PM6675AS datasheet(27/48 Pages) STMICROELECTRONICS

27 / 48 page

PM6675AS

Device description

27/48

7.1.5

Current sensing and current limit

The PM6675AS switching controller employes a valley current sensing algorithm to properly

handle the current limit protection and the inductor current zero-crossing information. The

current is sensed during the conduction time of the low-side MOSFET. The current sensing

element is the low-side MOSFET on-resistance. The sensing scheme is visible in Figure 30.

Figure 30.

Current sensing scheme

An internal 100

µA current source is connected to CSNS pin that is also the non-inverting

input of the positive current limit comparator. When the voltage drop developed across the

sensing parameter equals the voltage drop across the programming resistor RILIM, the

controller skips subsequent cycles until the overcurrent is detected or the output UV

protection latches off the device (see par. Chapter 7.1.4 UV and OV Protections).

Referring to Figure 30, the RDSon sensing technique is tailored to all low cost, high

efficiency applications.

It must be taken into account that the current limit circuit actually regulates the inductor

valley current. This means that RILIM must be calculated to set a limit threshold given by the

maximum DC output current plus half of the inductor ripple current:

Equation 21

where RSENSE is the sensing device (RDSon).

The PM6675AS provides also a fixed negative current limit to prevent excessive reverse

inductor current when the switching section sinks current from the load in forced-PWM (3rd

quadrant working conditions). This negative current limit threshold is measured between

PHASE and PGND pins, comparing the drop magnitude on PHASE pin with an internal

110mV fixed voltage.

RILIM

HGATE

PHASE

CSNS

LGATE

PGND

SENSE

ILIM

100

⋅

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