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参数资料
| 型号： | LTC3859EUHF#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 27/42页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BST PWM CM 38-QFN |
| 标准包装： | 52 |
| PWM 型： | 电流模式，Burst Mode? |
| 输出数： | 3 |
| 频率 - 最大： | 850kHz |
| 占空比： | 100% |
| 电源电压： | 4.5 V ~ 38 V |
| 降压： | 是 |
| 升压： | 是 |
| 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： | 38-WFQFN 裸露焊盘 |
| 包装： | 管件 |
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LTC3859
APPLICATIONS INFORMATION
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (PLLIN/MODE
= INTV CC ) at maximum V IN .
When the voltage applied to EXTV CC rises above 4.7V, the
V BIAS 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 5.4V, so while EXTV CC
is less than 5.4V, the LDO is in dropout and the INTV CC
voltage is approximately equal to EXTV CC . When EXTV CC
is greater than 5.4V, up to an absolute maximum of 14V,
INTV CC is regulated to 5.4V.
Using the EXTV CC LDO allows the MOSFET driver and
control power to be derived from one of the LTC3859’s
switching regulator outputs (4.7V ≤ V OUT ≤ 14V) dur-
ing normal operation and from the V BIAS LDO when the
output is out of regulation (e.g., startup, short-circuit). If
2. EXTV CC connected directly to the output voltage of one
of the buck regulators. This is the normal connection
for a 5V to 14V regulator and provides the highest ef-
?ciency.
3. EXTV CC connected to an external supply. If an external
supply is available in the 5V to 14V range, it may be
used to power EXTV CC providing it is compatible with the
MOSFET gate drive requirements. Ensure that EXTV CC
< V IN .
4. EXTV CC connected to an output-derived boost network
off one of the buck regulators. For 3.3V and other low
voltage buck 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. This
can be done with the capacitive charge pump shown in
Figure 9. Ensure that EXTV CC < V IN .
more current is required through the EXTV CC LDO than
is speci?ed, an external Schottky diode can be added
between the EXTV CC and INTV CC pins. In this case, do
LTC3859
V IN1,2
C1
BAT85
BAT85
not apply more than 6V to the EXTV CC pin and make sure
than EXTV CC ≤ V BIAS .
Signi?cant ef?ciency and thermal gains can be realized
by powering INTV CC from the buck output, since the V IN
current resulting from the driver and control currents will
be scaled by a factor of (Duty Cycle)/(Switcher Ef?ciency).
EXTV CC
TG
SW
BG
MTOP
MBOT
L
R SENSE
BAT85
V OUT1,2
For 5V to 14V regulator outputs, this means connecting
the EXTV CC pin directly to V OUT . Tying the EXTV CC pin to
a 8.5V supply reduces the junction temperature in the
previous example from 125°C to:
T J = 70°C + (40mA)(8.5V)(34°C/W) = 82°C
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 5.4V regulator result-
ing in an ef?ciency penalty of up to 10% at high input
voltages.
PGND
3859 F09
Figure 9. Capacitive Charge Pump for EXTV CC
Topside MOSFET Driver Supply (C B , D B )
External bootstrap capacitors C B connected to the BOOST
pins supply the gate drive voltages for the topside MOSFETs.
Capacitor C B in the Functional Diagram is charged though
external diode D B from INTV CC when the SW pin is low.
When one of the topside MOSFETs is to be turned on, the
driver places the C B voltage across the gate-source of the
desired MOSFET. This enhances the MOSFET and turns
on the topside switch. The switch node voltage, SW, rises
to V IN for the buck channels (V OUT for the boost channel)
and the BOOST pin follows. With the topside MOSFET
3859fa
27
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