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参数资料
| 型号： | LTC3785EUF-1#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 13/20页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BST PWM VM 24-QFN |
| 标准包装： | 2,500 |
| PWM 型： | 电压模式 |
| 输出数： | 2 |
| 频率 - 最大： | 1MHz |
| 占空比： | 99% |
| 电源电压： | 2.7 V ~ 10 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 ~ 85°C |
| 封装/外壳： | 24-WFQFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
LTC3785-1
APPLICATIONS INFORMATION
(switches B and C shown in Figure 1). Important param-
eters for the power MOSFETs are the breakdown voltage
Switch C operates in boost mode as the control switch. Its
power dissipation at maximum current is given by:
V BR(DSS) , threshold voltage V GS(TH) , on-resistance R DS(ON) ,
reverse transfer capacitance C RSS and maximum current
I DS(MAX) . The drive voltage is set by the 4.35V V CC supply.
PC(BOOST) =
2
( V OUT – V IN ) ? V OUT ?I
V IN
2
OUT(MAX)
? ρ T
Consequently, logic-level threshold MOSFETs must be used
in LTC3785-1 applications. If the input voltage is expected
to drop below 5V, then sub-logic threshold MOSFETs should
? R DS(ON) + k ? V OUT 3 ?
I OUT(MAX)
V IN
? C RSS ? f
? V
?
PA(BOOST) = ? OUT ?I OUT(MAX) ? ? ρ T ? R DS(ON)
PD ( BOOST ) = OUT ?I OUT(MAX)2 ? ρ T ? R DS(ON)
beconsidered.InordertoselectthepowerMOSFETs,the
power dissipated by the device must be known.
For switch A, the maximum power dissipation happens
in boost mode, when it remains on all the time. Its maxi-
mum power dissipation at maximum output current is
given by:
2
? V IN ?
where ρ T is a normalization factor (unity at 25°C) ac-
counting for the signi?cant variation in on-resistance with
temperature, typically about 0.4%/°C as shown in Figure 4.
For a maximum junction temperature of 125°C, using a
value ρ T = 1.5 is reasonable.
Switch B operates in buck mode as the synchronous
recti?er. Its power dissipation at maximum output current
is given by:
where C RSS is usually speci?ed by the MOSFET manufactur-
ers. The constant k, which accounts for the loss caused by
reverse recovery current, is inversely proportional to the
gate drive current and has an empirical value of 1.0.
For switch D, the maximum power dissipation happens in
boost mode when its duty cycle is higher than 50%. Its
maximum power dissipation at maximum output current
is given by:
V
V IN
Typically, switch A has the highest power dissipation and
switch B has the lowest power dissipation unless a short
occurs at the output. From a known power dissipated
in the power MOSFET, its junction temperature can be
obtained using the following formula:
T J = T A + P ? R TH(JA)
PB(BUCK) = IN
2.0
1.5
1.0
0.5
V – V OUT
V IN
?I OUT(MAX)2 ? ρ T ? R DS(ON)
The R TH(JA) to be used in the equation normally includes
the R TH(JC) for the device plus the thermal resistance from
the case to the ambient temperature (R TH(CA) ). This value
of T J can then be compared to the original, assumed value
used in the iterative calculation process.
SCHOTTKY DIODE (D1, D2) SELECTION
Optional Schottky diodes D1 and D2 shown in the Block
Diagram conduct during the dead time between the conduc-
tion of the power MOSFET switches. They are intended to
prevent the body diode of synchronous switches B and D
from turning on and storing charge during the dead time.
0
–50
0 50 100
JUNCTION TEMPERATURE (°C)
150
37851 F04
In particular, D2 signi?cantly reduces reverse recovery
current between switch D turn off and switch C turn on,
which improves converter ef?ciency and reduces switch C
voltage stress. In order for D2 to be effective, it must be
Figure 4. Normalized R DS(ON) vs Temperature
located in very close proximity to SWD.
37851fa
13
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