The TPS61299 differs from the TPS61022 pre-charge method. The TPS61299 applies a switching charge method called down mode to charge the output when output voltage drops below the input. Current control requires creating a voltage higher than input so that volt-second balance can be implemented.

TPS61299 serves as a typical example of PMOS down mode. Like pre-charge devices such as TPS61022, TPS61299 switches the source to the SW pin to block the previously mentioned inrush current. Figure 2-11 depicts the down mode behavior of TPS61299.

Similar to the boost mode switching process, a down mode switching cycle divides into a low-side phase and a high-side phase. During the low-side phase, the low-side switch turns on and inductor current rises with slope VIN/L. Then during the high-side phase, the low-side switch turns off while the high-side switch connects the gate to the input. As shown in Figure 2-12, inductor current charges the gate capacitor and raises the voltage on the SW pin.

As V_gs rises, the high-side enters the saturation region and charges the output capacitor with Id. Current Ig continues charging Cg and raising Vsw until Id matches IL. At this point, Equation 17 gives the voltage on the SW pin.

With this SW voltage higher than Vin, volt-second balance can be implemented and the device can be controlled the same way as boost mode. Figure 2-13 shows the TPS61299 start-up waveform.

Down mode devices overcome the drawback of pre-charge in low Vin start-up capability. For pre-charge devices, the V_gs of the high-side switch receives supply from and cannot exceed the input voltage. If the input voltage lacks sufficient level for the switch to form a channel, the device cannot start-up. However, with Vgs charged by inductor current instead of input current, down mode devices can boot up with much lower input voltage. This makes down mode devices better for low-Vin products.

Also, for high-current devices that use NMOS as the high-side switch, pre-charge cannot be applied. For an NMOS upper switch, the gate voltage must exceed the input voltage to get the interface inverted. Without switching, the bootstrap capacitor cannot be charged and the gate voltage becomes impossible to provide.

However, compared with pre-charge devices, down mode devices have worse efficiency. Power loss for down mode is given by Equation 18 and power loss for pre-charge is given by Equation 19. Obviously, down mode devices suffer from more loss than pre-charge devices due to higher voltage across the high-side MOSFET.