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參數(shù)資料
| 型號(hào)： | LM2647MTC |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | Dual Synchronous Buck Regulator Controller |
| 中文描述： | DUAL SWITCHING CONTROLLER, 345 kHz SWITCHING FREQ-MAX, PDSO28 |
| 封裝： | TSSOP-28 |
| 文件頁數(shù)： | 12/25頁 |
| 文件大?。?/td> | 763K |
| 代理商： | LM2647MTC |
Operation Descriptions
(Continued)
Note:
A common question is: can one change from FPWM to SKIP Mode ‘on
the fly’ That means that the voltage on the FPWM pin would be
changed while the converter is operating normally (with outputs in
regulation). This is generally not recommended. The designer must
realize that doing so would in essence represent a fundamental
change applied to the system. The pulse widths would need to re-
adjust suddenly and in the process momentary imbalances can be
created. For example, there is an observed negative surge current
passing from Drain to Source of the lower FET. It must be kept in mind
that though the LM2647 has current limiting for current passing in the
‘positive’ direction (positive with regards to the inductor, i.e. passing
from Source to Drain of the lower FET), there is no set limit for reverse
currents. The amount of reverse current when the FPWM pin is toggled
‘on the fly’ can be very high. This current is determined by several
factors. One key factor is the output capacitance. Large output capaci-
tances will lead to higher peak reverse currents. The reverse swing will
be worse for lighter loads because of the bigger difference between the
duty cycles/average frequency in the two modes. See
Figure 5
for a
plot of what happened in going from SKIP to FPWM mode at 0A load
(worst case). The peak reverse current was as high as 3A, lasting
about 0.1ms. The inductor could also saturate severely at this point if
designed for light loads. In general, if the designer wants to toggle the
FPWM pin while the converter is operating, both the low side FET
rating and the inductor peak current rating must be closely evaluated
under this condition.
SOFT-START
The maximum output voltage of the error amplifier is limited
during start-up by the voltage on the 0.1μF capacitor con-
nected between the SS pin and ground. When the controller
is enabled (by taking EN pin high) the following steps may
occur. First the SS capacitor is discharged (if it has a pre-
charge) by a 1.8 k
internal resistor (R
SS_DCHG
, see Elec-
trical Characteristics). This ensures that reset is obtained.
Note that reset is said to occur only when the voltage on
both
the SS pins falls below 100mV (V
SS_RESET
, see Electrical
Characteristics table). Then a charging current source
I
SS_CHG
of 11μA is applied at this pin to bring up the voltage
of the Soft-start capacitor voltage gradually. This causes the
(maximum allowable) duty cycle to increase slowly, thereby
limiting the charging current into the output capacitor and
also ensuring that the inductor does not saturate. The Soft-
start capacitor will eventually charge up close to the 5V input
rail. When EN is pulled low the Soft-start capacitor is dis-
charged by the same 1.8 k
internal resistor and the con-
troller is shutdown. Now the sequence is allowed to repeat
the next time EN is taken high.
The above Soft-start sequence is actually initiated not only
whenever EN is taken high, but also under a normal
power-up or during recovery from a fault condition (more on
this later).
As mentioned in the section ‘Forced-PWM Mode and Pulse-
skip Mode’under startup, since the currents are high until the
output capacitors have charged up, there will be no observ-
able difference in the shape of the ramp-up of the output rails
in either SKIP mode or FPWM mode. The design has thus
forced the startup waveforms to be identical irrespective of
whether the FPWM mode or the SKIP mode has been
selected.
SHUTDOWN/SOFT-SHUTDOWN
When the EN pin is driven low, the LM2647 initiates shut-
down by turning OFF both upper and lower FETs completely
(this occurs irrespective of FPWM or SKIP modes). See
Figure 6
for a typical shutdown plot and note that the LDRV
20056312
FIGURE 4. Inductor Current in FPWM Mode
20056313
CH1: PGOOD, CH2: Vo, CH3: LDRV, CH4: I
L
(1A/div)
Output 1V
@
0A, VIN = 10V, L = 10μH, f = 300kHz
FIGURE 5. SKIP to FPWM ’On The Fly’
L
www.national.com
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM2647MTC/NOPB | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
| LM2647MTCX | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
| LM2647MTCX/NOPB | 功能描述:DC/DC 開關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK |
| LM2648 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Two-Phase, Synchronous Step-Down 3-Channel Switching Regulator Controller |
| LM26480 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Externally Programmable Dual High-Current Step-Down DC/DC and Dual Linear Regulators |
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