L99LD21 datasheet(16/73 Pages) STMICROELECTRONICS

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Boost controller

L99LD21

16/73

DS11130 Rev 5

2.7

Soft start

The L99LD21 features an internal soft start function, which gradually increases the boost

mosfet current limit in 8 steps, in order to avoid a voltage overshoot of the boost output. The

threshold of the current limitation reaches its nominal value after a specified soft start time

(tSS).

A soft-start phase is initiated at the activation of the boost controller:

•

after leaving standby mode;

•

after deactivation of the boost controller due to a VS under voltage;

•

after a previous de-activation of the boost by SPI (see bit <1> on Table 16: CR#3:

Control Register 3);

•

after deactivation of the boost controller due to a BST_FB_FAIL.

2.8

Slope compensation

Slope compensation is needed to ensure loop stability with all possible values of duty cycle:

D = TON / T (0 < D < 1) especially when duty cycle is greater than 0.5. The slope of the

additional ramp is proportional to converter inductor current slope during the turn off phase.

The L99LD21 generates an internal peak current value, ISLOPE, which is added to the

sensing signal at the output of the OTA. The percentage of slope compensation is achieved

by choosing a proper value of the RSC resistor (see Figure 2 for RSC resistor proper

connection).

Figure 6. Slope compensation

2.9

Operation together with the buck converters

Right after a power on reset (POR) of the device or after a fault event leading to a latch-off of

the boost controller (VS under voltage), a soft start phase is initiated and the boost is

activated.

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