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参数资料
| 型号： | LTC3869IGN-2#TRPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, PDSO28 |
| 封装： | 0.150 INCH, LEAD FREE, PLASTIC, SSOP-28 |
| 文件页数： | 13/40页 |
| 文件大小： | 2705K |
| 代理商： | LTC3869IGN-2#TRPBF |
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LTC3869/LTC3869-2
20
3869f
APPLICATIONS INFORMATION
INTVCC Regulators and EXTVCC
The LTC3869 features a true PMOS LDO that supplies
power to INTVCC from the VIN supply. INTVCC powers the
gate drivers and much of the LTC3869’s internal circuitry.
The linear regulator regulates the voltage at the INTVCCpin
to 5V when VIN is greater than 5.5V. EXTVCC connects to
INTVCC through a P-channel MOSFET and can supply the
needed power when its voltage is higher than 4.7V. Each
of these can supply a peak current of 100mA and must
be bypassed 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 supply the high transient currents required
by the MOSFET gate drivers and to prevent interaction
between the channels.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3869 to be
exceeded. The INTVCC current, which is dominated by the
gatechargecurrent,maybesuppliedbyeitherthe5Vlinear
regulator or EXTVCC. When the voltage on the EXTVCC pin
is less than 4.7V, the linear regulator 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 3 of the
ElectricalCharacteristics.Forexample,theLTC3869INTVCC
current is limited to less than 42mA from a 38V supply in
the UFD package and not using the EXTVCC supply:
TJ = 70°C + (42mA)(38V)(34°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operatingincontinuousconductionmode(MODE/PLLIN=
SGND) at maximum VIN. When the voltage applied to EXT-
VCC rises above 4.7V, the INTVCC linear regulator is turned
offandtheEXTVCCisconnectedtotheINTVCC.TheEXTVCC
remainsonaslongasthevoltageappliedtoEXTVCCremains
above 4.5V. Using the EXTVCC allows the MOSFET driver
and control power to be derived from one of the LTC3869’s
switching regulator outputs during normal operation and
from the INTVCC when the output is out of regulation
(e.g., start-up, 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 at all times.
Significant efficiency and thermal gains can be realized by
powering INTVCC from the output, since the VIN current
resultingfromthedriverandcontrolcurrentswillbescaled
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 3.3V and other low voltage outputs, addi-
tional 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 5V regulator
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. EXTVCC connected to an external supply. If a 5V
external 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 net-
work. 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.
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