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| 型号： | LTC3890MPUH#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 21/40页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 32-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： | 99% |
| 电源电压： | 4 V ~ 60 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： | -55°C ~ 150°C |
| 封装/外壳： | 32-WFQFN 裸露焊盘 |
| 包装： | 管件 |
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LTC3890
APPLICATIONS INFORMATION
INTV CC Regulators
The LTC3890 features two separate internal P-channel low
dropout linear regulators (LDO) that supply power at the
INTV CC pin from either the V IN supply pin or the EXTV CC
pin depending on the connection of the EXTV CC pin.
INTV CC powers the gate drivers and much of the LTC3890’s
internal circuitry. The V IN LDO and the EXTV CC LDO regulate
INTV CC 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 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 LTC3890 to be
exceeded. The INTV CC current, which is dominated by
the gate charge current, may be supplied by either the
V IN LDO or the EXTV CC LDO. When the voltage on the
EXTV CC pin is less than 4.7V, the V IN 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 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 INTV CC current
is limited to less than 32mA from a 40V supply when not
using the EXTV CC supply at a 70°C ambient temperature:
T J = 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
= INTV CC ) at maximum V IN .
When the voltage applied to EXTV CC rises above 4.7V, the
V IN LDO is turned off and the EXTV CC LDO is enabled. The
EXTV CC LDO remains on as long as the voltage applied to
EXTV CC remains above 4.5V. The EXTV CC LDO attempts
to regulate the INTV CC voltage to 5.1V, so while EXTV CC
is less than 5.1V, the LDO is in dropout and the INTV CC
voltage is approximately equal to EXTV CC . When EXTV CC
is greater than 5.1V, up to an absolute maximum of 14V,
INTV CC is regulated to 5.1V.
Using the EXTV CC LDO allows the MOSFET driver and
control power to be derived from one of the LTC3890’s
switching regulator outputs (4.7V ≤ V OUT ≤ 14V) during
normal operation and from the V IN LDO when the output
is out of regulation (e.g., start-up, short-circuit). If more
current is required through the EXTV CC LDO than is speci-
fied, an external Schottky diode can be added between the
EXTV CC and INTV CC pins. In this case, 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 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).
For 5V to 14V regulator outputs, this means connecting
the EXTV CC pin directly to V OUT . Tying the EXTV CC pin to
an 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
T J = 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 INTV CC power from the output.
3890fc
21
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