10 μA.
Once ACDET rises above 0.6 V and PVCC is above 8 V, all the bias circuits are enabled and VREF goes to 3.3
V; and REGN output goes to 6 V if CE is HIGH. IADAPT becomes valid to proportionally reflect the adapter
current.
When ACDET keeps rising and passes 2.4 V, a valid AC adapter is present. 8 ms later, charge is allowed to turn
on.
Programming the PWM Switching Frequency
To program the PWM switching frequency, place a resistor from the FSET pin to ground, according to the
following formula:
R_FSET
=
41´10³
F_s
- 6.25
(kW)
(5)
Where R_FSET (kΩ) is the resistor from the FSET pin to ground, and F_s (kHz) is the desired switching frequency.
The switching frequency should be programmed between 300 kHz and 800 kHz.
Enable and Disable Charging
The following conditions must be valid before the charge function is enabled:
• CE is HIGH
• Adapter is detected
• Adapter voltage is higher than PVCC-BAT threshold
• Adapter is not over voltage
• The VREF and REGEN regulators are above 90% of the final values
Copyright © 2009, Texas Instruments Incorporated
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