The boost converter of the TPS61162A, TPS61163A integrates a 1.5A, 40V low-side switch MOSFET and has a fixed switching frequency of 1.2MHz. The control architecture is based on traditional current-mode Pulse Width Modulation (PWM) control. For operation see the Section 6.2. Two current sinks regulate the dual-channel current, and the boost output is automatically set by regulating voltage on the IFBx pin. The output of error amplifier and the sensed current of switch MOSFET are applied to a control comparator to generate the boost switching duty cycle; slope compensation is added to the current signal to allow stable operation for duty cycles larger than 50%.

The forward voltages of two LED strings are normally different due to the LED diode forward voltage inconsistency; thus, the IFB1 and IFB2 voltages are normally different. The TPS61162A, TPS61163A can select out the IFBx pin which has a lower voltage than the other and regulate its voltage to a very low value (90mV typical), which is enough for the two current sinks' headroom. In this way, the output voltage of the boost converter is automatically set and adaptive to LED strings' forward voltages, and the power dissipation of the current sink regulators can be reduced remarkably with this very low headroom voltage.

In order to improve the boost efficiency at light load, Pulse Frequency Modulation (PFM) mode is automatically enabled under light load conditions. When the load current decreases along with the dimming duty, the output of gm amplifier — COMP pin voltage decreases until it is clamped at an internal reference voltage. Because COMP pin voltage controls the inductor peak current, when it is clamped the inductor peak current is also clamped and cannot decrease. As a result, more energy than needed is transferred to the output stage, and the output voltage and IFBx pin voltage increase. An internal hysteresis comparator detects the minimum IFBx pin voltage. When the minimum IFBx voltage is detected as higher than the regulation voltage 90mV by around 120mV, the boost stops switching. Then the output voltage, as well as IFBx pin voltage, decrease. When the minimum IFBx voltage is lower than the hysteresis (around 40mV), the boost switches again. Thus, during PFM mode the boost output trips between the low and high thresholds. When the load increases along with the dimming duty, the COMP pin voltage will exit from the clamped status, and the boost will exit the PFM mode and return to the PWM operation, during which the minimum IFBx pin voltage is regulated at 90mV again. Refer to Figure 7-10 and Figure 7-11 for PFM mode operation.