Not Recommended for New Designs
SLUSB76B – FEBRUARY 2013 – REVISED MAY 2015
8.3.14 Boost Mode Operation
In HOST mode, when OTG pin is high (and OTG_EN bit is high thereby enabling OTG functionality) or the
operation mode bit (OPA_MODE) is set to 1, the device operates in boost mode and delivers the power to VBUS
from the battery. In normal boost mode, the device converts the battery voltage to VBUS-B (about 5.05 V) and
delivers a current as much as IBO (about 375 mA for bq24157S) to support other USB OTG devices connected to
the USB connector.
126.96.36.199 PWM Controller in Boost Mode
Similar to charge mode operation, in boost mode, the IC provides an integrated, fixed 3-MHz frequency voltage-
mode controller to regulate output voltage at PMID pin (VPMID). The voltage control loop is internally
compensated using a Type-III compensation scheme that provides enough phase margin for stable operation
with a wide load range and battery voltage range.
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.
188.8.131.52 Boost Start Up
To prevent the inductor saturation and limit the inrush current, a soft-start control is applied during the boost start
184.108.40.206 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 when the
inductor current is less than 75 mA, 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.
220.127.116.11 Protection in Boost Mode
18.104.22.168.1 Output Overvoltage Protection
The IC provides built-in overvoltage protection to protect the device and other components against damage if the
VBUS voltage goes too high. When an overvoltage 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. When VBUS
drops to the normal level, the boost starts after host sets OPA_MODE to 1 or OTG pin stays in active status.
22.214.171.124.2 Output Overload Protection
The IC provides built-in overload protection to prevent the device and battery from damage when VBUS is
overloaded. After the overload condition is detected, Q1 operates in linear mode to limit the output current. If the
overload condition lasts for more than 30 ms, the overload fault is detected. When an overload 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.
126.96.36.199.3 Battery Overvoltage Protection
During boosting, when the battery voltage is above the battery overvoltage threshold, VBATMAX, or below the
minimum battery voltage threshold, VBATMIN, 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. After the battery voltage goes above VBATMIN, the boost
will start after the host sets OPA_MODE to 1 or OTG pin stays in active status.
188.8.131.52 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.
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