L9396 datasheet(19/109 Pages) STMICROELECTRONICS

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DS12539 Rev 6

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L9396

Power supply

108

3.2

Boost regulator

The boost regulator can be enabled or disabled via SPI depending on the needs of the

application with respect to the operating battery level. It features an integrated power stage

and operates at 2 MHz to allow the use of external low cost 2.2 µH inductor. The current

capability should be enough to grant full I/O pin supply and minimal µC operation.

When not used, BSTSW pin can be connected to ground and VBST directly to the protected

battery line. The device enables or keeps disabled the boost converter at start-up depending

on the external circuitry: if BSTSW pin is shorted to ground, the boost is disabled at power

up and kept disabled; in case the BSTSW experiences a high voltage at power up, given by

battery connection through the inductor, the boost is enabled. This condition is reported via

SPI with bit BOOST_KEPT_OFF of SUPPLY_CONTROL_2 register (it means that boost

has been kept off and will not operate).

Boost converter diagnostics include under voltage, reported via SPI and FAULT pin (if the

regulator is enabled). The integrated FET featuring the boost switch is protected against

short to battery by means of a thermal shutdown circuit. When thermal fault is detected the

FET is switched off and latched in this state until the related fault flag is read. In case of loss

of ground the FET is switched off and automatically reactivated as soon as ground

connection is restored. Over-voltage protection from load-dump and inductive flyback is

provided via an active clamp and a disable circuitry. A dedicated circuitry is implemented to

keep the boost off at start-up till the voltage difference between VB and VBST pins is lower

than BST_DISABLETH in order to reduce in-rush current and diagnose VBST pin loss

condition or diode loss. An SPI bit is present to report output of this comparator (bit

BOOST_READY of SUPPLY_CONTROL_2 register goes high when VBST>=VB-

BST_DISABLETH).

State of boost regulator is reported via SPI bit BOOST_ON_FLAG in register

SUPPLY_CONTROL_2. In case boost is disabled due to diagnostic or battery voltage

above output regulation voltage this bit is cleared to 0.

Figure 3. Boost regulator block diagram

Comp

BST

control

CLAMP

enable

BSTSW

VBST

GNDBST

GADG1801171332PS

BST_DISABLE

driver &

CLAMP_EN

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