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参数资料
| 型号： | LTC3787MPGN#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 22/36页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BST PWM CM 28-SSOP |
| 标准包装： | 49 |
| 系列： | PolyPhase® |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 850kHz |
| 占空比： | 100% |
| 电源电压： | 4.5 V ~ 38 V |
| 降压： | 无 |
| 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： | -55°C ~ 150°C |
| 封装/外壳： | 28-SSOP（0.154"，3.90mm 宽） |
| 包装： | 管件 |
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LTC3787
APPLICATIONS INFORMATION
EXTV CC remains above 4.55V. 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 6V,
INTV CC is regulated to 5.4V.
Significant thermal gains can be realized by powering
INTV CC from an external supply. Tying the EXTV CC pin
to a 5V supply reduces the junction temperature in the
previous example from 125°C to 79°C in a QFN package:
T J = 70°C + (32mA)(5V)(43°C/W) = 77°C
and from 125°C to 74°C in an SSOP package:
T J = 70°C + (15mA)(5V)(90°C/W) = 77°C
If more current is required through the EXTV CC LDO than
is specified, an external Schottky diode can be added be-
tween the EXTV CC and INTV CC pins. Make sure that in all
cases EXTV CC ≤ VBIAS (even at start-up and shutdown).
The following list summarizes possible connections for
EXTV CC :
EXTV CC Grounded. This will cause INTV CC to be powered
from the internal 5.4V regulator resulting in an efficiency
penalty at high input voltages.
EXTV CC Connected to an External Supply. If an external
supply is available in the 5V to 6V range, it may be used
to provide power. Ensure that EXTV CC is always lower
than VBIAS.
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 Block 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 and
source of the desired MOSFET. This enhances the MOSFET
and turns on the topside switch. The switch node volt-
age, SW, rises to V OUT and the BOOST pin follows. With
the topside MOSFET on, the boost voltage is above the
output voltage: V BOOST = V OUT + V INTVCC . The value of
the boost capacitor C B needs to be 100 times that of the
total input capacitance of the topside MOSFET(s). The
reverse breakdown of the external Schottky diode must
be greater than V OUT(MAX) .
The external diode D B can be a Schottky diode or silicon
diode, but in either case it should have low leakage and fast
recovery. Pay close attention to the reverse leakage at high
temperatures where it generally increases substantially.
Each of the topside MOSFET drivers includes an internal
charge pump that delivers current to the bootstrap capaci-
tor from the BOOST pin. This charge current maintains
the bias voltage required to keep the top MOSFET on
continuously during dropout/overvoltage conditions. The
Schottky/silicon diodes selected for the topside drivers
should have a reverse leakage less than the available output
current the charge pump can supply. Curves displaying
the available charge pump current under different operat-
ing conditions can be found in the Typical Performance
Characteristics section.
A leaky diode D B in the boost converter can not only
prevent the top MOSFET from fully turning on but it can
also completely discharge the bootstrap capacitor C B and
create a current path from the input voltage to the BOOST
pin to INTV CC . This can cause INTV CC to rise if the diode
leakage exceeds the current consumption on INTV CC .
This is particularly a concern in Burst Mode operation
where the load on INTV CC can be very small. The external
Schottky or silicon diode should be carefully chosen such
that INTV CC never gets charged up much higher than its
normal regulation voltage.
Fault Conditions: Overtemperature Protection
At higher temperatures, or in cases where the internal
power dissipation causes excessive self heating on-chip
(such as an INTV CC short to ground), the overtemperature
shutdown circuitry will shut down the LTC3787. When the
junction temperature exceeds approximately 170°C, the
overtemperature circuitry disables the INTV CC LDO, causing
the INTV CC supply to collapse and effectively shut down
3787fc
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