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| 型号： | LTC3835EUFD#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A SWITCHING CONTROLLER, 580 kHz SWITCHING FREQ-MAX, PQCC20 |
| 封装： | 4 X 5 MM, LEAD FREE, PLASTIC, MO-220, QFN-20 |
| 文件页数： | 3/30页 |
| 文件大小： | 1775K |
| 代理商： | LTC3835EUFD#PBF |
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LTC3835
11
3835fd
OPERATION (Refer to Functional Diagram)
Light Load Current Operation (Burst Mode Operation,
Pulse-Skipping, or Continuous Conduction)
(PLLIN/MODE Pin)
The LTC3835 can be enabled to enter high efficiency Burst
Modeoperation,constant-frequencypulse-skippingmode,
orforcedcontinuousconductionmodeatlowloadcurrents.
To select Burst Mode operation, tie the PLLIN/MODE pin
to a DC voltage below 0.8V (e.g., SGND). To select forced
continuousoperation,tiethePLLIN/MODEpintoINTVCC.To
selectpulse-skippingmode,tiethePLLIN/MODEpintoaDC
voltage greater than 0.8V and less than INTVCC – 0.5V.
When the LTC3835 is enabled for Burst Mode operation,
the peak current in the inductor is set to approximately
one-tenth of the maximum sense voltage even though the
voltage on the ITH pin indicates a lower value. If the aver-
age inductor current is lower than the load current, the
error amplifier EA will decrease the voltage on the ITH pin.
When the ITH voltage drops below 0.4V, the internal sleep
signalgoeshigh(enabling“sleep”mode)andbothexternal
MOSFETs are turned off. The ITH pin is then disconnected
from the output of the EA and “parked” at 0.425V.
In sleep mode, much of the internal circuitry is turned off,
reducing the quiescent current that the LTC3835 draws to
only 80A. In sleep mode, the load current is supplied by
the output capacitor. As the output voltage decreases, the
EA’s output begins to rise. When the output voltage drops
enough, the ITH pin is reconnected to the output of the
EA, the sleep signal goes low, and the controller resumes
normal operation by turning on the top external MOSFET
on the next cycle of the internal oscillator.
When the LTC3835 is enabled for Burst Mode operation,
the inductor current is not allowed to reverse. The reverse
current comparator (RICMP) turns off the bottom external
MOSFET just before the inductor current reaches zero,
preventing it from reversing and going negative, thus
operating in discontinuous operation.
In forced continuous operation, the inductor current is
allowed to reverse at light loads or under large transient
conditions. The peak inductor current is determined by the
voltage on the ITH pin, just as in normal operation. In this
mode, the efficiency at light loads is lower than in Burst
Mode operation. However, continuous operation has the
advantages of lower output ripple and less interference
to audio circuitry. In forced continuous mode, the output
ripple is independent of load current.
When the PLLIN/MODE pin is connected for pulse-skipping
modeorclockedbyanexternalclocksourcetousethephase-
locked loop (see Frequency Selection and Phase-Locked
Loopsection),theLTC3835operatesinPWMpulse-skipping
modeatlightloads.Inthismode,constant-frequencyopera-
tion is maintained down to approximately 1% of designed
maximum output current. At very light loads, the current
comparator ICMP may remain tripped for several cycles and
forcetheexternaltopMOSFETtostayoffforthesamenumber
of cycles (i.e., skipping pulses). The inductor current is not
allowed to reverse (discontinuous operation). This mode,
like forced continuous operation, exhibits low output ripple
as well as low audio noise and reduced RF interference as
compared to Burst Mode operation. It provides higher low
current efficiency than forced continuous mode, but not
nearly as high as Burst Mode operation.
Frequency Selection and Phase-Locked Loop
(PLLLPF and PLLIN/MODE Pins)
The selection of switching frequency is a tradeoff between
efficiency and component size. Low frequency opera-
tion increases efficiency by reducing MOSFET switching
losses, but requires larger inductance and/or capacitance
to maintain low output ripple voltage.
The switching frequency of the LTC3835’s controllers can
be selected using the PLLLPF pin.
If the PLLIN/MODE pin is not being driven by an external
clocksource,thePLLLPFpincanbefloated,tiedtoINTVCC,
or tied to SGND to select 400kHz, 530kHz, or 250kHz,
respectively.
A phase-locked loop (PLL) is available on the LTC3835
to synchronize the internal oscillator to an external clock
source that is connected to the PLLIN/MODE pin. In this
case,aseriesR-CshouldbeconnectedbetweenthePLLLPF
pin and SGND to serve as the PLL’s loop filter. The LTC3835
phase detector adjusts the voltage on the PLLLPF pin to
align the turn-on of the external top MOSFET to the rising
edge of the synchronizing signal.
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