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參數(shù)資料
| 型號： | LTC3766MPUFD#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 316 kHz SWITCHING FREQ-MAX, PQCC28 |
| 封裝： | 4 X 5 MM, LEAD FREE, PLASTIC, MO-220, QFN-28 |
| 文件頁數(shù)： | 41/60頁 |
| 文件大?。?/td> | 607K |
| 代理商： | LTC3766MPUFD#PBF |
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LTC3766
46
3766f
APPLICATIONS INFORMATION
For the circuit of Figure 32, the overcurrent VDS on the
SG MOSFET is given by:
VOC = VREV
R1
+R2
R2
–IREV R1+R3
R1
+R2
R2
In addition to producing the desired VOC threshold, there
are three constraints on the selection of resistors R1, R2
and R3 that must be simultaneously met: 1) R1 and R2
must divide the maximum VSW plateau voltage down to
40V or less, 2) the impedance at the SW pin must be kept
as low as possible to reduce the delay in sensing the VSW
voltage, and 3) the power dissipation in R1 and R2 must
be kept reasonably low. The last two constraints can be
met by choosing a maximum power (PR) to be dissipated
in the sum of R1 and R2. Typically, setting PR = 0.25W is a
reasonable compromise that keeps the time constant low
while not greatly impacting converter efficiency.
The selection of R1, R2 and R3 is made using the follow-
ing procedure:
1. Calculate R1 and R2 based on a maximum power (PR
= 0.25W) and a divider ratio that will produce exactly
40V maximum on the SW pin:
R1
=
NS
NP
VOUTVIN(MAX)
PR
–
40VOUT
PR
R2
=
40 R1
NS
NP
VIN – 40
2. IfthevalueforVOCcalculatedusingR1andR2fromstep
1) is greater than the target VOC value, then choose R3
such that IREV R3(R1+ R2)/R2 equals the difference
between the calculated and target VOC values.
3. If the value for VOC calculated using R1 and R2 from
step 1) is less than the target value, then R3 = 0. Re-
calculate R1 and R2 based on maximum power (PR =
0.25W) and the desired target VOC value:
R1
=
BIREV – AVOC + AVOC +BIREV
(
)2 –4ABVREVIREV
2AIREV
R2
=
B – AR1
A
where A = PR(NP/NS) and B = VOUTVIN(MAX).
ForthecircuitofFigure32,the%errorintheSGovercurrent
trip can be estimated using:
VOC =
100
VOC
IREV R1+K R3
(
)
6
2
+ K
VREV
14
2
where K = (R1 + R2)/R2.
RC Snubbers
Most applications will make use of an RC snubber to
reduce the overshoot and ringing on the SW and SWB
pins, as shown in Figure 33. The snubber capacitor is
chosen to limit the peak voltage overshoot on SW or SWB
by absorbing the energy in the leakage inductance of the
main transformer. The snubber resistor is then chosen to
provide optimum damping so as to minimize ringing. A
larger snubber capacitor reduces the overshoot, but at the
expense of increased power dissipation in the snubber
resistor. In general, the snubber on the SWB node has far
less energy to absorb and can therefore be smaller than
that on the SW node. In some cases, the snubber on SWB
can be eliminated entirely.
TheprecisevaluesneededfortheRCsnubberswilldepend
upon the specifics of each application, and should be
optimized in the lab. Typical values for CS1 and CS2 range
from 1nF to 4.7nF, and RS1 and RS2 are typically 1Ω to
50Ω. Always use a high quality ceramic (X7R) capacitor
and resistors with a high power rating (1/4W to 1/2W) for
and an RC snubber.
RS2
RS1
3766 F33
VSWB
VSW
CS2
CS1
VOUT
NS
NP
MAIN
XFMR
Figure 33. Using RC Snubbers
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