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参数资料
| 型号： | LTC3891EUDC#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 900 kHz SWITCHING FREQ-MAX, PQCC20 |
| 封装： | 3 X 4 MM, LEAD FREE, PLASTIC, QFN-20 |
| 文件页数： | 13/32页 |
| 文件大小： | 1931K |
| 代理商： | LTC3891EUDC#PBF |
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LTC3891
20
3891f
INTVCC Regulators
The LTC3891 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power
at the INTVCC pin from either the VIN supply pin or the
EXTVCC pin depending on the connection of the EXTVCC
pin. INTVCC powers the gate drivers and much of the
LTC3891’s internal circuitry. The VIN LDO and the EXTVCC
LDO regulate INTVCC to 5.1V. Each of these can supply a
peak current of at least 50mA and must be bypassed to
ground with a minimum of 2.2μF ceramic capacitor. No
matter what type of bulk capacitor is used, an additional
1μFceramiccapacitorplaceddirectlyadjacenttotheINTVCC
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.
HighinputvoltageapplicationsinwhichlargeMOSFETsare
being driven at high frequencies may cause the maximum
junctiontemperatureratingfortheLTC3891tobeexceeded.
TheINTVCCcurrent,whichisdominatedbythegatecharge
current, may be supplied by either the VIN LDO or the
EXTVCC LDO. When the voltage on the EXTVCC pin is less
than4.7V,theVINLDOisenabled.Powerdissipationforthe
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 Electrical Characteristics.
For example, the LTC3891 INTVCC current is limited to less
than 32mA from a 40V supply when not using the EXTVCC
supply at a 70°C ambient temperature:
TJ = 70°C + (32mA)(40V)(43°C/W for QFN) = 125°C
To prevent the maximum junction temperature from be-
ing exceeded, the input supply current must be checked
while operating in forced continuous mode (PLLIN/MODE
= INTVCC) at maximum VIN.
When the voltage applied to EXTVCC rises above 4.7V, the
VIN LDO is turned off and the EXTVCC LDO is enabled. The
EXTVCC LDO remains on as long as the voltage applied to
EXTVCC remains above 4.5V. The EXTVCC LDO attempts
to regulate the INTVCC voltage to 5.1V, so while EXTVCC
is less than 5.1V, the LDO is in dropout and the INTVCC
voltage is approximately equal to EXTVCC. When EXTVCC
is greater than 5.1V, up to an absolute maximum of 14V,
INTVCC is regulated to 5.1V.
UsingtheEXTVCCLDOallowstheMOSFETdriverandcontrol
power to be derived from one of the LTC3891’s switching
regulator outputs (4.7V ≤ VOUT ≤ 14V) during normal
operation and from the VIN LDO when the output is out of
regulation (e.g., start-up, short-circuit). If more current
is required through the EXTVCC LDO than is specified, an
external Schottky diode can be added between the EXTVCC
and INTVCC pins. In this case, 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 current
resultingfromthedriverandcontrolcurrentswillbescaled
by a factor of (Duty Cycle)/(Switcher Efficiency).
For 5V to 14V regulator outputs, this means connecting
the EXTVCC pin directly to VOUT. Tying the EXTVCC pin to
an 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
TJ = 70°C + (32mA)(8.5V)(43°C/W) = 82°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. EXTVCCGrounded.ThiswillcauseINTVCCtobepowered
fromtheinternal5.1Vregulatorresultinginanefficiency
penalty of up to 10% at high input voltages.
2. EXTVCC Connected Directly to VOUT. This is the normal
connection for a 5V to 14V regulator and provides the
highest efficiency.
3. EXTVCC Connected to an External Supply. If an external
supply is available in the 5V to 14V range, it may be
used to power EXTVCC providing it is compatible with
the MOSFET gate drive requirements. Ensure that
EXTVCC < VIN.
APPLICATIONS INFORMATION
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