BQ25700A Datasheet, PDF (23/82 Pages) Texas Instruments – SMBus Multi-Chemistry Battery Buck-Boost Charge Controller With System Power Monitor and Processor Hot Monitor

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bq25700A
SLUSCQ8 – MAY 2017
8.3.3 USB On-The-Go (OTG)
The device supports USB OTG operation to deliver power from the battery to other portable devices through
USB port. The OTG mode output voltage is set in REG0x3B(). The OTG mode output current is set in
REG0x3C(). The OTG operation can be enabled if the conditions are valid:
• Valid battery voltage is set REG0x15()
• OTG output voltage is set in REG0x3B() and output current is set in REG0x3C()
• EN_OTG pin is HIGH and REG0x32[12] = 1
• VBUS is below V_{VBUS_UVLO}
• 10 ms after the above conditions are valid, converter starts and VBUS ramps up to target voltage. CHRG_OK
pin goes HIGH if REG0x12[11] = 1.
8.3.4 Converter Operation
The charger employs a synchronous buck-boost converter that allows charging from a standard 5-V or a high-
voltage power source. The charger operates in buck, buck-boost and boost mode. The buck-boost can operate
uninterruptedly and continuously across the three operation modes.
MODE
Q1
Q2
Q3
Q4
Table 2. MOSFET Operation
BUCK
Switching
Switching
OFF
ON
BUCK-BOOST
Switching
Switching
Switching
Switching
BOOST
ON
OFF
Switching
Switching
8.3.4.1 Inductor Setting through IADPT Pin
The charger reads the inductor value through the IADPT pin.
Table 3. Inductor Setting on IADPT Pin
INDUCTOR IN USE
1 µH
2.2 µH
3.3 µH
RESISTOR ON IADPT PIN
93 kΩ
137 kΩ
169 kΩ
8.3.4.2 Continuous Conduction Mode (CCM)
With sufficient charge current, the inductor current does not cross 0, which is defined as CCM. The controller
starts a new cycle with ramp coming up from 200 mV. As long as error amplifier output voltage is above the ramp
voltage, the high-side MOSFET (HSFET) stays on. When the ramp voltage exceeds error amplifier output
voltage, HSFET turns off and lowside MOSFET (LSFET) turns on. At the end of the cycle, ramp gets reset and
LSFET turns off, ready for the next cycle. There is always break-before-make logic during transition to prevent
cross-conduction and shoot-through. During the dead time when both MOSFETs are off, the body-diode of the
low-side power MOSFET conducts the inductor current.
During CCM, the inductor current always flows and creates a fixed two-pole system. Having the LSFET turn-on
keeps the power dissipation low and allows safe charging at high currents.
8.3.4.3 Pulse Frequency Modulation (PFM)
In order to improve converter light-load efficiency, the bq25700A switches to PFM control at light load when
inductor current is less than 500 mA. The effective switching frequency will decrease accordingly when system
load decreases. The minimum frequency can be limit to 25 kHz (ChargeOption0() bit[10]=1). To have higher light
load efficiency, set EN_OOA bit low (Chargeoption0() bit[10] = 0).
Copyright © 2017, Texas Instruments Incorporated
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