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參數(shù)資料
| 型號： | LTC3856EUH#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, PQCC38 |
| 封裝： | 5 X 5 MM, LEAD FREE, PLASTIC, MO-220WHHD-X, QFN-32 |
| 文件頁數(shù)： | 21/40頁 |
| 文件大?。?/td> | 2222K |
| 代理商： | LTC3856EUH#PBF |
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LTC3856
3856f
applicaTions inForMaTion
short circuit). If more current is required 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 the output, since the VIN cur-
rent resulting from the driver and control currents will be
scaled by a factor of (duty cycle)/(switcher efficiency).
Tying the EXTVCC pin to a 5V supply reduces the junction
temperature in the previous example from 125°C to:
TJ = 70°C + (42mA)(5V)(34°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. EXTVCCleftopen(orgrounded).ThiswillcauseINTVCC
to be powered from the internal 5V 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.
For applications where the main input power is 5V, tie 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 13) 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.
Topside MOSFET Driver Supply (CB, DB)
External bootstrap capacitors, CB, connected to the
BOOST pins supply the gate drive voltages for the top-
side MOSFETs. Capacitor CB in the Functional Diagram
is charged though external diode DB from INTVCC when
the SW pin is low. When one of the topside MOSFETs is
to be turned on, the driver places the CB voltage across
the gate source of the desired MOSFET. This enhances
the MOSFET and turns on the topside switch. The switch
node voltage, SW, rises to VIN and the BOOST pin follows.
With the topside MOSFET on, the boost voltage is above
the input supply:
VBOOST = VIN + VINTVCC
The value of the boost capacitor, CB, needs to be 100 times
thatofthetotalinputcapacitanceofthetopsideMOSFET(s).
The reverse breakdown of the external Schottky diode
must be greater than VIN(MAX). When adjusting the gate
drive level, the final arbiter is the total input current for
the regulator. If a change is made and the input current
decreases, then the efficiency has improved. If there is
no change in input current, then there is no change in
efficiency.
Figure 13. Set-Up for a 5V Input
INTVCC
LTC3856
RVIN
1
CIN
3856 F13
CINTVCC
4.7F
5V
+
VIN
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