PM6675AS datasheet(41/48 Pages) STMICROELECTRONICS

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PM6675AS

Application information

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8.1.5

Diode selection

A rectifier across the synchronous switch is recommended. The rectifier works as a voltage

clamp across the synchronous rectifier and reduces the negative inductor swing during the

dead time between turning the high-side MOSFET off and the synchronous rectifier on.

Moreover it increases the efficiency of the system.

The reverse voltage should be greater than the maximum input voltage VINmax and a

minimum recovery reverse charge is preferable. Table 17 shows some evaluated diodes.

8.1.6

VOUT current limit setting

The valley current limit is set by RCSNS and must be chosen to support the maximum load

current. The valley of the inductor current ILvalley is:

Equation 45

The output current limit depends on the current ripple as shown in Figure 38:

Figure 38.

Valley current limit waveforms

Being fixed the valley threshold, the more the current ripple is greater, the more the DC

output current is greater. If an output current limit greater than over all the input voltage

range is required, the minimum current ripple must be considered in the previous formula.

Then the resistor RCSNS is:

Equation 46

Table 17.

Evaluated free-wheeling rectifiers

Manufacturer

Type

Forward voltage (V)

Rated reverse

voltage (V)

STPS1L30M

0.34

STPS1L30A

0.34

STPS1L60A

0.56

(max)

LOAD

Lvalley

∆

−

Inductor current

Valley current limit

MAX LOAD 1

MAX LOAD 2

Current

Time

Inductor current

100

Lvalley

DSon

CSNS

⋅

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