JAJSEC4E September 2012 – January 2018 BQ24157
PRODUCTION DATA.
The IC provides an integrated, fixed 3 MHz frequency voltage-mode controller to regulate charge current or voltage. This type of controller is used to improve line transient response, thereby, simplifying the compensation network used for both continuous and discontinuous current conduction operation. The voltage and current loops are internally compensated using a Type-III compensation scheme that provides enough phase margin for stable operation, allowing the use of small ceramic capacitors with a low ESR. The device operates between 0% to 99.5% duty cycles.
The IC has back to back common-drain N-channel FETs at the high side and one N-channel FET at low side. The input N-FET (Q1) prevents battery discharge when VBUS is lower than VCSOUT. The second high-side N-FET (Q2) is the switching control switch. A charge pump circuit is used to provide gate drive for Q1, while a bootstrap circuit with an external bootstrap capacitor is used to supply the gate drive voltage for Q2.
Cycle-by-cycle current limit is sensed through the FETs Q2 and Q3. The threshold for Q2 is set to a nominal 2.4-A peak current. The low-side FET (Q3) also has a current limit that decides if the PWM Controller will operate in synchronous or non-synchronous mode. This threshold is set to 100mA and it turns off the low-side N-channel FET (Q3) before the current reverses, preventing the battery from discharging. Synchronous operation is used when the current of the low-side FET is greater than 100mA to minimize power losses.