參數(shù)資料
型號(hào)： L5988D
廠商： STMICROELECTRONICS
元件分類： 穩(wěn)壓器
英文描述： 5.6 A SWITCHING REGULATOR, 1000 kHz SWITCHING FREQ-MAX, PDSO16
封裝： LEAD FREE, HTSSOP-16
文件頁(yè)數(shù)： 14/52頁(yè)
文件大小： 1858K
代理商： L5988D
L5988D
Functional description
Doc ID 15324 Rev 3
21/52
5.5.2
Current limiting
The current limiting feature acts in different ways depending on the operative conditions.
●
In case an overcurrent detection happens after the soft-start phase, the internal logic
will trigger the “HICCUP” mode. Both switches are turned off and the soft-start
capacitor is discharged with a constant current of 22 μA (typ.). When the SS/INH
voltage drops below the INH threshold a new SS cycle will start.
●
During the soft-start phase the overcurrent information is used to provide a constant
current protection. In this way additional current is available to charge the output
capacitor during power up.
The most common way is to sense the current flowing through the power MOSFETs.
However, due to the noise created by the switching activity of the power MOSFETs, the
current sense is disabled during the initial phase of the conduction time. This avoids an
erroneous detection of a fault condition. This interval is generally known as “masking time”
or “blanking time”. For this reason, the current cannot be sensed through the high-side
MOSFET in the case of extremely low duty cycles, nor through the low-side MOSFET in the
case of very high duty cycles.
The L5988D assures the effective protection sensing the current flowing in both embedded
switches. The protection achieved by sensing the current in the high-side MOSFET is called
“peak overcurrent protection”, while the protection achieved by sensing the current in the
low-side MOSFET is called “valley overcurrent protection”. When the current limit is reached
during normal operation, the so called HICCUP mode is triggered, and the soft-start cap is
discharged and recharged. However, during the start-up phase, additional current is
required to charge the output capacitor. This could continuously trigger the HICCUP
intervention preventing the system from reaching a steady working condition. For this
reason the HICCUP feature is disabled during the start-up phase and a constant current
mode is active to charge the output capacitor. In this case, when the peak current limit is
triggered after a conduction time equal to the “masking time”, the high-side MOSFET is
turned off and the low side MOSFET is kept on until the flowing current goes below the
“valley” current limit. If necessary, some switching pulses are skipped, as illustrated in
Figure 11. Thus, the combination of the “peak” and “valley” current limits assure the
effectiveness of the overcurrent protection even in extreme duty cycle conditions. The
current threshold of the low side is designed higher than the high side one to guarantee the
proper protection.
The constant current mode during the soft-start phase limits the maximum current up to:
The overcurrent limit protection is adjustable (higher or lower than the nominal value)
through an external resistor. To guarantee effective protection, both thresholds (valley and
peak) are in tracking.
The typical active thresholds in case of ILIM-ADJ pin left floating are IPEAK_TH = 4.0 A,
IVALLEY_TH = 4.7 A. The dimensioning of the pull up resistor versus VREF to decrease the
peak (and valley) thresholds follows the formula:
In the same way the pull down resistor is selected using the following formula to increase the
maximum current thresholds:
I
MAX
I
VALLEY_TH
V
IN
V
OUT
–
L
------------------------------ T
MASK
+
=
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