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參數(shù)資料
| 型號： | LTC3805HMSE#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 700 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封裝： | LEAD FREE, PLASTIC, MSOP-10 |
| 文件頁數(shù)： | 8/20頁 |
| 文件大?。?/td> | 260K |
| 代理商： | LTC3805HMSE#TRPBF |
LTC3805
3805fe
Figure 7. Circuit to Increase the Overcurrent
Threshold for Small Switch Currents
LTC3805
OC
GATE
ISENSE
RSENSE1
RSENSE2
RSLOPE
GND
3805 F07
IOC = 10A
ISLOPE
be slowed by using a larger value of SSFLT capacitor. It is
also possible to trip overcurrent protection during a load
step especially if the trip threshold is lowered by making
ROC > ROC(CRIT).
Another overcurrent protection strategy is keep the con-
verter running as current limiting reduces the duty cycle
and the output voltage sags. In this case, the goal is often
keeptheconverterinnormaloperationoveraswidearange
as possible, including current limiting, and to trigger the
overcurrent trip only to prevent damage. To implement
this strategy use a value of ROC smaller than ROC(CRIT).
Thisalsoreducessensitivitytoovercurrenttripscausedby
transient operation. In the limit, set ROC = 0 and connect
the OC pin directly to RSENSE. This causes an overcurrent
trip near minimum duty cycle or around 6%.
In some cases it may be desirable to increase the trip
threshold even further. In this strategy, the converter is
allowed to operate all the way down to minimum duty
cycle at which point the cycle-by-cycle current limit of
the ISENSE pin is lost and switch current goes up propor-
tionally to the output current. Figures 7 and 8 show two
ways to do this. Figure 7 is for relatively low currents with
relatively large values of RSENSE. Using this circuit the
overcurrent trip threshold is increased from 100mV to:
VOC =
RSENSE1 + RSENSE2
RSENSE1
100mV
where it is assumed that the values of RSENSE1 and
RSENSE2 are so small that the IOC = 10A threshold adjust-
ment current produces a negligible change in VOC.
For larger currents, values of the current sense resistors
must be very small and the circuit of Figure 7 becomes
impractical. The circuit of Figure 8 can be substituted
and the current sense threshold is increased from
100mV to:
VOC =
R1
+ R2
R1
100mV
where the values of R1 and R2 should be kept below 10Ω
to prevent the IOC = 10A threshold adjustment current
from producing a shift in VOC.
External Soft-Start Fault Timeout
The external soft-start is programmed by a capacitor CSS
from the SSFLT pin to GND. At the initiation of soft-start
the voltage on the SSFLT pin is quickly charged to 0.7V
at which point GATE begins switching. From that point,
a 6A current charges the voltage on the SSFLT pin until
the voltage reaches about 2.25V at which point soft-start
is over and the converter enters closed-loop regulation.
The soft-start time tSS(EXT) as a function of the soft-start
capacitor, CSS, is therefore:
tSS(EXT) = CSS
2.25
0.7V
6A
After soft-start is complete, the voltage on the SSFLT
pin continues to charge to about a final value of 4.75V.
Note that choosing a value of CSS less than 5.8nF has
no effect since it would attempt to program an external
soft-start time tSS(EXT) less than the mandatory minimum
Figure 8. Circuit to Increase the Overcurrent
Threshold for Large Switch Currents
LTC3805
OC
GATE
ISENSE
R1
R2
RSLOPE
GND
3805 F08
RSENSE
IOC = 10A
ISLOPE
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