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參數(shù)資料
| 型號： | LTC3833IUDC#PBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING CONTROLLER, 2000 kHz SWITCHING FREQ-MAX, PQCC20 |
| 封裝： | 3 X 3 MM, LEAD FREE, PLASTIC, QFN-20 |
| 文件頁數(shù)： | 11/36頁 |
| 文件大?。?/td> | 513K |
| 代理商： | LTC3833IUDC#PBF |
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LTC3833
19
3833f
APPLICATIONS INFORMATION
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 maxi-
mum junction temperature rating for the LTC3833 to be
exceeded, especially if the LDO is active and provides
INTVCC. Power dissipation for the IC in this case is high-
est and is approximately 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 equa-
tions given in Note 2 of the Electrical Characteristics. For
example, when using the LDO, LTC3833’s INTVCC current
is limited to less than 38mA from a 38V supply at TA =70°C
in the FE package:
TJ = 70°C + (38mA)(38V)(38°C/W) ≈ 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode at maximum
VIN.
When the voltage applied to EXTVCC pin rises above 4.6V,
the INTVCC LDO is turned off and the EXTVCC is connected
to INTVCC with an internal switch. This switch remains on
as long as the voltage applied to EXTVCC remains above
4.4V. Using the EXTVCC allows the MOSFET driver and
control power to be derived from the LTC3833’s switching
regulator output during normal operation and from the
LDO when the output is out of regulation (e.g., start-up,
short circuit). If more than 50mARMS current is required
through EXTVCC, then 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 this
external voltage source is less than VIN.
Significant efficiency and thermal gains can be realized
by powering INTVCC from the switching regulator output,
since the VIN current resulting from the driver and control
currentswillbescaledbyafactorof(DutyCycle)/(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 + (38mA)(5V)(38°C/W) ≈ 77°C
However, for 3.3V and other low voltage outputs, addi-
tional circuitry is required to derive EXTVCC power from
the regulator output.
The following list summarizes the four possible connec-
tions for EXTVCC:
1. EXTVCCleftopen(orgrounded).ThiswillcauseINTVCC
to be powered from the internal 5.3V LDO resulting
in an efficiency penalty of up to 10% at high input
voltages.
2. EXTVCCconnecteddirectlytoswitchingregulatoroutput
VOUT > 4.6V. This provides the highest efficiency.
3. EXTVCC connected to an external supply. If a 4.6V or
greater external supply is available, it may be used to
power EXTVCC providing that the external supply is
sufficient enough for MOSFET gate drive requirements.
4. EXTVCC connected to an output-derived boost network.
For 3.3V and other low voltage converters, efficiency
gains can still be realized by connecting EXTVCC to an
output-derived voltage that has been boosted to greater
than 4.6V.
For applications where the main input power is less than
5.3V, tie the VIN and INTVCC pins together and tie the com-
bined pins to the VIN input with an optional 1Ω or 2.2Ω
resistor as shown in Figure 5 to minimize the voltage drop
caused by the gate charge current. This will override the
INTVCCLDOandwillpreventINTVCCfromdroppingtoolow
due to the dropout voltage. Make sure the INTVCC voltage
exceeds the RDS(ON) test voltage for the external MOSFET
which is typically at 4.5V for logic-level devices.
INTVCC
LTC3833
VIN
CVCC
RVIN
3833 F05
VIN
CIN
Figure 5. Setup for VIN ≤ 5V
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