Application guidelines

PM8841

10/13

DocID027119 Rev 1

6.3

Driving switches

The IN pin truth table is reported in Table 6.

Differential MOSFET's driving strength is seldom necessary in topologies such as flybacks

or boost controlled in the peak current mode. A lower driving current is used to turn on the

MOSFET in order to reduce the EMI produced by the Miller capacitance activation, while

a stronger turn-off action is suggested to minimize the turn-off delay and, consequently the

deviation between theoretical and practical behaviors.

The same asymmetrical driving strength is required when the IGBT switch is used: in fact

the driving strength control is mandatory to avoid latch-up phenomena intrinsically related

with this kind of the switch. The asymmetrical driving can be realized using a diode and

resistance as illustrated in typical application diagrams (refer to the PM8851 device when

accurate control of the asymmetrical driving current is required).

When low switching frequencies are required and propagation delays can be compensated,

it is possible to drive contemporary the IN pin and the IN_TH pin to exploit the relevant

UVLO threshold of the device (typ. 1.5 V) using the PM8841 as a fixed threshold device

without any external component: care has to be taken to consider an additional propagation

delay (typ. 300 ns) after the falling edge of the input signal.

6.4

Power dissipation

Overall power dissipation can be evaluated considering two main contributions: the device

related consumption (PD) and the gate driving power demand (PG):

Equation 1

The device power consumption can be found in Figure 6 on page 6: it represents the power

required by the device to supply internal structures and pull-downs resistors.

The gate driving power dissipation is the power required to deliver to and from the

MOSFET's gate the required gate charge:

Equation 2

can considered equal to VCC.

Table 6. PM8841 truth table

IN

PM8841

High

High

Low

Low