The LM51770 features four internal logic-level nMOS gate drivers. The drivers maintain the high frequency switching of both half bridges needed for a buck-boost operation. If the device is in boost or buck mode, the other half bridge high-side switch needs to be permanent on. The internal gate drivers support this by sharing the current from the other half bridge, which is switching. Therefore, a minimum of quiescent current can be assured as no additional char pump is needed. Due to the high drive current, it can support a wide range of external power FETs as well as a parallel operation of them.

The LO and HO outputs are protected with a shoot-through protection, which ensures that both outputs are not turned on at the same time. If the PWM modulation logic of the buck-boost turns the LOx pin off, the HOx pin is not turned on until the following are all true (AND not OR) true:

1. A minimum internal transition time (t_t(dead)) is reached.
2. The voltage on the LOx pin drops below the detection threshold V_TH(GATEOUT).

The high-side supply voltage for the gate driver are monitored by an additional bootstrap UVLO comparator. This comparator monitors the differential voltage between SWx and HBx. If the voltage drops below the threshold the buck-boost converter operation turns off. The device restarts automatically once the positive going threshold is reached with the soft-start scheme.

Additionally, the LM51770 monitors the upper voltage between SWx and HBx. If this voltage exceeds the threshold voltage of the clamping circuit, it activates a internal current source to pull the voltage down.

Figure 8-21 Functional Block Diagram Gate Driver