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| 型号： | LM3686TLX-AAEF/NOPB |
| 厂商： | National Semiconductor |
| 文件页数： | 15/27页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK SYNC .9A TRPL 12USMD |
| 标准包装： | 3,000 |
| 类型： | 降压（降压） |
| 输出类型： | 固定 |
| 输出数： | 3 |
| 输出电压： | 1.2V，1.8V，3V |
| 输入电压： | 2.7 V ~ 5.5 V |
| PWM 型： | 电流/电压模式 |
| 频率 - 开关： | 3MHz |
| 电流 - 输出： | 900mA |
| 同步整流器： | 是 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 12-WFBGA |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | 12-uSMD |
| 其它名称： | LM3686TLX-AAEF |
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V SW
I L
V BATT = 3.6V
SNVS520E – AUGUST 2008 – REVISED MAY 2013
V OUT = 1.8V
I OUT = 20 mA
V OUT
TIME (1 μs/DIV)
Figure 21. Typical PFM Operation
During PFM operation, the DC-DC converter positions the output voltage slightly higher than the nominal output
voltage during PWM operation, allowing additional headroom for voltage drop during a load transient from light to
heavy load. The PFM comparators sense the output voltage via the feedback pin and control the switching of the
output FETs such that the output voltage ramps between ~0.2% and ~1.8% above the nominal PWM output
voltage. If the output voltage is below the ‘high’ PFM comparator threshold, the PMOS power switch is turned on.
It remains on until the output voltage reaches the ‘high’ PFM threshold or the peak current exceeds the I PFM level
set for PFM mode. The typical peak current in PFM mode is: I PFM = 112 mA + V BATT / 20 ? .
Once the PMOS power switch is turned off, the NMOS power switch is turned on until the inductor current ramps
to zero. When the NMOS zero-current condition is detected, the NMOS power switch is turned off. If the output
voltage is below the ‘high’ PFM comparator threshold (see Figure 22 ), the PMOS switch is again turned on and
the cycle is repeated until the output reaches the desired level. Once the output reaches the ‘high’ PFM
threshold, the NMOS switch is turned on briefly to ramp the inductor current to zero and then both output
switches are turned off and the part enters an extremely low power mode. Quiescent supply current during this
‘sleep’ mode is 28μA (typ), which allows the part to achieve high efficiency under extremely light load conditions.
If the load current should increase during PFM mode (see Figure 22 ) causing the output voltage to fall below the
‘low2’ PFM threshold, the part will automatically transition into fixed-frequency PWM mode.
When V BATT = 2.7V the part transitions from PWM to PFM mode at ~35 mA output current and from PFM to
PWM mode at ~95 mA , when V BATT = 3.6V, PWM to PFM transition happens at ~42 mA and PFM to PWM
transition happens at ~115 mA, when V BATT = 4.5V, PWM to PFM transition happens at ~60 mA and PFM to
PWM transition happens at ~135 mA.
PFM Mode at Light Load
Load current
High PFM Threshold
~1.017*Vout
increases
Low1 PFM Threshold
~1.006*Vout
Current load
PFET on
until
I PFM limit
reached
NFET on
drains
conductor
current
until
I inductor=0
High PFM
Voltage
Threshold
reached,
go into
sleep mode
Low PFM
Threshold,
turn on
PFET
increases,
draws Vout
towards
Low2 PFM
Threshold
Low2 PFM Threshold
Vout
Low2 PFM Threshold,
switch back to PWMmode
PWM Mode at
Moderate to Heavy
Loads
Figure 22. Operation in PFM Mode and Transfer to PWM Mode
Copyright ? 2008–2013, Texas Instruments Incorporated
Product Folder Links: LM3686
15
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