BQ24157S Datasheet, PDF (17/45 Pages) Texas Instruments – Full USB Compliance and USB-OTG Support

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bq24157S
SLUSB76B – FEBRUARY 2013 – REVISED MAY 2015
Feature Description (continued)
8.3.2.2 Battery Short Protection
During the normal charging process, if the battery voltage is lower than the short-circuit threshold, V_SHORT, the
charger operates in short circuit mode with a lower charge rate of I_SHORT.
8.3.2.3 Battery Detection in HOST Mode
For applications with removable battery packs, the IC provides a battery absent detection scheme to reliably
detect insertion or removal of battery packs.
During the normal charging process with HOST control and termination enabled, when the voltage at the CSOUT
pin is above the battery recharge threshold, V_OREG – V_RCH, and the termination charge current is detected, the IC
turns off the PWM charge and enables a discharge current, I_DETECT, for a period of t_DETECT, (262-ms typical), then
checks the battery voltage. If the battery voltage is still above the recharge threshold after t_DETECT, the battery is
present. On the other hand, if the battery voltage is below the battery recharge threshold, the battery is absent.
Under this condition, the charge parameters (such as input current limit) are reset to the default values and
charge resumes after a delay of t_INT. This function ensures that the charge parameters are reset whenever the
battery is replaced.
8.3.3 DEFAULT Mode
After the battery and input bus voltages are removed from the IC and replaced, the bq24157S enters DEFAULT
mode until I²C communication begins.
8.3.4 USB Friendly Power Up
Prior to POR, if the host continues to write the TMR_RST bit to 1, to stay in HOST mode, then at POR, the
charger enters normal charge mode (using the desired control bits). If not in HOST mode at POR, the charge will
operate with default bit values, until the host updates the control registers.
The default control bits set the charging current and regulation voltage low as a safety feature to avoid violating
USB specifications and overcharging any of the Li-Ion chemistries, while the host has lost communication. The
input current limiting is described in the following sections.
8.3.5 Input Current Limiting at Power Up
The input current sensing circuit and control loop are integrated into the IC. When operating in DEFAULT mode,
the OTG pin logic level sets the input current limit to 100 mA for a logic low and 500 mA for a logic high. In
HOST mode, the input current limit is set by the programmed control bits in register 01H.
8.3.6 Factory Mode
The factory mode can be enabled only when the battery is removed. This can be done through an I²C register 05
bit 6 (see Table 9). The purpose of the mode is to operate the phone in a GSM phone call with no-battery
connected and do a calibration of the system. Setting the factory mode bit enables the following changes:
• 20X ICHG amp is disabled – that is, output current limit is disabled
• Cycle-by-cycle HS current limit threshold is doubled (current typical is 2.4 A, shifts to 4.8 A)
• CCM mode is always enabled (because the current could go from 0 to full GSM pulse)
8.3.7 Spread Spectrum Mode
The purpose of the spread spectrum clock modulation is to reduce EMI. In the spread spectrum mode, the
switching frequency is not fixed to 3 MHz. It is instead shifted by ±10% from the fixed 3-MHz switching
frequency. The shift is happening in eight steps, four steps in the upper range and four steps in the lower range
every 170 µs. By modulating the clock frequency, the energy of the switching converter’s EMI is distributed over
a wider range of frequencies thereby lowering the magnitude of EMI at 3 MHz ±10% as well as harmonic
frequencies.
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