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| 型号： | LTC3721EGN-1#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 11/16页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR ISO PWM CM 16-SSOP |
| 标准包装： | 100 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1MHz |
| 占空比： | 50% |
| 电源电压： | 10V（最小值） |
| 降压： | 无 |
| 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-SSOP（0.154"，3.90mm 宽） |
| 包装： | 管件 |
| 产品目录页面： | 1335 (CN2011-ZH PDF) |
LTC3721-1
OPERATIO
that the positive voltage square wave is produced while the
output inductor is freewheeling. An advantage of this
technique over the previous is that it does not require a
separate filter inductor and since the voltage is derived
from the well-regulated output voltage, it is also well
controlled. One disadvantage is that this winding will
require the same safety isolation that is required for the
main transformer. Another disadvantage is that a much
larger V CC filter capacitor is needed, since it does not
generate a voltage as the output is first starting up, or
during short-circuit conditions.
Programming the LTC3721-1 Oscillator
The high accuracy LTC3721-1 oscillator circuit provides
flexibility to program the switching frequency and slope
compensation required for current mode control. The
oscillator circuit produces a 2.35V peak-to-peak ampli-
tude ramp waveform on C T . Typical maximum duty cycles
of 49% are possible. The oscillator is capable of operation
up to 1MHz by the following equation:
C T = 1/(14.8k ? F OSC )
Note that this is the frequency seen on C T . The output
drivers switch at 1/2 of this frequency. Also note that
higher switching frequency and added driver dead-time
via DPRG will reduce the maximum duty cycle.
The LTC3721-1 derives a compensating slope current
from the oscillator ramp waveform and sources this
current out of CS. The desired level of slope compensation
Current Sensing and Overcurrent Protection
Current sensing provides feedback for the current mode
control loop and protection from overload conditions. The
LTC3721-1 is compatible with either resistive sensing or
current transformer methods. Internally connected to CS
are two comparators that provide pulse-by-pulse and
overcurrent shutdown functions respectively, (Figure 6).
The pulse-by-pulse comparator has a 300mV nominal
threshold. If the 300mV threshold is exceeded, the PWM
cycle is terminated. The overcurrent comparator is set
approximately 2x higher than the pulse-by-pulse level. If
the current signal exceeds this level, the PWM cycle is
terminated, the soft-start capacitor is quickly discharged
and a soft-start cycle is initiated. If the overcurrent condi-
tion persists, the LTC3721-1 halts PWM operation and
waits for the soft-start capacitor to charge up to approxi-
mately 4V before a retry is allowed. The soft-start capaci-
tor is charged by an internal 13 μ A current source. If the
fault condition has not cleared when soft-start reaches 4V,
the soft-start pin is again discharged and a new cycle is
initiated. This is referred to as hiccup mode operation. In
normal operation and under most abnormal conditions,
the pulse-by-pulse comparator is fast enough to prevent
hiccup mode operation. In severe cases, however, with
high input voltage, very low R DS(ON) MOSFETs and a
shorted output, or with saturating magnetics, the
overcurrent comparator provides a means of protecting
the power converter.
is selected with an external resistor connected between CS
and the external current sense resistor, (Figure 5).
CS
PULSE BY PULSE
CURRENT LIMIT
PWM
+
–
300mV
PWM
LATCH
Q
S
PWM
LOGIC
UVLO
ENABLE
S Q
R
H = SHUTDOWN
OUTPUTS
Q
OVERLOAD
V(C T )
650mV
C T
LTC3721-1
I=
33k
CS
R SLOPE
SWITCH
CURRENT
R CS
CURRENT LIMIT
+
–
UVLO
ENABLE
S Q
R
4.1V
0.4V
13 μ A
SS
C SS
33k
ADDED
SLOPE
R CS
Q
CURRENT SENSE
WAVEFORM
37211 F05
37211 F06
Figure 5. Slope Compensation Circuitry
Figure 6. Current Sense/Fault Circuitry Detail
sn37211 37211fs
11
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| LTC3721EUF-1 | 功能描述:IC REG CTRLR ISO PWM CM 16-QFN RoHS:否 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:- 标准包装:4,500 系列:PowerWise® PWM 型:控制器 输出数:1 频率 - 最大:1MHz 占空比:95% 电源电压:2.8 V ~ 5.5 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:无 工作温度:-40°C ~ 125°C 封装/外壳:6-WDFN 裸露焊盘 包装:带卷 (TR) 配用:LM1771EVAL-ND - BOARD EVALUATION LM1771 其它名称:LM1771SSDX |
| LTC3721EUF-1#PBF | 功能描述:IC REG CTRLR ISO PWM CM 16-QFN RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:- 标准包装:4,500 系列:PowerWise® PWM 型:控制器 输出数:1 频率 - 最大:1MHz 占空比:95% 电源电压:2.8 V ~ 5.5 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:无 工作温度:-40°C ~ 125°C 封装/外壳:6-WDFN 裸露焊盘 包装:带卷 (TR) 配用:LM1771EVAL-ND - BOARD EVALUATION LM1771 其它名称:LM1771SSDX |
| LTC3721EUF-1#TR | 功能描述:IC REG CTRLR ISO PWM CM 16-QFN RoHS:否 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:- 标准包装:4,500 系列:PowerWise® PWM 型:控制器 输出数:1 频率 - 最大:1MHz 占空比:95% 电源电压:2.8 V ~ 5.5 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:无 工作温度:-40°C ~ 125°C 封装/外壳:6-WDFN 裸露焊盘 包装:带卷 (TR) 配用:LM1771EVAL-ND - BOARD EVALUATION LM1771 其它名称:LM1771SSDX |
| LTC3721EUF-1#TRPBF | 功能描述:IC REG CTRLR ISO PWM CM 16-QFN RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:- 标准包装:4,500 系列:PowerWise® PWM 型:控制器 输出数:1 频率 - 最大:1MHz 占空比:95% 电源电压:2.8 V ~ 5.5 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:无 工作温度:-40°C ~ 125°C 封装/外壳:6-WDFN 裸露焊盘 包装:带卷 (TR) 配用:LM1771EVAL-ND - BOARD EVALUATION LM1771 其它名称:LM1771SSDX |
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