LT3750 查看數據表（PDF） - Linear Technology
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LT3750 Datasheet PDF : 16 Pages
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LT3750
APPLICATIO S I FOR ATIO
Table 4. Recommended NMOS Transisitors
MANUFACTURER
PART NUMBER
Philips Semiconductor
(www.semiconductors.philips.com)
PHM21NQ15T
PHK12NQ10T
PHT6NQ10Y
PSMN038-100K
International Rectifier
(www.irf.com)
IRF7488
IRF7493
IRF6644
ID (A)
22.2
11.6
6.5
6.3
6.3
9.3
10.3
VDS(MAX) (V)
150
100
100
100
80
80
100
VGS(MAX) (V)
20
20
20
20
20
20
20
RDS(ON) (mΩ)
55
28
90
38
29
15
10.7
PACKAGE
HVSON8
SO-8
SOT223
SO-8
SO-8
SO-8
DirectFET
The transistor’s continuous drain current rating must
exceed IAVG,M.
Table 4 lists recommended NMOS transistors.
Setting Current Limit
A sense resistor from the SOURCE pin to GND implements
current limit. The current limit is nominally 78mV/RSENSE.
The average power dissipation rating of the current sense
resistor must exceed:
PRESISTOR
≥
IPK2
•
RSENSE
3
⎛
⎝⎜
VOUT(PK)
VOUT(PK) + N • VTRANS
⎞
⎠⎟
Additionally, there is approximately a 100ns propagation
delay from the time that peak current limit is detected to
when the gate transitions to the low state. This delay
increases the peak current limit by (VTRANS)(tDELAY)/LPRI.
Setting The Target Output Voltage
The parameters that determine the target output voltage
are the resistors RVOUT and RBG, the turns ratio of the
transformer (N), and the voltage drop across the output
diode (VDIODE). The target output voltage is set according
to the following equation:
VOUT
=
⎛
⎝⎜
1.24V
• RVOUT
RBG
• N⎞⎠⎟
– VDIODE
Use at least 1% tolerance resistors for RVOUT and RBG.
Choosing large value resistors for RBG decreases the
amount of current that charges the parasitic internal
capacitances and degrades the response time of the VOUT
comparator. This may result in overcharging of the output
capacitor. The maximum recommended value for RBG is
2.5k for typical applications.
When high primary currents are used, a voltage spike
can prematurely trip the output voltage comparator. A
33pF to 100pF capacitor in parallel with RBG is sufficient to
filter this spike for most applications. Always check that
the voltage waveform on RBG does not overshoot and that
it reaches a plateau at maximum VOUT.
Discontinuous Mode Detection
The RDCM resistor stands off voltage transients on the
drain node. A 43k, 5% resistor is recommended for 300V
applications. Higher output voltages will require a larger
resistor.
In order for the LT3750 to properly detect discontinuous
mode and start a new charge cycle, the reflected voltage to
the primary winding must exceed the discontinuous mode
comparator threshold which is nominally 36mV. The
worst-case condition occurs when VOUT is shorted to
ground. When this occurs, the reflected voltage is simply
the diode forward voltage drop divided by N.
3750fa
10
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