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AN2738

The below-ground voltage on the OUT pin

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The below-ground voltage on the OUT pin

A typical phenomenon found in a great number of power applications is the below-ground

voltage experienced by the OUT pin, which sometimes can be very high. Contrary to

common opinion, the real problem of the below-ground voltage is not in the maximum

absolute value of the OUT pin, but rather the voltage on BOOT pin and the over-charging of

the bootstrap capacitance. In the paragraph that follows the root causes of the below-ground

voltages will be explained in detail, and the description of the real

issues which could result

from the phenomenon will be provided.

11.1

The below-ground voltage phenomenon

In power applications using half-bridge topologies and typically driving Ioads with a

significant inductive component, the output of the power half-bridge experiences,

systematically, a below-ground voltage transition, which can be distinct in a dynamic

contribution consisting of a greater undershoot spike and in a static contribution, which is a

below-ground static voltage with lower absolute value (Figure 35b). This phenomenon

occurs when the bridge carries out a so-called hard switching transition towards a low

voltage level and the load current is outgoing (from the bridge to the load). When the high

side switch turns off, the output current tends to remain quite constant due to the inductive

component of the load, and then has to flow through the low side freewheeling diode, which

turns on going from a high voltage reverse condition to a forward condition. It is evident that

until the output bridge voltage has reached the “zero” value, the diode is turned off, so the

output transition is dominated by the high side turn-off commutation. After the output voltage

reaches the zero-voltage level, the diode can turn on and it begins to bring the entire load

current in a very brief time, so the high dIF/dt causes the well-known forward peak voltage,

which is the main contribution to the undershoot spikes. Other contributions to dynamic

below-ground voltage are the spikes due to the high dI/dt experienced by the parasitic

inductances in series with the free-wheeling diode located along the turn-off current path of

the half-bridge (Figure 35).

Figure 35.

below-ground voltages in L6390

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