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參數(shù)資料
| 型號： | 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)： | 11/24頁 |
| 文件大?。?/td> | 265K |
| 代理商： | LTC3765IMSE#TRPBF |
LTC3765
19
3765f
APPLICATIONS INFORMATION
the voltage on the ISMAG pin is less than –1V, the active
clamp PMOS is turned off. Similarly for the alternative
configuration of Figure 7b, if the voltage on the ISMAG pin
is less than (VCC – 1V), the active clamp PMOS is turned
off. The ISMAG pin therefore directly monitors and limits
the magnetizing current to prevent core saturation in the
negative direction.
Choose the magnetizing current sense resistor value to
limit the transformer saturation current (ISAT):
RMAG =
1V
ISAT
where the saturation current is calculated from the maxi-
mum flux density (BMAX), area of the core in cm2 (AC),
number of turns on the primary (NP), and magnetizing
inductance (LMAG) from the following formula:
ISAT =
BMAX AC NP
108 LMAG
For a transformer designed for 2000 gauss operating flux
density, which is typical for a ferrite core, set BMAX to 2700
gausstokeepsufficientlyfarfromsaturationovertempera-
ture.ForthePulsePA08xxseriespowertransformersused
in the Typical Applications section, AC = 0.59cm2. For the
Pulse PA09xx series power transformers, AC = 0.81cm2.
WhentheNMOSison,themagnetizingcurrentandreflected
inductor current are both flowing through the NMOS. The
inductor current is generally much larger than the mag-
netizing current, which makes the magnetizing current
difficult to measure directly. Therefore, when the NMOS
is on the LTC3765 internally replicates the magnetizing
currentbasedontransformercoreparameters,thevoltage
on the ISMAG pin at the end of the previous reset cycle,
and the sensed input voltage on the RUN pin. The RUN
pin must be connected to a resistive divider from VIN
to ground for proper operation of the Direct Flux Limit.
At the end of the reset cycle, the voltage on the ISMAG
pin is sampled and held internally. This voltage is an ac-
curate measurement of the magnetizing current. When
the NMOS turns on, an internal ramp proportional to the
RUN pin voltage divided by the RCORE resistor increases
the internal replicated magnetizing current. If this internal
voltage exceeds 1V (or VCC + 1V for the alternative AG
configuration of Figure 7b), then the NMOS is turned off
to prevent core saturation.
When the NMOS is turned off due to a Direct Flux Limit, the
secondary-sideswitchnodefalls.TheLTC3766detectsthis
prematurely falling switch node and turns off the forward
gate to allow the transformer core to reset. This switch
node behavior is indistinguishable from a primary-side
shutdown; therefore, if the switch node falls prematurely
for 19 consecutive cycles, the LTC3766 concludes that a
primary-side shutdown has occurred and will fault.
Choose RCORE based on the RUN pin divider network and
the transformer core parameters:
RCORE =
R2
R1
+R2
BMAX AC NP
0.030
– 2k
where R1 and R2 comprise the divider network on the
RUN pin, with R1 from VIN to the RUN pin and R2 from
the RUN pin to ground. BMAX is typically 2700 gauss for
a transformer designed to operate at 2000 gauss, AC is
the area of the core in cm2, and NP is the number of turns
on the primary winding of the transformer.
The internal approximation of the magnetizing current is
linear, which is accurate if the transformer flux density is
kept sufficiently far from saturation. As the flux density
approaches saturation, the magnetizing inductance of
the transformer decreases and the magnetizing current
increasesrapidly.Dependingontheparticularcoreproper-
ties, it may be necessary to additionally decrease BMAX in
the equations above.
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