L99LD21 datasheet(13/73 Pages) STMICROELECTRONICS

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DS11130 Rev 5

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L99LD21

Boost controller

2.1

General description

The L99LD21 integrates one boost controller, which is based on a fixed frequency, peak

current mode architecture. It drives the gate of an external n-channel mosfet in order to step

up the VS input voltage to a higher stabilized output voltage.

2.2

Frequency selection

The boost controller operates at a fixed frequency which can range from 100 kHz to 450

kHz. The switching frequency is set by a SPI control register (CR#3<9:7>, see Section 5.4:

Registers description).

2.3

Output voltage setting

The control loop regulates the voltage at the FB pin to a reference voltage, which value,

according table 2, is configurable by the control register CR#3<11:10> (see Section 5.4:

Registers description). Connect the resistor divider tap, top and bottom respectively to the

FB pin, to output of the boost controller and to the bottom to SGND.

The resulting boost output voltage is given by:

2.4

Overvoltage protection

The peak current mode requires a minimum on-time, because of the noise generated right

after the turn-on of the switching mosfet. At light load (very low output current), this

minimum on-time, in combination with the selected switching frequency is no longer able to

regulate the output voltage to the requested voltage. The device enters in overvoltage

protection (OVP), in order to prevent an excessive rise of the boost output voltage above the

target voltage.

Table 2. Reference voltage configuration

VFB_REF [V]

0.596

0.895

1.242

1.496

BOOST

FB_REF b1 b0

[] 1

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