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参数资料
| 型号： | LM25011AQ1MY |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING REGULATOR, 2000 kHz SWITCHING FREQ-MAX, PDSO10 |
| 封装： | MSOP-10 |
| 文件页数： | 6/20页 |
| 文件大小： | 458K |
| 代理商： | LM25011AQ1MY |
inductor is rated for this peak current. The minimum ripple
current, which occurs at minimum V
IN, calculates to 200 mAp-
p.
R
S: The minimum current limit threshold is calculated at max-
imum load current, using the minimum ripple current calcu-
lated above. The current limit threshold is the lower peak of
the inductor current waveform when in current limit (see Fig-
ure 2).
I
LIM = 1.5A – (0.2 A/2) = 1.4A
Current limit detection occurs when the voltage across the
sense resistor (R
S) reaches the current limit threshold. To al-
low for tolerances, the sense resistor value is calculated using
the minimum threshold specification:
R
S = 115 mV/1.4A = 82 m
The next smaller standard value, 80 m
, is selected. The next
step is to ensure that sufficient ripple voltage occurs across
R
S with this value sense resistor. As mentioned in the Ripple
Requirements section, a minimum of 15 mVp-p voltage ripple
is required across the R
S sense resistor during the off-time to
ensure the regulation circuit operates properly. The ripple
voltage is the product of the inductor ripple current amplitude
and the sense resistor value. In this case, the minimum ripple
voltage calculates to:
V
RIPPLE = ΔI x RS = 200 mA x 0.080 = 16 mV
If the ripple voltage had calculated to less than 15 mVp-p the
inductor value would have to be reduced to increase the ripple
current amplitude. This would have required a recalculation
of I
LIM and RS in the above equations. Since the minimum
requirement is satisfied in this case no change is necessary.
The nominal current limit threshold calculates to 1.63A. The
minimum and maximum thresholds calculate to 1.44A and
1.83A respectively, using the minimum and maximum limits
for the current limit threshold specification. The load current
is equal to the threshold current plus one half the ripple cur-
rent. Under normal load conditions, the maximum power dis-
sipation in R
S occurs at maximum load current, and at
maximum input voltage where the on-time duty cycle is min-
imum. In this design example, the minimum on-time duty
cycle is:
At maximum load current, the power dissipation in R
S is equal
to:
P
(RS) = (1.5A)
2
x 0.080
x (1 – 0.139) = 155 mW
When in current limit the maximum power dissipation in R
S
calculates to
P
(RS) = (1.83A + 0.472A/4)
2
x 0.080
= 304 mW
Duty cycle is not included in this power calculation since the
on-time duty cycle is typically <5% when in current limit.
C
OUT: The output capacitor should typically be no smaller than
3.3 F, although that is dependent on the frequency and the
desired output characteristics. C
OUT should be a low ESR
good quality ceramic capacitor. Experimentation is usually
necessary to determine the minimum value for C
OUT, as the
nature of the load may require a larger value. A load which
creates significant transients requires a larger value for
C
OUT than a non-varying load.
C
IN and CBYP: The purpose of C
IN is to supply most of the
switch current during the on-time, and limit the voltage ripple
at V
IN, since it is assumed the voltage source feeding VIN has
some amount of source impedance. When the buck switch
turns on, the current into V
IN suddenly increases to the lower
peak of the inductor’s ripple current, then ramps up to the up-
per peak, then drops to zero at turn-off. The average current
during the on-time is the average load current. For a worst
case calculation, C
IN must supply this average load current
during the maximum on-time, without letting the voltage at the
VIN pin drop below a minimum operating level of 5.5V. For
this exercise 0.5V is chosen as the maximum allowed input
ripple voltage. Using the maximum load current, the minimum
value for C
IN is calculated from:
(9)
where t
ON is the maximum on-time, and ΔV is the allowable
ripple voltage at V
IN. The purpose of CBYP is to minimize tran-
sients and ringing due to long lead inductance leading to the
VIN pin. A low ESR 0.1 F ceramic chip capacitor is recom-
mended, and C
BYP must be located close to the VIN and
SGND pins.
C
BST: The recommended value for C
BST is 0.1 F. A high
quality ceramic capacitor with low ESR is recommended as
C
BST supplies a surge current to charge the buck switch gate
at each turn-on. A low ESR also helps ensure a complete
recharge during each off-time.
C
SS: The capacitor at the SS pin determines the soft-start
time, i.e. the time for the output voltage to reach its final value
(t1 in Figure 1). For a soft-start time of 5 ms, the capacitor
value is determined from the following:
D1:
A Schottky diode is recommended. Ultra-fast recovery
diodes are not recommended as the high speed transitions at
the SW pin may affect the regulator’s operation due to the
diode’s reverse recovery transients. The diode must be rated
for the maximum input voltage, the maximum load current,
and the peak current which occurs when the current limit and
maximum ripple current are reached simultaneously. The
diode’s average power dissipation is calculated from:
P
D1 = VF x IOUT x (1 - D)
where V
F is the diode’s forward voltage drop, and D is the on-
time duty cycle.
FINAL CIRCUIT
The final circuit is shown in Figure 5, and its performance is
shown in Figure 6 and Figure 7. The current limit measured
approximately 1.62A at Vin = 8V, and 1.69A at Vin = 36V.
www.national.com
14
LM25011/LM25011QLM25011A/LM25011AQ
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM25011AQ1MY/NOPB | 功能描述:直流/直流开关转换器 RoHS:否 制造商:STMicroelectronics 最大输入电压:4.5 V 开关频率:1.5 MHz 输出电压:4.6 V 输出电流:250 mA 输出端数量:2 最大工作温度:+ 85 C 安装风格:SMD/SMT |
| LM25011AQ1MYX | 制造商:TI 制造商全称:Texas Instruments 功能描述:42V 2A Constant On-Time Switching Regulator With Adjustable Current Limit |
| LM25011AQ1MYX/NOPB | 功能描述:直流/直流开关转换器 RoHS:否 制造商:STMicroelectronics 最大输入电压:4.5 V 开关频率:1.5 MHz 输出电压:4.6 V 输出电流:250 mA 输出端数量:2 最大工作温度:+ 85 C 安装风格:SMD/SMT |
| LM25011MY | 制造商:NSC 制造商全称:National Semiconductor 功能描述:42V, 2A Constant On-Time Switching Regulator with Adjustable Current Limit |
| LM25011MY/NOPB | 功能描述:直流/直流开关转换器 42V,2A CONSTANT ON- TIME SWITCHING REG RoHS:否 制造商:STMicroelectronics 最大输入电压:4.5 V 开关频率:1.5 MHz 输出电压:4.6 V 输出电流:250 mA 输出端数量:2 最大工作温度:+ 85 C 安装风格:SMD/SMT |
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