L99LD02 datasheet(15/66 Pages) STMICROELECTRONICS

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DS12448 Rev 3

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L99LD02

Boost controller

3.6

Soft start

The L99LD02 features an internal soft start function, which gradually increases the boost

current in 9 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 deactivation of the boost by SPI (see CR#3 Bit[2:1], BST_DIS in

Table 24: CR#3: Control Register 3);



after deactivation of the boost controllers due to a BST_OUT_FAIL;



after deactivation of the boost controllers due to OVT.

3.7

Stability – Error Amplifier characteristics

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 L99LD02 generates an internal peak current value, ISLOPE, which is added to the

positive sensing signal at the output of the OTA (see the following figure).

Figure 4. Resistor ladder for slope compensation and input current limitation

To avoid the use of external slope compensation resistor, thus limiting the coupling noise on

the sense positive path, the compensation ramp slope of each boost controller can be

configured by 6 bits (see Bit [23÷18], BST_SLP_CTRL, in Table 23: CR#2: Control Register

2) in the range [from 0.45 to 13.05] kΩ with a granularity step of 0.2 kΩ.

To keep the same compensation loop in multi-phase operation (same gain / phase-margin /

band-width), without any hardware change, the OTA gain value can be configurable in 4

steps according to Table 3.

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