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参数资料
| 型号： | LTC1700EMS#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 8/16页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BST PWM CM 10-MSOP |
| 标准包装： | 2,500 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 680kHz |
| 占空比： | 92% |
| 电源电压： | 0.9 V ~ 6 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： | -40°C ~ 85°C |
| 封装/外壳： | 10-TFSOP，10-MSOP（0.118"，3.00mm 宽） |
| 包装： | 带卷 (TR) |
LTC1700
APPLICATIO N S I N FOR M ATIO N
Power MOSFET Selection
The LTC1700 requires two external power MOSFETs, one
for the main switch (N-channel) and one for the synchro-
nous rectifier (P-channel). Since the voltage operating
range of the LTC1700 is limited to less than 6V, the
breakdown voltage of the MOSFETs is not a concern.
Therefore the MOSFETs parameters that should be used
for selecting the power MOSFETs are threshold voltage
V GS(TH) , on-resistance R DS(ON) , reverse transfer capaci-
tance C RSS and maximum current I D(MAX) .
The gate drive voltage is set by the output voltage, V OUT .
Since the LTC1700 exits the start-up mode at 2.3V, sub-
logic level threshold MOSFETs should be used in LTC1700
applications. Newer MOSFETs with guaranteed R DSON at
gate voltage of 1.8V are now available and will work very
well with the LTC1700.
The MOSFETs on-resistance is chosen based on the
required load current. The maximum average output
current I O(MAX) is :
I O(MAX) = (I PK – 0.5 ? I)(1 – DC)
where:
For 25 ° C operating condition, set ? V SENSE = 65mV. For
conditions that vary over the full temperature range, set
? V SENSE = 55mV.
The ρ T is a normalized term accounting for the significant
variation in R DS(ON) with temperature, typically about
0.375%/ ° C as shown in Figure 2. Junction to case tem-
perature T JC is around 10 ° C in most applications. For a
maximum ambient temperature of 70 ° C, using ρ 80 ° C ? 1.2
in the above equation is a reasonable choice. This equation
is plotted in Figure 3 to illustrate the dependence of
maximum output current on R DS(ON) , assuming
? I = 0.4I O(MAX) .
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
I PK = Peak Inductor Current
–55 –35 –15
5 25 45 65
TEMPERATURE ( ° C)
85 105 125
? I = Inductor Ripple Current
DC = Duty Cycle
The peak inductor current is inherently limited in a
current mode controller. The maximum V DS sense volt-
age of the main MOSFET is limited to 78mV. The LTC1700
5.0
4.5
4.0
1700 F02
Figure 2. R DS(ON) vs Temperature
will not allow peak inductor current to exceed 78mV/
R DS(ON)(N-CHANNEL) . The following equation is a good
guide for determining the required R DS(ON)(MAX) , allow-
ing some margin for ripple current, current limit and
variations in the LTC1700 and external component val-
ues:
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
DUTY CYCLE = 10%
DUTY CYCLE = 50%
DUTY CYCLE = 80%
( )
? + ? I L ? ρ T
? 1 – DC
?
R DS ( ON )( MAX ) ?
? V SENSE
? I O ( MAX ) 1 ?
2
0 10 20 30 40 50 60 70 80 90 100
R DS(ON) (m ? )
1700 F03
Figure 3. Maximum Current vs R DS(ON)
Power dissipated by the main and synchronous
MOSFETs depends upon their respective duty cycles and
1700fa
8
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