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| 型号： | LT1952IGN-1#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 17/28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： | 560kHz |
| 占空比： | 90% |
| 电源电压： | 6.82 V ~ 25 V |
| 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 ~ 125°C |
| 封装/外壳： | 16-SSOP（0.154"，3.90mm 宽） |
| 包装： | 管件 |
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LT1952/LT1952-1
APPLICATIONS INFORMATION
Example calculation for (2)
For R T = 35.7k, R B = 100k, V REF = 2.5V,
R DELAY = 40k, f OSC = 200kHz and SD_V SEC = 1.32V,
this gives SS_MAXDC(DC) = 1.84V, t DELAY = 40ns
and k = 1
Maximum Duty Cycle Clamp
= 1 ? 0.522(1.84/1.32) – (40ns ? 200kHz)
= 0.728 – 0.008 = 0.72 (Duty Cycle Clamp = 72%)
Note 1: To achieve the same maximum duty cycle clamp at
100kHz as calculated for 200kHz, the SS_MAXDC voltage
should be reprogrammed by:
SS_MAXDC(DC) (100kHz)
= SS_MAXDC(DC) (200kHz) ? k (200kHz)/k (100kHz)
= 1.84 ? 1.0/1.055 = 1.74V (k = 1.055 for 100kHz)
Note 2 : To achieve the same maximum duty cycle clamp
while synchronizing to an external clock at the SYNC pin,
the SS_MAXDC voltage should be re-programmed as:
maximum switch duty cycle clamp, determine soft-start
timing (Figure 11).
A soft-start event is triggered for the following faults:
(1) V IN < 8.75V, or
(2) SD_V SEC < 1.32V (UVLO), or
(3) OC > 107mV (overcurrent condition)
When a soft-start event is triggered, switching at SOUT
and OUT is stopped immediately. A soft-start latch is set
and SS_MAXDC pin is discharged. The SS_MAXDC pin can
only recharge when the soft-start latch has been reset.
Note: A soft-start event caused by (1) or (2) above, also
causes V REF to be disabled and to fall to ground.
Soft-start latch reset requires all of the following:
SS_MAXDC
SS_MAXDC (DC) (fsync)
= SS_MAXDC (DC) (200kHz) ? [(fosc/fsync) +
0.09(fosc/200kHz)0.6]
SOFT-START
EVENT TRIGGERED
0.8V (ACTIVE THRESHOLD)
0.45V (RESET THRESHOLD)
TIMING (A): SOFT START FAULT REMOVED
BEFORE SS_MAXDC FALLS TO 0.45V
For SS_MAXDC (DC) (200kHz) = 1.84V for 72%
duty cycle
SS_MAXDC (DC) (fsync = 250kHz) for 72%
duty cycle
= 1.84 ? [(200kHz/250kHz) + 0.09(1)0.6]
= 1.638V
Programming Soft-Start Timing
SS_MAXDC
0.8V (ACTIVE THRESHOLD)
0.45V (RESET THRESHOLD)
0.2V
TIMING (B): SOFT-START FAULT REMOVED
AFTER SS_MAXDC FALLS PAST 0.45V 1952 F10
Figure 10. Soft-Start Timing
R CHARGE
V REF
TheLT1952/LT1952-1havebuilt-insoft-startcapabilityto
provide low stress controlled start-up from a list of fault
conditions that can occur in the application (see Figure 1
and Figure 10). The LT1952/LT1952-1 provide true PWM
soft-start by using the SS_MAXDC pin to control soft-start
timing. The proportional relationship between SS_MAXDC
voltage and switch maximum duty cycle clamp allows
the SS_MAXDC pin to slowly ramp output voltage by
ramping the maximum switch duty cycle clamp —until
switch duty cycle clamp seamlessly meets the natural duty
cycle of the converter. A capacitor C SS on the SS_MAXDC
pin and the resistor divider from V REF used to program
C SS
SS_MAXDC(DC)
LT1952/
LT1952-1
SS_MAXDC
R T
C SS
R B
SS_MAXDC CHARGING MODEL
SS_MAXDC(DC) = V REF [R B /(R T + R B )]
R CHARGE = [R T ? R B /(R T + R B )]
Figure 11. Programming Soft-Start Timing
LT1952/
LT1952-1
SS_MAXDC
1952 F11
19521fe
17
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| LT1952IGN-1TR | 制造商:LINER 制造商全称:Linear Technology 功能描述:Single Switch Synchronous Forward Controller |
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