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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)： | 13/25頁 |
| 文件大?。?/td> | 763K |
| 代理商： | LM2647MTC |
Operation Descriptions
(Continued)
goes to zero (and stays there). Though not displayed, Power
Good also goes low within less than 100ns of the EN pin
going low (
t
SD
, see Electrical Characteristics table). There-
fore in this case, the controller is NOT waiting for the output
to actually fall out of the Power Good window before it
signals Power Not Good. Note that since there is a constant
current 2A load applied at the output, the stored charge on
the output capacitor continues to be discharged into the
load. From
V/
t=i/C=2A/330μF it can be seen that the
output voltage (say 1V) will fall to zero in about 165μs, as will
be observed.
But if the load is very close to zero, the only means for the
output capacitor to discharge is through the resistive divider
on the feedback pin (if any) and any internal bleeder resistor
present. In fact there is such an internal bleeder resistor in
the LM2647 and it performs Soft-shutdown by discharging
the output capacitors gradually. Its value is about 20
and it
is internally connected between the SENSE pin and ground
whenever the EN pin is taken low. Note that this will be
perceivable only when the external load is small, and pro-
vided a normal shutdown is being carried out. Normal shut-
down as being defined here calls for the Enable pin to be the
cause of the outputs being disabled. In a shutdown provoked
by a fault, the situation is very different as will be explained
later.
POWER GOOD/NOT GOOD SIGNALING
PGOOD is an open-Drain output pin with an external pull-up
resistor connected to 5V. It goes high (non-conducting) when
both the outputs are within the regulation band as deter-
mined by the Power Good window detector stage on the
feedback pin (see Block Diagram). PGOOD goes low (con-
ducting) when either of the two outputs falls out of this
window. This signal is referred to as Power Not Good here.
A glitch filter of 7μs filters out noise, and helps prevent
spurious PGOOD responses. So Power Not Good is not
asserted until 7μs after either of the two outputs have fallen
out of the Power Good window (see
t
PG_NOK
in Electrical
Characteristics table). With the feedback pin voltage rising
towards regulation value, there is a 20μs delay between both
the outputs being in regulation and the signaling of Power
Good (see
t
in Electrical Characteristics table).
Power Not Good is signaled within 100ns of the Enable pin
being pulled low (see
t
in Electrical Characteristics
table), irrespective of the fact that the outputs could still be in
regulation. The Soft-start capacitor is also then discharged
as explained earlier.
VIN POWER-OFF
The LM2647 has an internal comparator that also looks at
VIN. If VIN falls to about 4.5V (roughly), switching ceases.
The response is slightly different under FPWM or SKIP
modes, but the final result is the same. In both cases ulti-
mately, LDRV is latched high and so the output capacitors
are discharged through the lower FETs. Power Not Good
has meanwhile already been signaled and a fault condition is
asserted shortly thereafter.
In
Figure 7
and
Figure 8
the situation where the connection
to the input DC power source is abruptly removed is shown
for two cases.
In the first case (FPWM mode,
Figure 7
), LDRV goes high
immediately, as soon as VIN falls to about 4.5V. For the
second case (SKIP mode,
Figure 8
), the output starts to
discharge into the load resistor. Then Power Not Good is
signaled. Finally, when the output falls below the Under-
voltage threshold a fault condition is asserted. This is ac-
companied by LDRV latching high. The output then suddenly
collapses just as it does for FPWM mode. Note that once
VIN reaches 4.5V, it does not fall quickly thereafter. The
reason is that there is no applied external voltage dragging it
low (in our case as it is described), nor is there any signifi-
cant consumption from the VIN rail since the converter has
stopped switching.
20056314
CH1: LDRV, CH2: Vo, CH3: SW, CH4: I
L
(1A/div)
Output 1V
@
2A, VIN = 10V, FPWM/SKIP, L = 10μH, f = 300kHz, C
OUT
=
330μF
FIGURE 6. Shutdown
20056315
CH1: PGOOD, CH2: VIN, CH3: LDRV, CH4: Vo
Output 1V
@
0.02A, VIN = 9.75V, FPWM, L = 10μH, f = 300kHz, C
OUT
=
660μF
FIGURE 7. VIN Removal in FPWM Mode
L
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| 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 |
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