參數(shù)資料
型號(hào)： LLT3837EFE
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, 110 kHz SWITCHING FREQ-MAX, PDSO16
封裝： 4.40 MM, PLASTIC, TSSOP-16
文件頁(yè)數(shù)： 10/28頁(yè)
文件大?。?/td> 255K
代理商： LLT3837EFE
LT3837
18
3837fa
APPLICATIONS INFORMATION
with slope compensation and system stability. Keep the
sync pulse width greater than 500ns.
Selecting Timing Resistors
There are three internal “one-shot” times that are pro-
grammed by external application resistors: minimum
on-time, enable delay time and primary MOSFET turn-on
delay. These are all part of the isolated yback control
technique, and their functions are previously outlined in
the Theory of Operation section.
The following information should help in selecting and/or
optimizing these timing values.
Minimum On-Time (tON(MIN))
Minimum on-time is the programmable period during
which current limit is blanked (ignored) after the turn
on of the primary side switch. This improves regulator
performance by eliminating false tripping on the leading
edge spike in the switch, especially at light loads. This
spike is due to both the gate/source charging current and
the discharge of drain capacitance. The isolated yback
sensing requires a pulse to sense the output. Minimum
on-time ensures that there is always a signal to close the
feedback loop. The LT3837 does not employ cycle skipping
at light loads. Therefore, minimum on-time along with
synchronous rectication sets the switch over in forced
continuous mode operation.
The tON(MIN) resistor is set with the following equation:
Rk
tns
tON MIN
ON MIN
()
() –
.
Ω=
104
1 063
Keep RtON(MIN) greater than 70k. A good starting value
is 160k.
Enable Delay Time (ENDLY)
Enable delay time provides a programmable delay between
turn-off of the primary gate drive node and the subsequent
enabling of the feedback amplier. As discussed earlier, this
delay allows the feedback amplier to ignore the leakage
inductance voltage spike on the primary side.
The worst-case leakage spike pulse width is at maximum
load conditions. So set the enable delay time at these
conditions.
While the typical applications for this part use forced
continuous operation, it is conceivable that a secondary-
side controller might cause discontinuous operation at
light loads. Under such conditions the amount of energy
stored in the transformer is small. The yback waveform
becomes “l(fā)azy” and some time elapses before it indicates
the actual secondary output voltage. The enable delay time
should be made long enough to ignore the “irrelevant”
portion of the yback waveform at light load.
Even though the LT3837 has a robust gate drive, the gate
transition-time slows with very large MOSFETs. Increase
delay time is as required when using such MOSFETs.
The enable delay resistor is set with the following
equation:
Rk
tns
ENDLY
()
() –
.
Ω=
30
2 616
Keep RENDLY greater than 40k. A good starting point
is 56k.
COSCAP (pF)
30
50
f OSC
(kHz)
100
200
300
100
200
3837 F02
Figure 3. fOSC vs OSC Capacitor Values
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