參數(shù)資料
型號(hào)： LTC3890EUH#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類： 穩(wěn)壓器
英文描述： DUAL SWITCHING CONTROLLER, 585 kHz SWITCHING FREQ-MAX, PQCC32
封裝： 5 X 5 MM, 0.75 MM HEIGHT, LEAD FREE, PLASTIC, MO-220WHHD, QFN-32
文件頁數(shù)： 14/38頁
文件大?。?/td> 381K
代理商： LTC3890EUH#PBF
LTC3890
21
3890fa
APPLICATIONS INFORMATION
INTVCC Regulators
The LTC3890 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 LTC3890’s
internal circuitry. The VINLDOandtheEXTVCCLDOregulate
INTVCC to 5.1V. Each of these can supply a peak current of
50mA and must be bypassed to ground with a minimum
of 4.7μF ceramic capacitor. No matter what type of bulk
capacitor is used, an additional 1μF 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 LTC3890 to be
exceeded. The INTVCC current, which is dominated by
the gate charge current, may be supplied by either the
VIN LDO or the EXTVCC LDO. When the voltage on the
EXTVCC pin is less than 4.7V, the VIN 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 Consid-
erations section. The junction temperature can be estimated
by using the equations given in Note 3 of the Electrical
Characteristics. For example, the LTC3890 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) = 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.
Using the EXTVCC LDO allows the MOSFET driver and
control power to be derived from one of the LTC3890’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 speci-
fied, 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 cur-
rent resulting from the driver and control currents will be
scaled 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, additional
circuitry is required to derive INTVCCpowerfromtheoutput.
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