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参数资料
| 型号： | LTC3858EUH-2#TRPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | DUAL SWITCHING CONTROLLER, 900 kHz SWITCHING FREQ-MAX, PQCC32 |
| 封装： | 5 X 5 MM, LEAD FREE, PLASTIC, MO-220WHHD, QFN-32 |
| 文件页数： | 16/40页 |
| 文件大小： | 432K |
| 代理商： | LTC3858EUH-2#TRPBF |
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LTC3858-2
23
38582f
APPLICATIONS INFORMATION
INTVCC Regulators
The LTC3858-2 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 EXT-
VCC pin depending on the connection of the EXTVCC pin.
INTVCC powers the gate drivers and much of the internal
circuitry. The VIN LDO and the EXTVCC LDO regulate IN-
TVCC 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 low ESR capacitor. Regardless of 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 LTC3858-2 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 dissipa-
tion 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 Electrical Char-
acteristics. For example, the LTC3858-2 INTVCC current
is limited to less than 32mA from a 40V supply when not
using the EXTVCC supply at 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 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 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.
The following list summarizes the four possible connec-
tions for EXTVCC:
1. EXTVCC Grounded.ThiswillcauseINTVCCtobepowered
from the internal 5.1V 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 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. Ensure that EXTVCC < VIN.
4. EXTVCCConnectedtoanOutput-DerivedBoostNetwork.
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. This can be done with the capacitive charge
pump shown in Figure 9. Ensure that EXTVCC < VIN.
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