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参数资料
| 型号： | LTC3869EUFD#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 21/42页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 28-QFN |
| 标准包装： | 73 |
| 系列： | PolyPhase® |
| PWM 型： | 电流模式 |
| 输出数： | 2 |
| 频率 - 最大： | 850kHz |
| 占空比： | 95% |
| 电源电压： | 4 V ~ 38 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 28-WFQFN 裸露焊盘 |
| 包装： | 管件 |
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LTC3869/LTC3869-2
APPLICATIONS INFORMATION
INTV CC Regulators and EXTV CC
The LTC3869 features a true PMOS LDO that supplies
power to INTV CC from the V IN supply. INTV CC powers the
gate drivers and much of the LTC3869’s internal circuitry.
The linear regulator regulates the voltage at the INTV CC pin
to 5V when V IN is greater than 5.5V. EXTV CC connects to
INTV CC 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 4.7μF ceramic
capacitor or low ESR electrolytic capacitor. No matter
what type of bulk capacitor is used, an additional 0.1μF
ceramic capacitor placed directly adjacent to the INTV CC
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 INTV CC current, which is dominated by the
gate charge current, may be supplied by either the 5V linear
regulator or EXTV CC . When the voltage on the EXTV CC 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 V IN ? I INTVCC . 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 LTC3869 INTV CC
current is limited to less than 42mA from a 38V supply in
the UFD package and not using the EXTV CC supply:
T J = 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
operating in continuous conduction mode (MODE/PLLIN =
SGND) at maximum V IN . When the voltage applied to EXT-
V CC rises above 4.7V, the INTV CC linear regulator is turned
off and the EXTV CC is connected to the INTV CC . The EXTV CC
remains on as long as the voltage applied to EXTV CC remains
above 4.5V. Using the EXTV CC 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 INTV CC when the output is out of regulation
(e.g., start-up, short-circuit). If more current is required
through the EXTV CC than is specified, an external Schottky
diode can be added between the EXTV CC and INTV CC pins.
Do not apply more than 6V to the EXTV CC pin and make
sure that EXTV CC < V IN at all times.
Significant efficiency and thermal gains can be realized
by powering INTV CC from the output, since the V IN cur-
rent resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Efficiency).
Tying the EXTV CC pin to a 5V supply reduces the junction
temperature in the previous example from 125°C to:
T J = 70°C + (42mA)(5V)(34°C/W) = 77°C
However, for 3.3V and other low voltage outputs, additional
circuitry is required to derive INTV CC power from the output.
The following list summarizes the four possible connec-
tions for EXTV CC :
1. EXTV CC left open (or grounded). This will cause
INTV CC to be powered from the internal 5V regulator
resulting in an efficiency penalty of up to 10% at high
input voltages.
2. EXTV CC connected directly to V OUT . This is the
normal connection for a 5V regulator and provides
the highest efficiency.
3. EXTV CC connected to an external supply. If a 5V
external supply is available, it may be used to power
EXTV CC providing it is compatible with the MOSFET
gate drive requirements.
4. EXTV CC connected to an output-derived boost net-
work. For 3.3V and other low voltage regulators,
efficiency gains can still be realized by connecting
EXTV CC to an output-derived voltage that has been
boosted to greater than 4.7V.
38692fa
For more information www.linear.com/LTC3869
21
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