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| 型号： | LTC3704IMS#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 16/28页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR INV PWM CM 10-MSOP |
| 标准包装： | 50 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1MHz |
| 占空比： | 97% |
| 电源电压： | 2.5 V ~ 36 V |
| 降压： | 无 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 是 |
| 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： | 10-TFSOP，10-MSOP（0.118"，3.00mm 宽） |
| 包装： | 管件 |
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LTC3704
APPLICATIO S I FOR ATIO
200
150
100
50
2.0
1.5
1.0
0.5
0
0
0.2
0.4
0.5
0.8
1.0
0
– 50
0
50
100
150
DUTY CYCLE
3704 F11
Figure 11. Maximum SENSE Threshold Voltage vs Duty Cycle
Another method of choosing which power MOSFET to use
is to check the maximum output current for a given
R DS(ON) , since MOSFET on-resistances are generally
available in discrete values.
JUNCTION TEMPERATURE ( ° C)
3704 F12
Figure 12. Normalized R DS(ON) vs Temperature
As a result, some iterative calculation is normally required
to determine a reasonably accurate value. Since the
controller is using the MOSFET as both a switching and a
sensing element, care should be taken to ensure that the
converter is capable of delivering the required load current
?
1 + ? ? R DS ( ON ) ? ρ Τ
I O ( MAX ) = – V SENSE ( MAX ) ?
?
?
1 – D MAX
χ ?
2 ?
over all operating conditions (line voltage and tempera-
ture), and for the worst-case specifications for V SENSE(MAX)
and the R DS(ON) of the MOSFET listed in the manufacturer’s
data sheet.
For the case where a conventional sense resistor is used,
The power dissipated by the MOSFET in a positive-to-
negative converter is:
? 1 + ? ? I O ( MAX )
? – I O ( MAX ) ?
P FET = ? ? ? R DS ( ON ) ? D MAX ? ρ T
R SENSE = V SENSE ( MAX ) ?
? χ ?
? 2 ?
D MAX – 1
? 1 – D MAX ?
2
Sense resistors are generally low TC and are available with
different ranges of tolerance depending on price. The
+ k ? ( V IN – V O ) 1 . 85 ?
I O ( MAX )
1 – D MAX
? C RSS ? f
= I
power dissipated in the sense resistor is:
2
P SENSE SW ( PEAK ) ? R SENSE ? D MAX
Calculating Power MOSFET Switching and Conduction
Losses and Junction Temperatures
In order to calculate the junction temperature of the power
MOSFET, the power dissipated by the device must be
known. This power dissipation is a function of the duty
cycle, the load current and the junction temperature itself
(due to the positive temperature coefficient of its R DS(ON) ).
where I O(MAX) and V O are negative numbers.
The first term in the equation above represents the
I 2 R losses in the device, and the second term, the switch-
ing losses. The constant, k = 1.7, is an empirical factor
inversely related to the gate drive current and has the
dimension of 1/current.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
T J = T A + P FET ? R TH(JA)
3704fb
16
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