PM6675STR datasheet(31/53 Pages) STMICROELECTRONICS

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PM6675S

Device description

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7.1.5

Current sensing and current limit

The PM6675S switching controller uses a valley current sensing algorithm to properly

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

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

element is the on-resistance of the low-side switch. The sensing scheme is visible in

Figure 40

Figure 40.

Current sensing scheme

An internal 100

µA current source is connected to C

SNS 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 condition is detected or the output

UV protection latches off the device (see Section 7.1.11: Switching section OV and UV

protections on page 34

Referring to Figure 40, the RDS(on) sensing technique allows high efficiency performance

without the need for an external sensing resistor. The on-resistance of the MOSFET is

affected by temperature drift and nominal value spread of the parameter itself; this must be

considered during the RILIM setting resistor design.

Table 8.

Mode-of-operation settings summary

VSEL

NOSKIP

VOUT

Operating mode

VVSEL > 4.3 V

VNOSKIP > 4.2 V

1.5 V

Forced-PWM

1V <VNOSKIP < 3.5 V

Non-audible pulse-skip

< 0.5 V

Pulse-skip

VVSEL < 3.7 V

VNOSKIP > 4.2 V

ADJ

Forced-PWM

1V <VNOSKIP < 3.5 V

Non-audible pulse-skip

VNOSKIP < 0.5 V

Pulse-skip

HGATE

PHASE

CSNS

LGATE

PGND

PM6675

100µA·

ILIM

OUT

VALLEY·RDSon

HGATE

PHASE

CSNS

LGATE

PGND

PM6675

100µA·

ILIM

OUT

VALLEY·RDSon

PM6675S

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