參數(shù)資料
型號(hào)： LTC3728LZEUH
廠(chǎng)商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： 3 A DUAL SWITCHING CONTROLLER, 590 kHz SWITCHING FREQ-MAX, PQCC28
封裝： 5 X 5 MM, PLASTIC, MO-220WHHD, QFN-28
文件頁(yè)數(shù)： 2/32頁(yè)
文件大?。?/td> 467K
代理商： LTC3728LZEUH
LTC3728LZ
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top MOSFET continuously. The dropout detector detects
this and forces the top MOSFET off for about 400ns every
tenth cycle to allow CB to recharge.
The main control loop is shut down by pulling the RUN/SS
pin low. Releasing RUN/SS allows an internal 1.2μA cur-
rent source to charge soft-start capacitor CSS. When CSS
reaches 1.5V, the main control loop is enabled with the ITH
voltage clamped at approximately 30% of its maximum
value. As CSS continues to charge, the ITH pin voltage is
gradually released allowing normal, full-current operation.
When both RUN/SS1 and RUN/SS2 are low, all controller
functions are shut down, including the 5V regulator.
Low Current Operation
The FCB pin is a multifunction pin providing two func-
tions: 1) to provide regulation for a secondary winding
by temporarily forcing continuous PWM operation on
both controllers; and 2) to select between two modes of
low current operation. When the FCB pin voltage is below
0.8V, the controller forces continuous PWM current mode
operation. In this mode, the top and bottom MOSFETs are
alternately turned on to maintain the output voltage inde-
pendent of direction of inductor current. When the FCB pin
is below VINTVCC – 1V but greater than 0.8V, the controller
enters Burst Mode operation. Burst Mode operation sets
a minimum output current level before inhibiting the top
switch and turns off the synchronous MOSFET(s) when
the inductor current goes negative. This combination of
requirements will, at low currents, force the ITH pin below
a voltage threshold that will temporarily inhibit turn-on of
both output MOSFETs until the output voltage drops. There
is 60mV of hysteresis in the burst comparator B tied to
the ITH pin. This hysteresis produces output signals to the
MOSFETs that turn them on for several cycles, followed by
a variable “sleep” interval depending upon the load current.
The resultant output voltage ripple is held to a very small
value by having the hysteretic comparator after the error
amplier gain block.
Frequency Synchronization
The phase-locked loop allows the internal oscillator to
be synchronized to an external source via the PLLIN pin.
The output of the phase detector at the PLLFLTR pin is
also the DC frequency control input of the oscillator that
operates over a 260kHz to 550kHz range corresponding
to a DC voltage input from 0V to 2.4V. When locked, the
PLL aligns the turn on of the top MOSFET to the rising
edge of the synchronizing signal. When PLLIN is left
open, the PLLFLTR pin goes low, forcing the oscillator to
minimum frequency.
Constant Frequency Operation
When the FCB pin is tied to INTVCC, Burst Mode opera-
tion is disabled and the forced minimum output current
requirement is removed. This provides constant frequency,
discontinuous current (preventing reverse inductor cur-
rent) operation over the widest possible output current
range. This constant frequency operation is not as efcient
as Burst Mode operation, but does provide a lower noise
operation.
Continuous Current (PWM) Operation
Tying the FCB pin to ground will force continuous current
operation. This is the least efcient operating mode, but
may be desirable in certain applications. The output can
source or sink current in this mode. When sinking current
while in forced continuous operation, current will be forced
back into the main power supply.
INTVCC/EXTVCC Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTVCCpin.When
the EXTVCC pin is left open, an internal 5V low dropout
linear regulator supplies INTVCC power. If EXTVCC is taken
above 4.7V, the 5V regulator is turned off and an internal
switch is turned on connecting EXTVCC to INTVCC. This
allows the INTVCCpowertobederivedfromahighefciency
external source such as the output of the regulator itself
or a secondary winding, as described in the Applications
Information section.
Output Overvoltage Protection
An overvoltage comparator, OV, guards against transient
overshoots (>7.5%) as well as other more serious condi-
tions that may overvoltage the output. In this case, the top
MOSFET is turned off and the bottom MOSFET is turned
on until the overvoltage condition is cleared.
Power Good (PGOOD) Pin
The PGOOD pin is connected to an open drain of an internal
MOSFET. The MOSFET turns on and pulls the pin low when
OPERATION (Refer to Functional Diagram)
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