BQ24157 Datasheet, PDF (22/39 Pages) Texas Instruments – Fully Integrated Switch-Mode Charger With USB Compliance and USB-OTG Support

bq24157
SLUSB80 – SEPTEMBER 2012
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In boost mode, the input N-FET (Q1) prevents battery discharge when VBUS pin is over loaded. Cycle-by-cycle
current limit is sensed through the internal sense FET for Q3. The cycle-by-cycle current limit threshold for Q3 is
set to a nominal 1.0-A peak current. Synchronous operation is used in PWM mode to minimize power losses.
Boost Start Up
To prevent the inductor saturation and limit the inrush current, a soft-start control is applied during the boost start
up.
PFM Mode at Light Load
In boost mode, under light load conditions, the IC operates in pulse skipping mode (PFM mode) to reduce the
power loss and improve the converter efficiency. During boosting, the PWM converter is turned off once the
inductor current is less than 75mA; and the PWM is turned back on only when the voltage at PMID pin drops to
about 99.5% of the rated output voltage. A unique pre-set circuit is used to make the smooth transition between
PWM and PFM mode.
Protection in Boost Mode
Output Overvoltage Protection
The IC provides a built-in over-voltage protection to protect the device and other components against damage if
the VBUS voltage goes too high. When an over-voltage condition is detected, the IC turns off the PWM
converter, resets OPA_MODE bit to 0, sets fault status bits, and sends out a fault pulse from the STAT pin. Once
VBUS drops to the normal level, the boost starts after host sets OPA_MODE to “1” or OTG pin stays in active
status.
Output Overload Protection
The IC provides a built-in over-load protection to prevent the device and battery from damage when VBUS is
over loaded. Once the over load condition is detected, Q1 operates in linear mode to limit the output current. If
the over load condition lasts for more than 30ms, the over-load fault is detected. When an over-load condition is
detected, the IC turns off the PWM converter, resets OPA_MODE bit to 0, sets fault status bits and sends out
fault pulse in STAT pin. The boost will not start until the host clears the fault register.
Battery Overvoltage Protection
During boosting, when the battery voltage is above the battery over voltage threshold, V_BATMAX, or below the
minimum battery voltage threshold, V_BATMIN, the IC turns off the PWM converter, resets OPA_MODE bit to 0, sets
fault status bits and sends out fault pulse in STAT pin. Once the battery voltage goes above V_BATMIN, the boost
will start after the host sets OPA_MODE to “1” or OTG pin stays in active status.
STAT Pin in Boost Mode
During normal boosting operation, the STAT pin behaves as a high impedance (open-drain) output. Under fault
conditions, a 128-μs pulse is sent out to notify the host.
HIGH IMPEDANCE (Hi-Z) MODE
In Hi-Z mode, the charger stops charging and enters a low quiescent current state to conserve power. Taking the
CD pin high causes the charger to enter Hi-Z mode. When in default mode and the CD pin is low, the charger
automatically enters Hi-Z mode if
1. VBUS > UVLO and a battery with V_BAT > V_LOWV is inserted, or
2. VBUS falls below UVLO.
When in HOST mode and the CD is low, the charger can be placed into Hi-Z mode if the HZ-MODE control bit is
set to “1” and OTG pin is not in active status.
In order to exit Hi-Z mode, the CD pin must be low, VBUS must be higher than UVLO and the HOST must write
a "0" to the HZ-MODE control bit.
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