L99LD21 datasheet(14/73 Pages) STMICROELECTRONICS

14 / 73 page

Boost controller

L99LD21

14/73

DS11130 Rev 5

This mode is activated when the voltage on FB pin is higher than the selected internal

reference voltage of a specified threshold value (VFB_OV_ON).

The switching activity is resumed as soon as the voltage on FB pin decreases to the

selected internal reference voltage (VFB_REF[xx]).

In case of FB voltage increases above VFB_OV_ON, an output digital flag, called BST_OVP,

is set.

As soon as feedback voltage decreases down to target value (VFB_REF[xx]), the bit is reset

after tBST_OVP_RST delay time. This delay time is implemented in order to eliminate the

diagnostic ambiguity (toggling of the OVP flag) during permanent no load / light load

operation.

BST_OVP bit is not set in case of boost disabled or boost feedback failure.

2.5

Feedback failure protection

L99LD21 is protected in case of boost controller feedback pin failure. More in detail, a

specific bit, called BST_FB_FAIL, is set in case of feedback pin is shorted to ground.

When this bit is set:

•

If device is OFF, boost controller does not start;

•

If device is ON in single phase configuration, boost controller is immediately switched

OFF;

•

If device is ON in dual phase configuration and it is in Active mode: both boost

controllers are switched off when the failure is recognized on Master side; only Slave

controller is switched off when the failure is recognized on Slave side while the Master

is managed by the microcontroller;

•

If device is ON in dual phase configuration and it is in Limp Home: both boost

controllers are switched off when the failure is recognized on Master side; only Slave

controller is switched off when the failure is recognized on Slave side, while the Master

is forced to work at minimum duty cycle.

The reset of FB failure bit is demanded to the microcontroller (in Active mode) or to an auto-

restart function (in Limp Home) that cyclically clears this bit with a period equal to

tAUTORESTART.

This bit is not set if L99LD21 internal boost controller is not used (in this case, BST_DIS bit

is set).

If left floating, feedback pin will be pulled up internally. In this case, BST_OVP bit will be set

permanently and boost gate driver will be permanently off. Since the feedback pin voltage is

in any case high, N_PWR_GOOD flag is reset in such condition and shall be ignored.

Note:

Setting this bit doesn't imply any action on buck converters.

2.6

Operation in dual phase interleaved mode

It is possible to combine the boost controllers of two L99LD21, for high power applications,

in dual phase configuration. In this configuration, the switching mosfets of the boost

controllers are driven at 180° out of phase. By sharing the current between two phases, the

conduction losses (which are proportional the square of the conducted current) are reduced

and the efficiency of the boost stage increases, in comparison to a single-phase.

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