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參數(shù)資料
| 型號(hào)： | LM25010 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 42V, 1.0A Step-Down Switching Regulator |
| 中文描述： | 42V的1.0安培降壓型開關(guān)穩(wěn)壓器 |
| 文件頁數(shù)： | 15/17頁 |
| 文件大?。?/td> | 776K |
| 代理商： | LM25010 |
Applications Information
(Continued)
In Figure 10, Cff is added across R1 to AC-couple the ripple
at V
directly to the FB pin. This allows the ripple at V
to be reduced, in some cases considerably, by reducing R3.
In the circuit of Figure 6, the ripple at V
OUT
ranged from 50
mVp-p at V
= 6V to 285 mVp-p at V
1000 pF capacitor at Cff and reducing R3 to 0.75
, the V
ripple was reduced by 50%, ranging from 25 mVp-p to 142
mVp-p.
To reduce V
OUT
ripple further, the circuit of Figure 11 can be
used. R3 has been removed, and the output ripple amplitude
is determined by C2’s ESR and the inductor ripple current.
RA and CA are chosen to generate a 40-50 mVp-p sawtooth
at their junction, and that voltage isAC-coupled to the FB pin
via CB. In selecting RA and CA, V
is considered a virtual
ground as the SW pin switches between V
IN
and -1V. Since
the on-time at SW varies inversely with V
, the waveform
amplitude at the RA/CA junction is relatively constant. R1
and R2 must typically be increased to more than 5k each to
not significantly attenuate the signal provided to FB through
CB. Typical values for the additional components are RA =
200k, CA = 680 pF, and CB = 0.01 μF.
INCREASING THE CURRENT LIMIT THRESHOLD
The current limit threshold is nominally 1.25A, with a mini-
mum guaranteed value of 1.0A. If, at maximum load current,
the lower peak of the inductor current (I
in
Figure 5
)
exceeds 1.0A, resistor R
must be added between SGND
and ISEN to increase the current limit threshold to equal or
exceed that lower peak current. This resistor diverts some of
the recirculating current from the internal sense resistor so
that a higher current level is needed to switch the internal
current limit comparator. I
PK-
is calculated from:
(14)
where I
is the maximum load current, and I
is
the minimum ripple current calculated using Equation 13.
R
CL
is calculated from:
(15)
where 0.11
is the minimum value of the internal resistance
from SGND to ISEN. The next smaller standard value resis-
tor should be used for R
CL
. With the addition of R
CL
, and
when the circuit is in current limit, the upper peak current out
of the SW pin (I
PK
in Figure 4) can be as high as:
where I
OR(max)
is calculated using Equation 12. The inductor
L1 and diode D1 must be rated for this current. If I
PK
exceeds
2A , the inductor value must be increased to reduce the
ripple amplitude. This will necessitate recalculation of
I
OR(min)
, I
PK-
, and R
CL
.
Increasing the circuit’s current limit will increase power dis-
sipation and the junction temperature within the LM25010.
See the next section for guidelines on this issue.
PC BOARD LAYOUT and THERMAL CONSIDERATIONS
The LM25010 regulation, over-voltage, and current limit
comparators are very fast, and will respond to short duration
noise pulses. Layout considerations are therefore critical for
optimum performance. The layout must be as neat and
compact as possible, and all the components must be as
close as possible to their associated pins. The two major
current loops have currents which switch very fast, and so
the loops should be as small as possible to minimize con-
ducted and radiated EMI. The first loop is that formed by C1,
through the VIN to SW pins, L1, C2, and back to C1. The
second loop is that formed by D1, L1, C2, and the SGND and
ISEN pins. The ground connection from C2 to C1 should be
as short and direct as possible, preferably without going
through vias. Directly connect the SGND and RTN pin to
each other, and they should be connected as directly as
possible to the C1/C2 ground line without going through vias.
The power dissipation within the IC can be approximated by
determining the total conversion loss (P
IN
- P
OUT
), and then
subtracting the power losses in the free-wheeling diode and
the inductor. The power loss in the diode is approximately:
P
D1
= I
O
x V
F
x (1-D)
where Io is the load current, V
is the diode’s forward voltage
drop, and D is the duty cycle. The power loss in the inductor
is approximately:
P
L1
= I
O2
x R
L
x 1.1
where R
is the inductor’s DC resistance, and the 1.1 factor
is an approximation for the AC losses. If it is expected that
the internal dissipation of the LM25010 will produce high
junction temperatures during normal operation, good use of
the PC board’s ground plane can help considerably to dissi-
pate heat. The exposed pad on the IC package bottom
should be soldered to a ground plane, and that plane should
both extend from beneath the IC, and be connected to
exposed ground plane on the board’s other side using as
many vias as possible. The exposed pad is internally con-
nected to the IC substrate. The use of wide PC board traces
at the pins, where possible, can help conduct heat away
from the IC. The four No Connect pins on the TSSOP
package are not electrically connected to any part of the IC,
and may be connected to ground plane to help dissipate
heat from the package. Judicious positioning of the PC
board within the end product, along with the use of any
available air flow (forced or natural convection) can help
reduce the junction temperature.
20172749
FIGURE 11. Low Output Ripple Using Ripple Injection
L
www.national.com
15
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| LM25010_08 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:42V, 1.0A Step-Down Switching Regulator |
| LM25010EVAL | 功能描述:電源管理IC開發(fā)工具 LM25010 EVAL BOARD RoHS:否 制造商:Maxim Integrated 產(chǎn)品:Evaluation Kits 類型:Battery Management 工具用于評(píng)估:MAX17710GB 輸入電壓: 輸出電壓:1.8 V |
| LM25010MH | 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 |
| LM25010MH/NOPB | 功能描述:直流/直流開關(guān)調(diào)節(jié)器 42V,1.0A STEP-DOWN SWITCHING REG RoHS:否 制造商:International Rectifier 最大輸入電壓:21 V 開關(guān)頻率:1.5 MHz 輸出電壓:0.5 V to 0.86 V 輸出電流:4 A 輸出端數(shù)量: 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:PQFN 4 x 5 |
| LM25010MHX | 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 |
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