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参数资料
| 型号： | LTC3728IG |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A DUAL SWITCHING CONTROLLER, 590 kHz SWITCHING FREQ-MAX, PDSO28 |
| 封装： | 0.209 INCH, PLASTIC, SSOP-28 |
| 文件页数： | 17/36页 |
| 文件大小： | 1661K |
| 代理商： | LTC3728IG |
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LTC3728
24
3728fg
APPLICATIONS INFORMATION
Minimum On-Time Considerations
Minimum on-time, tON(MIN), is the smallest time duration
that the LTC3728 is capable of turning on the top MOSFET.
It is determined by internal timing delays and the gate
charge required to turn on the top MOSFET. Low duty
cycle applications may approach this minimum on-time
limit and care should be taken to ensure that:
tON(MIN) <
VOUT
VIN(f)
If the duty cycle falls below what can be accommodated
by the minimum on-time, the LTC3728 will begin to skip
cycles. The output voltage will continue to be regulated,
but the ripple voltage and current will increase.
The typical tested minimum on-time of the LTC3728 is
100ns under an ideal condition without switching noise.
However, the minimum on-time can be affected by PCB
switching noise in the voltage and current loops. With
reasonably good PCB layout, minimum 30% inductor
current ripple and about 15mV sensing ripple voltage,
200ns minimum on-time is a conservative number to
start with.
FCB Pin Operation
The FCB pin can be used to regulate a secondary winding
or as a logic-level input. Continuous operation is forced
on both controllers when the FCB pin drops below 0.8V.
During continuous mode, current ows continuously in
the transformer primary. The secondary winding(s) draw
current only when the bottom, synchronous switch is on.
When primary load currents are low and/or the VIN/VOUT
ratio is low, the synchronous switch may not be on for a
sufcient amount of time to transfer power from the output
capacitor to the secondary load. Forced continuous opera-
tion will support secondary windings providing there is
sufcient synchronous switch duty factor. Thus, the FCB
input pin removes the requirement that power must be
drawn from the inductor primary in order to extract power
from the auxiliary windings. With the loop in continuous
mode, the auxiliary outputs may nominally be loaded
without regard to the primary output load.
The secondary output voltage, VSEC, is normally set (as
shown in Figure 6a) by the turns ratio N of the trans-
former:
VSEC (N + 1) VOUT
However, if the controller goes into Burst Mode operation
and halts switching due to a light primary load current, then
VSEC will droop. An external resistive divider from VSEC to
the FCB pin sets a minimum voltage VSEC(MIN):
VSEC(MIN) 0.8V 1+
R6
R5
where R5 and R6 are shown in Figure 2.
If VSEC drops below this level, the FCB voltage forces
temporary continuous switching operation until VSEC is
again above its minimum.
In order to prevent erratic operation if no external connec-
tions are made to the FCB pin, the FCB pin has a 0.18μA
internal current source pulling the pin high. Include this
current when choosing resistor values R5 and R6.
Table 1 summarizes the possible states available on the
FCB pin:
Table 1
FCB Pin
Condition
0V to 0.75V
Forced Continuous Both Controllers
(Current Reversal Allowed—Burst
Inhibited)
0.85V < VFCB < 4.0V
Minimum Peak Current Induces
Burst Mode Operation
No Current Reversal Allowed
Feedback Resistors
Regulating a Secondary Winding
>4.8V
Burst Mode Operation Disabled
Constant-Frequency Mode Enabled
No Current Reversal Allowed
No Minimum Peak Current
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相关代理商/技术参数 |
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| LTC3728IG#PBF | 功能描述:IC REG CTRLR BUCK PWM CM 28-SSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 切换控制器 系列:PolyPhase® 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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