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参数资料
| 型号： | LTC3867EUF#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 26/36页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 24-QFN |
| 产品培训模块： | LTC3867 Synchronous Step-Down DC/DC Controller |
| 标准包装： | 91 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1.2MHz |
| 占空比： | 98% |
| 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： | 24-WFQFN 裸露焊盘 |
| 包装： | 管件 |
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LTC3867
APPLICATIONS INFORMATION
INTV CC (LDO) and EXTV CC
The LTC3867 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 LTC3867’s internal circuitry.
The LDO regulates the voltage at the INTV CC pin to 5.3V
when V IN is greater than 5.8V. 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. Either 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 sup-
ply the high transient currents required by the MOSFET
gate drivers. High input voltage applications in which
large MOSFETs are being driven at high frequencies may
cause the maximum junction temperature rating for the
LTC3867 to be exceeded. The INTV CC current, which is
dominated by the gate charge current, may be supplied by
either the 5.3V LDO or EXTV CC . When the voltage on the
EXTV CC pin is less than 4.5V, the LDO 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 2 of the
Electrical Characteristics tables. For example, the LTC3867
INTV CC current is limited to less than 30mA from a 38V
supply in the UF package and not using the EXTV CC supply
with a 70°C ambient temperature:
T J = 70°C + (30mA)(38V)(47°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 = SGND)
at maximum V IN . When the voltage applied to EXTV CC rises
above 4.7V, the INTV CC LDO 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 an efficient switching regulator
output during normal operation. 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 .
Significant efficiency and thermal gains can be realized
by powering INTV CC from EXTV CC , since the V IN current
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 + (30mA)(5V)(47°C/W) = 77°C
However, for 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 LDO 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 network.
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.
For applications where the main input power is 5V, tie
the V IN and INTV CC pins together and tie the combined
pins to the 5V input with a 1Ω or 2.2Ω resistor as shown
in Figure 12 to minimize the voltage drop caused by the
gate charge current. This will override the INTV CC linear
regulator and will prevent INTV CC from dropping too low
due to the dropout voltage. Make sure the INTV CC voltage
is at or exceeds the R DS(ON) test voltage for the MOSFET
which is typically 4.5V for logic-level devices
3867f
26
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