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参数资料
| 型号： | LTC1698IS#TRPBF |
| 厂商： | Linear Technology |
| 文件页数： | 14/24页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR ISO PWM CM 16-SOIC |
| 标准包装： | 2,500 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 400kHz |
| 电源电压： | 6 V ~ 12.6 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 是 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 16-SOIC（0.154"，3.90mm 宽） |
| 包装： | 带卷 (TR) |
LTC1698
APPLICATIO S I FOR ATIO
R SECSEN =
R 6 =
2 ? π ? C 2 ?
A series RC network can be added in parallel with R2
(Figure 7) to provide a zero for the feedback loop fre-
quency compensation.
The opto driver will drive a capacitive load up to 0.1 μ F. For
optocouplers with a base pin, switching signal noise can
get into this high impedance node. Connect a large resis-
tor, 1M or 2M between the base and the emitter. This
increases the diode current and the overall feedback
bandwidth slightly, and decreases the optocoupler gain.
When designing the resistor in series with the optocoupler
diode, it is important to consider the part to part variations
in the current transfer ratio and its reduction over tem-
perature and aging. The bigger the biasing current, the
faster the aging. The LTC1698 opto driver is designed to
source up to 10mA of current and swing between 0.4V to
(V DD – 2.5V). This should meet the design consideration
of most optocouplers.
Besides the voltage feedback function, the LTC1698 opto
driver couples fault signals to the primary controller and
prevents catastrophic damage to the circuit. Upon current
limit or an overvoltage fault, the I LIM or OVP comparator
overrides the error amplifier output and forces the
OPTODRV pin high. This sources maximum current into
the external optodiode and reduces the forward converter
duty cycle.
Average Current Limit
The secondary current limit function is implemented by
measuring the negative voltage across the current sense
resistor R SECSEN . The current limit transconductance
amplifier I LIM has a – 25mV threshold. As shown in
Figure 8, if the secondary current is small, the I COMP pin
goes low and the transistor M ILIM shuts off. The potential
at V COMP determines the OPTODRV output. If the second-
ary current is large, I COMP pulls high and forces the tran-
sistor M ILIM to turn on hard. Thus the current limit circuit
overrides the voltage feedback and forces OPTODRV high
and injects maximum current into the external optocoupler.
The R ILIM resistor provides a linear relationship between
the current sensed and the OPTODRV output.
The I SNS and I SNSGND pins allow a true Kelvin current
sense measurement and offer true differential measure-
ment across the sense resistor. A differential lowpass
filter formed by R6 and C2 removes the pulse-to-pulse
inductor current ripple and generates the average sec-
ondary current which is equal to the load current. The
lowpass corner frequency is typically set to 1 to 2 orders
of magnitude below the switching frequency and follows
the relationship:
25mV
I LMAX
1
f SW
10
where:
R SECSEN = Secondary current sense resistor
I LMAX = Maximum allowed secondary current
f SW = Forward converter switching frequency
DRIVE
+
V REF
CG
2
Q4
T1
V OUT
5
OPTODRV
OPTO
–
20k
V COMP
FG
16
Q3
R CILM
C CILM
13
I COMP
100k
M ILIM
R ILIM
3k
I LIM
25mV
+ I SNS
I SNSGND
12
11
C2
R DIV
(OPTIONAL)
R6
R6
R SECSEN
LTC1698
1698 F08
Figure 8. Secondary Average Current Limit
1698f
14
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