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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)： | 8/60頁 |
| 文件大?。?/td> | 607K |
| 代理商： | LTC3766MPUFD#PBF |
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LTC3766
16
3766f
OPERATION
then adding the amplitude of this internal ramp to the ITH
clamp voltage on a cycle-by cycle basis. During the on
time, the slope of the inductor current is given by:
dIL
dt
=
VSW – VS+
L
The LTC3766 establishes a voltage on the IPK pin of (VSW
– VS+)/15, which is one-fifteenth of the voltage across the
output inductor during the on-time when SW is high. By
choosing a resistor RIPK that is proportional to the value of
the output inductor (RIPK = KL), the current flowing in RIPK
becomes proportional to the slope of the inductor current:
IRIPK =
VSW – VS+
15RIPK
=
VSW – VS+
15KL
During the time when SW is high, the LTC3766 uses the
RIPK current to create an internal ramp by charging an
on-chip capacitor CRIP. The slope of this internal ramp
voltage is given by:
dVRAMP
dt
=
IRIPK
CRIP
=
VSW – VS+
15KLCRIP
The amplitude of this internal ramp is then added to the
ITH clamp level dynamically. By choosing the appropri-
ate value of RIPK, therefore, the average current during
current limit will be essentially independent of changes
in ripple current.
As is the case with all DC/DC converters that maintain
constant frequency operation, a cycle by cycle current
limit is only effective at duty cycles where the on time is
greater than the minimum controllable on-time. Under
short-circuit conditions, for example, the LTC3766 limits
thecurrentusingaseparateovercurrentcomparator.When
thisovercurrentcomparatoristripped,theLTC3766gener-
ates a fault followed by a soft-start retry. This hiccup mode
overcurrent protection is highly effective at minimizing
power losses under short-circuit conditions.
Direct Flux Limit
In active clamp forward converters, it is essential to es-
tablish an accurate limit to the transformer flux density
in order to avoid core saturation during load transients or
when starting up into a pre-biased output. Although the
active clamp technique provides a suitable reset voltage
during steady-state operation, the sudden increase in duty
cycle caused in response to a load step can cause the
transformer flux to accumulate or “walk,” potentially lead-
ing to saturation. This occurs because the reset voltage on
the active clamp capacitor cannot keep up with the rapidly
changing duty cycle. This effect is most pronounced at low
input voltage, where the voltage loop demands a greater
increase in duty cycle due to the lower voltage available
to ramp up the current in the output inductor.
Traditionally,transformercoresaturationhasbeenavoided
either by limiting the maximum duty cycle of the converter
or by slowing down the loop to limit the rate at which the
dutycyclechanges.Limitingthemaximumdutycycledoes
help the converter avoid saturation for a load step at low
input voltage, since the duty cycle maximum is clamped;
however, transformer saturation can also easily occur at
higher input voltage where the maximum duty cycle clamp
is ineffective. Limiting the rate of duty cycle change such
that the active clamp capacitor can sufficiently track the
duty cycle change also helps to prevent saturation in many
situations, but results in a very poor transient response.
Neither of these traditional techniques is guaranteed to
prevent the transformer from saturating in all situations.
For example, saturation can easily occur using these
traditional techniques when starting up into a pre-biased
output, where the duty cycle can quickly change from 0%
to75%.Moreover,neitherofthesetraditionaltechniquesis
able to prevent saturation in the negative direction, which
can result from sudden decreases in duty cycle.
TheLTC3765andLTC3766implementanewuniquesystem
for monitoring and directly limiting the flux accumula-
tion in the transformer core. During a reset cycle, when
the active clamp PMOS is on, the magnetizing current is
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