PM6675 datasheet(40/47 Pages) STMICROELECTRONICS

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PM6675

40/47

7.1.6

VDDQ 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 46

The output current limit depends on the current ripple as shown in Figure 20:

Figure 20. Valley current limit waveforms

As the valley threshold is fixed, the greater the current ripple, the greater the DC output

current will be. If an output current limit greater than ILOAD(max) 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 47

where RDSon is the drain-source on-resistance of the low-side switch. Consider the

temperature effect and the worst case value in RDSon calculation (typically +0.4%/°C).

The accuracy of the valley current also depends on the offset of the internal comparator

(±5mV).

The negative valley-current limit (if the device works in forced-PWM mode) is given by:

Equation 48

(max)

LOAD

Lvalley

∆

−

Inductor current

Valley current limit

MAX LOAD 1

MAX LOAD 2

Inductor current

Current

Time

100

Lvalley

DSon

CSNS

⋅

DSon

NEG

120

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