參數(shù)資料
型號(hào)： LTC3766EUFD#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(yè)數(shù)： 11/60頁(yè)
文件大小： 607K
代理商： LTC3766EUFD#PBF
LTC3766
19
3766f
APPLICATIONS INFORMATION
Secondary-Side Bias and Start-Up
In most applications, the LTC3766 will receive its bias
voltage from a supply that is generated on the secondary
side. The manner in which the secondary bias is generated
depends upon the output voltage as well as the variation in
theinputvoltageoftheDC/DCconverter.Inallapplications,
however, the secondary bias must always come up before
the output reaches the regulation level. This is essential
to avoid an overvoltage condition on the output, since the
initial start-up is performed from the primary side in an
open-loop fashion. See Generating the Secondary-Side
Bias for more information.
Note that the LTC3766 will not begin a soft-start sequence
and initiate switching until the RUN pin is high, adequate
voltage is present on both the VIN and VCC pins, and
switching is detected on the SW pin. The LTC3766 looks
for switching on the SW pin to ensure that the LTC3765 is
active and ready for control hand-off. For switching to be
detected, the SW node waveform must have at least eight
consecutive pulses in the range of 50kHz to 700kHz. The
SW node waveform must also have a peak that is greater
than 1.4V and a valley that is less than 0.5V. In standalone
mode,theLTC3766beginsthesoft-startsequencewithout
waiting for a switching waveform to be detected on the
SW pin.
Linear Regulator Operation
TheLTC3766containstwolinearregulatorsthatareusedto
regulate the available bias voltage down to a level suitable
for driving MOSFETs. If the bias supply voltage is greater
than 15V, then the high voltage linear regulator controller
may be used. This makes use of an external N-type pass
device. Place a capacitor of 0.22μF or greater on VIN and
1μF or greater on VCC. If the bias supply connected to the
VIN pin has a relatively high output impedance, it may be
necessary to use a larger capacitor on VIN to prevent the
VIN pin voltage from dropping when the VCC capacitor is
being charged. The VCC charge rate during linear regulator
start-up is set by the LTC3766 to approximately 0.5V/μs,
which will create at charging current of (0.5 106) CVCC.
Care should be taken to ensure that this charging current
does not exceed the SOA of the N-type pass device, par-
ticularly when operating at higher VIN voltages. The VCC
regulation level can be set to either 7V or 8.5V as desired
using the MODE pin. See the section on VCC and Drive
Mode Selection for details.
The LTC3766 also contains a 15V internal bypass LDO. If
the voltage on the VAUX pin exceeds the VAUX switchover
threshold,thenthehighvoltagelinearregulatorisdisabled,
and an internal PMOS-pass LDO uses the VAUX voltage to
supply power to VCC. This allows the high voltage linear
regulator to be used for initial start-up and the higher ef-
ficiency bypass LDO to be used during normal operation.
Figure 3 illustrates such a configuration that uses both
linear regulators.
If the voltage on the VAUX pin is below the switchover
threshold, then the VAUX pin is internally loaded with a
resistance of approximately 920Ω. This internal load is
removed after the VAUX regulator is enabled, and is used
to ensure that the VAUX supply is reasonably stiff before
the bypass regulator is activated.
In some cases, it is desirable to use the high voltage linear
regulator only briefly during start-up, so as to limit the
temperatureriseintheexternalpassdevice.Toaccomplish
this, place a capacitor on the REGSD pin to ground (see
Figure 3) such that:
CRSD =
tHVREG 13A
(
)
1.21V
where tHVREG is the time that the high voltage regulator
will operate. When the high voltage regulator is operat-
ing, a 13μA current is sourced from the REGSD pin, and
when it is shut down (e.g., the bypass regulator is ac-
tive), a 3μA current is sinked into the REGSD pin. If the
REGSD voltage exceeds 1.21V, the high voltage regulator
is disabled. Choose a time tHVREG that is greater than the
normal start-up time. After start-up, if the voltage on the
VAUX pin drops, the high-voltage linear regulator will be
re-energized, but only for a limited time.
VIN
NDRV
LTC3766
OPTIONAL
VCC
CVAUX
CVCC
3766 F03
CVIN
CRSD
VAUX
REGSD
LV BIAS
SUPPLY
5V TO 15V
HV BIAS
SUPPLY
6V TO 32V
Figure 3. Typical Linear Regulator Connections
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