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参数资料
| 型号： | LTC3704IMS#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 17/28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
The R TH(JA) to be used in this 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.
Output Diode Selection
To maximize efficiency, a fast switching diode with low
forward drop and low reverse leakage is desired. The
1A/DIV
output diode in a positive-to-negative converter conducts
500ns/DIV
3704 F13
current during the switch off-time. The peak reverse
voltage that the diode must withstand is equal to V IN(MAX)
– V O . The average forward current in normal operation is
equal to the output current, and the peak current is equal
to the peak inductor current.
Figure 13. Ripple Current in the DC Coupling Capacitor
A low ESR and ESL, X5R- or X7R-type ceramic capacitor
is recommended here.
= – ? 1 +
? ? I O ( MAX )
I D ( PEAK )
?
?
χ ?
2 ?
1
1 – D MAX
Selecting the Output Capacitor
The output ripple voltage appears as a triangular wave-
form riding on V O , due to the ripple current of L2 (the DC
The power dissipated by the diode is:
P D = I O(MAX) ? V D
and the diode junction temperature is:
T J = T A + P D ? R TH(JA)
component of the current in L2 equals the output current).
This ripple current flows through the ESR and bulk capaci-
tance of the output capacitor to produce the overall ripple
voltage on this node. Using the off-time to calculate this
ripple current results in the following equation for Δ I L2 :
1 – D MAX V O
TheR TH(JA) tobeusedinthisequationnormallyincludes
the R TH(JC) for the device plus the thermal resistance from
Δ I L 2 = –
?
f L 2
Δ V O ( P – P ) =
1 – D MAX V O
the board to the ambient temperature in the enclosure.
Remember to keep the diode lead lengths short and to
observe proper switch-node layout (see Board Layout
Checklist) to avoid excessive ringing and increased
dissipation.
Selecting the DC Coupling Capacitor
The voltage on the coupling capacitor in a positive-to-
negative converter is V IN(MAX) – V O , plus any additional Δ V
due to the ripple currents in the inductors. Generally, the
DC coupling capacitor is dimensioned based on the high
RMS ripple which flows in it, as shown in Figure 13.
The minimum RMS current rating of this capacitor must
exceed:
where V O is a negative number. The output ripple voltage
is therefore:
?
f L 2
? 1 ?
? – ESR – ?
? 8 ? f ? C O ?
The ESR can be minimized by using high quality, X5R- or
X7R-dielectric ceramic capacitor in parallel with a larger
value tantalum or aluminum electrolytic bulk capacitor.
Depending upon the application, it may be that the ceramic
capacitor alone will be sufficient.
The RMS ripple current rating of the output capacitor
I RMS ( CAP ) = – I O ( MAX ) ?
D MAX
1 – D MAX
needs to be greater than:
3704fb
17
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