L6460 datasheet(50/139 Pages) STMICROELECTRONICS

50 / 139 page

Main switching regulator

L6460

50/139

Doc ID 17713 Rev 1

Main switching regulator

Main switching regulator is an asynchronous switching regulator intended to be the source

of the main voltage in the system. It implements a soft start strategy and could be a system

regulator so even if its output voltage VSWmain is not used to power any internal circuit,

L6460 will check that it is in the good value range before enabling all its internal functions.

When L6460 detects a system regulator under-voltage event with a duration longer than the

period defined by the deglitch filter (tprim_uv), it will enter in reset state signaling this event to

the microcontroller by pulling low the nRESET pin and disabling most of its internal block

(e.g. bridges, GPIOs, …).

The output voltage will be externally set by a divider network connected to feedback pin. To

reduce as much as possible the regulation voltage error L6460 has the possibility to choose

between four feedback voltage references (and, as a consequence, four under-voltage

thresholds) using the serial interface. The feedback reference voltage selection is made by

writing the SelFBRef bits in the MainSwCfg register.

Here after are summarized the primary features of this regulator:

–

Internal power switch.

–

Soft start circuitry to limit inrush current flow from primary supply.

–

Internally generated PWM (250 kHz switching frequency).

–

Nonlinear pulse skipping control.

–

Protected against load short circuit.

–

Cycle by cycle current limiting using internal current sensor.

–

Under voltage signal (both continuous and latched) accessible through SPI.

When L6460 is in “low power mode”, this regulator will be disabled.

In order to save external components and power when using two or more L6460 IC’s on the

same board, the primary switching regulator can be disabled by serial interface. Care must

be paid using this function because an under-voltage on this regulator, as previously seen,

will be read as a fault condition by L6460.

12.1

Pulse skipping operation

Pulse skipping is a well known, non linear, control strategy used in switching regulators.

In this technique (see Figure 11) the feedback comparator output is sampled at the

beginning of each switching cycle. At this time, if the sampled value shows that output

voltage is lower than requested one, the complete PWM duty cycle is applied to power

switch; otherwise no PWM is applied and the switching cycle is skipped. Once PWM is

applied to power element only a current limit event can disable the power switch before the

whole duty cycle is finished.

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