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参数资料
| 型号： | LTC3850IGN-2#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 19/36页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 28-SSOP |
| 产品培训模块： | LTC3850 Dual Output DC/DC Switching Regulator Controller |
| 标准包装： | 2,500 |
| 系列： | PolyPhase® |
| PWM 型： | 电流模式 |
| 输出数： | 2 |
| 频率 - 最大： | 860kHz |
| 占空比： | 97.2% |
| 电源电压： | 4 V ~ 30 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 ~ 85°C |
| 封装/外壳： | 28-SSOP（0.154"，3.90mm 宽） |
| 包装： | 带卷 (TR) |
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LTC3850-2
APPLICATIONS INFORMATION
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3850-2 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 high-
est and is equal to V IN ? I INTVCC . The gate charge current
is dependent on operating frequency as discussed in the
Ef?ciency Considerations section. The junction tempera-
ture can be estimated by using the equations given in
Note 3 of the Electrical Characteristics. For example, the
LTC3850-2 INTV CC current is limited to less than 24mA
from a 24V supply in the GN package and not using the
EXTV CC supply:
T J = 70°C + (24mA)(24V)(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 (MODE/PLLIN
= SGND) at maximum V IN . When the voltage applied to
EXTV 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 LTC3850-2’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 speci?ed,
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 .
Signi?cant ef?ciency and thermal gains can be realized by
However, for 3.3V and other low voltage outputs, addi-
tional 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 ef?ciency 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 ef?ciency.
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,
ef?ciency 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 8 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.
V IN
powering INTV CC from the output, since the V IN current
resulting from the driver and control currents will be scaled
LTC3850-2
INTV CC
R VIN
5V
by a factor of (Duty Cycle)/(Switcher Ef?ciency).
Tying the EXTV CC pin to a 5V supply reduces the junction
C INTVCC
4.7μF
1Ω
+
C IN
temperature in the previous example from 125°C to:
T J = 70°C + (24mA)(5V)(95°C/W) = 81°C
Figure 8. Setup for a 5V Input
38502 F08
38502f
19
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