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參數(shù)資料
| 型號： | LM34910SD |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | RADIATION HARDENED HIGH EFFICIENCY, 5 AMP SWITCHING REGULATORS |
| 中文描述： | 1.5 A SWITCHING REGULATOR, DSO10 |
| 封裝： | 4 X 4 MM, LLP-10 |
| 文件頁數(shù)： | 9/11頁 |
| 文件大?。?/td> | 583K |
| 代理商： | LM34910SD |
Thermal Shutdown
The LM34910 should be operated so the junction tempera-
ture does not exceed 125C. If the junction temperature
increases, an internal Thermal Shutdown circuit, which acti-
vates (typically) at 175C, takes the controller to a low power
reset state by disabling the buck switch and the on-timer,
and grounding the Softstart pin. This feature helps prevent
catastrophic failures from accidental device overheating.
When the junction temperature reduces below 155C (typical
hysteresis = 20C), the Softstart pin is released and normal
operation resumes.
Applications Information
EXTERNAL COMPONENTS
The following guidelines can be used to select the external
components.
R1 and R2:
The ratio of these resistors is calculated from:
R1/R2 = (V
OUT
/2.5V) - 1
R1 and R2 should be chosen from standard value resistors
in the range of 1.0 k
- 10 k
which satisfy the above ratio.
R
ON
:
The minimum value for R
ON
is calculated from:
Equation 1 can be used to select R
ON
if a specific frequency
is desired as long as the above limitation is met.
L1:
The main parameter affected by the inductor is the
output current ripple amplitude (I
OR
). The limits for I
OR
must
be determined at both the minimum and maximum nominal
load currents.
a) If the maximum load current is less than the current limit
threshold (1.25A), the minimum load current is used to de-
termine the maximum allowable ripple. To maintain continu-
ous conduction mode the lower peak should not reach 0 mA.
For this case, the maximum ripple current is:
I
OR(MAX1)
= 2 x I
O(min)
The ripple calculated in Equation 6 is then used in the
following equation:
(6)
(7)
where V
IN
is the maximum input voltage and Fs is deter-
mined from equation 1. This provides a minimum value for
L1. The next larger standard value should be used, and L1
should be rated for the I
PK
current level.
b) If the maximum load current is greater than the current
limit threshold (1.25A), the LM34910 ensures the lower peak
reaches 1.25Aeach cycle, requiring that I
be at least twice
the difference. The upper peak, however, must not exceed
3.5A. For this case, the ripple limits are:
I
OR(MAX2)
= 2 x (3.5A - I
O(max)
)
and
I
OR(MIN1)
= 2 x (I
O(max)
- 1.25A)
The lesser of Equations 8 and 9 is then used in Equation 7.
If I
is used, the maximum V
is used in Equation 7.
The next larger value should then be used for L1. If I
is used, the minimum V
IN
is used in Equation 7. The next
(8)
(9)
smaller value should then be used for L1. L1 must be rated
for the peak value of the current waveform (I
PK
in
Figure 7
).
C3:
The capacitor on the V
output provides not only noise
filtering and stability, but also prevents false triggering of the
V
UVLO at the buck switch on/off transitions. For this
reason, C3 should be no smaller than 0.1 μF, and should be
a good quality, low ESR, ceramic capacitor.
C2, and R3:
Since the LM34910 requires a minimum of 25
mV
p-p
of ripple at the FB pin for proper operation, the re-
quired ripple at V
OUT1
is increased by R1 and R2. This
necessary ripple is created by the inductor ripple current
acting on C2’s ESR + R3. The minimum ripple current is
calculated using equation 7, rearranged to solve for I
OR
at
minimum V
IN
. The minimum ESR for C2 is then equal to:
(10)
If the capacitor used for C2 does not have sufficient ESR, R3
is added in series as shown in
Figure 1
. Generally R3 is less
than 1
. C2 should generally be no smaller than 3.3 μF,
although that is dependent on the frequency and the allow-
able ripple amplitude at V
. Experimentation is usually
necessary to determine the minimum value for C2, as the
nature of the load may require a larger value. A load which
creates significant transients requires a larger value for C2
than a non-varying load.
D1:
The important parameters are reverse recovery time and
forward voltage. The reverse recovery time determines how
long the reverse current surge lasts each time the buck
switch is turned on. The forward voltage drop is significant in
the event the output is short-circuited as it is mainly this
diode’s voltage (plus the voltage across the current limit
sense resistor) which forces the inductor current to decrease
during the off-time. For this reason, a higher voltage is better,
although that affects efficiency. A reverse recovery time of
)
30 ns, and a forward voltage drop of
)
0.75V are preferred.
The reverse leakage specification is important as that can
significantly affect efficiency. D1’s reverse voltage rating
must be at least as great as the maximum V
IN
, and its
current rating must equal or exceed I
PK
Figure 7
.
C1 and C5:
C1’s purpose is to supply most of the switch
current during the on-time, and limit the voltage ripple at V
IN
,
on the assumption that the voltage source feeding V
IN
has
an output impedance greater than zero. If the source’s dy-
namic impedance is high (effectively a current source), it
supplies the average input current, but not the ripple current.
At maximum load current, when the buck switch turns on, the
current into V
suddenly increases to the lower peak of the
inductor’s ripple current, ramps up to the peak value, then
drop to zero at turn-off. The average current during the
on-time is the load current. For a worst case calculation, C1
must supply this average load current during the maximum
on-time. C1 is calculated from:
where Io is the load current, t
is the maximum on-time,
and
V is the allowable ripple voltage at V
. C5’s purpose is
to help avoid transients and ringing due to long lead induc-
tance at V
. A low ESR, 0.1 μF ceramic chip capacitor is
recommended, located close to the LM34910 .
L
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM34910SD/NOPB | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
| LM34910SDX | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
| LM34910SDX/NOPB | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
| LM34914 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Ultra Small 1.25A Step-Down Switching Regulator with Intelligent Current Limit |
| LM34914EVAL | 功能描述:電源管理IC開發(fā)工具 LM34914 EVAL BOARD RoHS:否 制造商:Maxim Integrated 產(chǎn)品:Evaluation Kits 類型:Battery Management 工具用于評估:MAX17710GB 輸入電壓: 輸出電壓:1.8 V |
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