Figure 6-1 shows a simplified block diagram of the power stage of the device. Inductor current is sensed in series with Q1 (the peak current) and Q4 (the valley current).

Figure 6-1 Power Stage Simplified Block Diagram

The device uses a trapezoidal inductor current to regulate its output under all operating conditions. Thus, the device only has one operating mode and does not display any of the mode-change transients or unpredictable switching displayed by many other buck-boost devices.

There are four phases of operation:

• Phase A – Q1 and Q3 are on and Q2 and Q4 are off.
• Phase B – Q1 and Q4 are on and Q2 and Q3 are off.
• Phase C – Q2 and Q4 are on and Q1 and Q3 are off.
• Phase D – Q2 and Q3 are on and Q1 and Q4 are off.

Figure 6-2 shows the inductor current waveform when V_I > V_O, Figure 6-3 shows the current waveform when V_I = V_O, and Figure 6-4 shows the current waveform when V_I < V_O.

Figure 6-2 through Figure 6-4 show the typical waveforms during continuous conduction mode (CCM) switching for three operating conditions. During discontinuous conduction mode (DCM), the typical inductor current waveforms look similar to CCM with Phase D at 0-A inductor current. In deep boost mode, where V_I << V_O, Phase C length gradually decreases to zero until the switching waveform becomes triangular.

Figure 6-2 Inductor Current Waveform when V_I > V_O (CCM)

Figure 6-3 Inductor Current Waveform when V_I = V_O (CCM)

Figure 6-4 Inductor Current Waveform when V_I < V_O (CCM)

The ideal relationship between V_I and V_O (that is, assuming no losses) is:

Equation 1.

where

• V_I is the input voltage.
• V_O is the output voltage.
• t_w(A) is the duration of phase A.
• t_w(B) is the duration of phase B.
• t_w(C) is the duration of phase C.

By varying relative duration of each phase, the device can regulate V_O to be less than, equal to, or greater than V_I.