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參數(shù)資料
| 型號(hào)： | LTC3867IUF#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 0.1 A SWITCHING CONTROLLER, 1200 kHz SWITCHING FREQ-MAX, PQCC24 |
| 封裝： | 4 X 4 MM, LEAD FREE, PLASTIC, MO-220WGGD, QFN-24 |
| 文件頁數(shù)： | 19/36頁 |
| 文件大?。?/td> | 380K |
| 代理商： | LTC3867IUF#PBF |
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LTC3867
26
3867f
INTVCC (LDO) and EXTVCC
The LTC3867 features a true PMOS LDO that supplies
power to INTVCC from the VIN supply. INTVCC powers the
gate drivers and much of the LTC3867’s internal circuitry.
The LDO regulates the voltage at the INTVCC pin to 5.3V
when VIN is greater than 5.8V. EXTVCC connects to INTVCC
through a P-channel MOSFET and can supply the needed
power when its voltage is higher than 4.7V. Either of these
cansupplyapeakcurrentof100mAandmustbebypassed
to ground with a minimum of 4.7F ceramic capacitor or
low ESR electrolytic capacitor. No matter what type of bulk
capacitor is used, an additional 0.1F ceramic capacitor
placed directly adjacent to the INTVCC and PGND pins is
highly recommended. Good bypassing is needed to sup-
ply the high transient currents required by the MOSFET
gate drivers. High input voltage applications in which
large MOSFETs are being driven at high frequencies may
cause the maximum junction temperature rating for the
LTC3867 to be exceeded. The INTVCC current, which is
dominated by the gate charge current, may be supplied by
either the 5.3V LDO or EXTVCC. When the voltage on the
EXTVCC pin is less than 4.5V, the LDO is enabled. Power
dissipation for the IC in this case is highest and is equal
to VIN IINTVCC. The gate charge current is dependent
on operating frequency as discussed in the Efficiency
Considerations section. The junction temperature can be
estimated by using the equations given in Note 2 of the
ElectricalCharacteristicstables.Forexample,theLTC3867
INTVCC current is limited to less than 30mA from a 38V
supply in the UF package and not using the EXTVCC supply
with a 70°C ambient temperature:
TJ = 70°C + (30mA)(38V)(47°C/W) 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operatingincontinuousconductionmode(MODE=SGND)
atmaximumVIN.WhenthevoltageappliedtoEXTVCCrises
above 4.7V, the INTVCC LDO is turned off and the EXTVCC
is connected to the INTVCC. The EXTVCC remains on as
long as the voltage applied to EXTVCC remains above 4.5V.
Using the EXTVCC allows the MOSFET driver and control
power to be derived from an efficient switching regulator
outputduringnormaloperation.Ifmorecurrentisrequired
through the EXTVCC than is specified, an external Schottky
diode can be added between the EXTVCC and INTVCC pins.
Do not apply more than 6V to the EXTVCC pin and make
sure that EXTVCC < VIN.
Significant efficiency and thermal gains can be realized
by powering INTVCC from EXTVCC, since the VIN current
resultingfromthedriverandcontrolcurrentswillbescaled
by a factor of (duty cycle)/(switcher efficiency). Tying the
EXTVCCpintoa5Vsupplyreducesthejunctiontemperature
in the previous example from 125°C to:
TJ = 70°C + (30mA)(5V)(47°C/W) = 77°C
However, for low voltage outputs, additional circuitry is
required to derive INTVCC power from the output.
The following list summarizes the four possible connec-
tions for EXTVCC:
1. EXTVCC left open (or grounded). This will cause
INTVCC to be powered from the internal LDO resulting
in an efficiency penalty of up to 10% at high input
voltages.
2. EXTVCC connected directly to VOUT. This is the normal
connection for a 5V regulator and provides the highest
efficiency.
3. EXTVCCconnectedtoanexternalsupply.Ifa5Vexternal
supply is available, it may be used to power EXTVCC
providing it is compatible with the MOSFET gate drive
requirements.
4. EXTVCC connected to an output-derived boost network.
For 3.3V and other low voltage regulators, efficiency
gains can still be realized by connecting EXTVCC to an
output-derived voltage that has been boosted to greater
than 4.7V.
the VIN and INTVCC pins together and tie the combined
pins to the 5V input with a 1Ω or 2.2Ω resistor as shown
in Figure 12 to minimize the voltage drop caused by the
gate charge current. This will override the INTVCC linear
regulator and will prevent INTVCC from dropping too low
due to the dropout voltage. Make sure the INTVCC voltage
is at or exceeds the RDS(ON) test voltage for the MOSFET
which is typically 4.5V for logic-level devices
APPLICATIONS INFORMATION
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