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參數(shù)資料
| 型號(hào)： | LTC3765IMSE#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 電源管理 |
| 英文描述： | 1-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO16 |
| 封裝： | LEAD FREE, PLASTIC, MSOP-16 |
| 文件頁數(shù)： | 6/24頁 |
| 文件大?。?/td> | 265K |
| 代理商： | LTC3765IMSE#TRPBF |
LTC3765
14
3765f
APPLICATIONS INFORMATION
In this equation, VTH is the maximum threshold voltage of
the external MOSFET. Table 1 below shows typical values
of RNDRV for common input voltage ranges.
Table 1. Typical RNDRV Values
VIN RANGE
VTH(MAX)
RNDRV RANGE TYPICAL RNDRV
8V to 36V
2V
70k to 180k
125k
36V to 72V
4V
60k to 1.4M
150k
Setting the Overcurrent Limit
TheovercurrentlimitfortheLTC3765isprincipallyasafety
feature to protect the converter. The current that flows in
series through the transformer primary winding and the
primary switch is sensed by a resistor (RSENSE)connected
between the source of the switch and ground. The voltage
across this resistor is sensed by the IS+ and IS– pins. If
the difference between IS+ and IS– exceeds 150mV, the
LTC3765 immediately turns off the primary NMOS and, if
SSFLTisnotgrounded,faults.Theovercurrentcomparator
is blanked for approximately 200ns after PG goes high to
avoid false trips due to noise.
Choose the overcurrent trip current ITRIP to be less than
the maximum pulsed drain current rating of the primary
NMOSbutgreaterthanthesumofthepeakinductorcurrent
at full load and the current required to charge the output
capacitor at start-up, reflected through the transformer.
The sense resistor value, RSENSE, can be calculated from
the 150mV trip threshold and the primary-side trip current
from the following equation:
RSENSE =
150mV
ITRIP
The RSENSE resistor should be verified to have sufficient
margin over the maximum operating current of the con-
verter, which typically occurs at start-up into a full load.
Inaself-startingapplicationatfullload,thelinearlyincreas-
ing duty cycle determined by the soft-start capacitor (CSS)
ramps the output voltage with a fixed rate independent
of the output capacitor value. Larger output capacitance
requires a proportionally larger charging current to main-
tain the output voltage ramp rate. Since additional output
capacitance is generally distributed through a system and
may not be exactly known, the sense resistor and soft-
start capacitor should be chosen with sufficient margin
to ensure that the overcurrent comparator does not trip
on start-up. An upper bound for the available current to
charge the output capacitor can be calculated as shown
in the following equation:
ICHG <
150mV
RSENSE NS/NP
– 1.4ILOAD(MAX)
where NS/NP is the turns ratio of the transformer and the
factor of 1.4 accounts for the typical 40% ripple current
in the inductor. To ensure that the overcurrent comparator
does not trip on start-up, the soft-start capacitor should
be chosen so that only a fraction of the charging cur-
rent calculated above is available to charge the output
capacitor. Using 10% of the maximum charging current
generally allows for sufficient margin. This establishes a
lower bound on the soft-start capacitor value, which can
be computed from:
CSS >60010
–9 V
IN(MAX) NS/ NP
COUT
0.1ICHG
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