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參數(shù)資料
| 型號(hào)： | LTC3835IUFD |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | 3 A SWITCHING CONTROLLER, 580 kHz SWITCHING FREQ-MAX, PQCC20 |
| 封裝： | 4 X 5 MM, PLASTIC, MO-220, QFN-20 |
| 文件頁數(shù)： | 9/30頁 |
| 文件大?。?/td> | 1775K |
| 代理商： | LTC3835IUFD |
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LTC3835
17
3835fd
APPLICATIONS INFORMATION
INTVCC Regulators
The LTC3835 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, respectively, depending on the connection of the
EXTVCC pin. INTVCC powers the gate drivers and much of
the LTC3835’s internal circuitry. The VIN LDO regulates
the voltage at the INTVCC pin to 5.25V and the EXTVCC
LDO regulates it to 7.5V. Each of these can supply a peak
current of 50mA and must be bypassed to ground with
a minimum of 4.7F tantalum, 10F special polymer, or
low ESR electrolytic capacitor. A ceramic capacitor with a
minimum value of 4.7F can also be used if a 1Ω resistor
isaddedinserieswiththecapacitor.Nomatterwhattypeof
bulk capacitor is used, an additional 1F ceramic capacitor
placed directly adjacent to the INTVCC and PGND IC pins is
highlyrecommended.Goodbypassingisneededtosupply
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 maximum
junctiontemperatureratingfortheLTC3835tobeexceeded.
The INTVCC current, which is dominated by the gate charge
current, may be supplied by either the 5V VIN LDO or the
7.5V 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.
Thegatechargecurrentisdependentonoperatingfrequency
as discussed in the Efficiency Considerations section.
The junction temperature can be estimated by using the
equations given in Note 2 of the Electrical Characteristics.
For example, the LTC3835 INTVCC current is limited to less
than 41mA from a 24V supply when in the G package and
not using the EXTVCC supply:
TJ = 70°C + (41mA)(36V)(95°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 (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 7.5V, so while EXTVCC
is less than 7.5V, the LDO is in dropout and the INTVCC
voltage is approximately equal to EXTVCC. When EXTVCC
is greater than 7.5V up to an absolute maximum of 10V,
INTVCC is regulated to 7.5V.
Using the EXTVCC LDO allows the MOSFET driver and
control power to be derived from the LTC3835 switching
regulator output (4.7V ≤ VOUT ≤ 10V) during normal
operation and from the VIN LDO when the output is out
of regulation (e.g., startup, short-circuit). If more cur-rent
is required through the EXTVCC LDO than is specified, an
external Schottky diode can be added between the EXTVCC
andINTVCCpins.Donotapplymorethan10VtotheEXTVCC
pin and make sure than 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
scaledbyafactorof(DutyCycle)/(SwitcherEfficiency).For
4.7V to 10V regulator outputs, this means connecting the
EXTVCC pin directly to VOUT. Tying the EXTVCC pin to a 5V
supply reduces the junction temperature in the previous
example from 125°C to:
TJ = 70°C + (24mA)(5V)(95°C/W) = 81°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 5.25V 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 an external
supply is available in the 5V to 7V range, it may be used
to power EXTVCC providing it is compatible with the
MOSFET gate drive requirements.
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