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参数资料
| 型号： | LM2797MM-1.8/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 0.12 A SWITCHED CAPACITOR REGULATOR, PDSO10 |
| 封装： | MSOP-10 |
| 文件页数： | 4/14页 |
| 文件大小： | 388K |
| 代理商： | LM2797MM-1.8/NOPB |
Application Information (Continued)
Capacitors with a Y5V or Z5U temperature characteristic are
generally not recommended for use with the LM2797/98.
These types of capacitors typically have wide capacitance
tolerance (+80%, -20%) and vary significantly over tempera-
ture (Y5V: +22%, -82% over -30oCto+85oC range; Z5U:
+22%, -56% over +10oCto+85oC range). Under some con-
ditions, a 1F-rated Y5V or Z5U capacitor could have a
capacitance as low as 0.1F. Such detrimental deviation is
likely to cause these Y5V and Z5U capacitors to fail to meet
the minimum capacitance requirements of the LM2797/98.
The table below lists some leading ceramic capacitor manu-
facturers.
Manufacturer
Contact Information
AVX
www.avx.com
Murata
www.murata.com
Taiyo-Yuden
www.t-yuden.com
TDK
www.component.tdk.com
Vishay-Vitramon
www.vishay.com
OUTPUT CAPACITOR
The output capacitor of the LM2797/98 greatly affect perfor-
mance of the circuit. In typical high-current applications, a
10F low-ESR (ESR = equivalent series resistance) ceramic
capacitor is recommended. For lighter loads, the output
capacitance may be reduced (as low as 1F for output
currents
≤ 60mA is usually acceptable). The performance of
the part should be evaluated with special attention paid to
efficiency and output ripple to ensure the capacitance cho-
sen on the output yields performance suitable for the appli-
cation. In extreme cases, excessive ripple could cause con-
trol loop instability, severely affecting the performance of the
part. If excessive ripple is present, the output capacitance
should be increased.
The ESR of the output capacitor affects charge pump output
resistance, which plays a role in determining output current
capability. Both output capacitance and ESR affect output
voltage ripple (See Output Voltage Ripple section, above).
For these reasons, a low-ESR X7R/X5R ceramic capacitor is
the capacitor of choice for the LM2797/98 output.
FLYING CAPACITORS
The flying capacitors (C
1 and C2) transfer charge from the
input to the output, and determine the strength of the charge
pump: the larger the capacitance, the greater the output
current capability. If capacitors are too small, the LM2797/98
could spend excessive amount of time gain hopping: de-
creasing efficiency, increasing output voltage ripple, and
possibly impeding the ability of the part to regulate. On the
other hand, if the flying capacitors are too large they could
potentially overwhelm the output capacitor, resulting in in-
creased output voltage ripple.
Low-ESR ceramic capacitors with X7R or X5R temperature
characteristic are strongly recommended for use here. The
flying capacitors C1 and C2 should be identical. As a general
rule, the capacitance value of each flying capacitor should
be 1/10th that of the output capacitor. ESR should be as low
as possible to minimize the output resistance of the charge
pump and give maximum output current capability. Polarized
capacitor (tantalum, aluminum electrolytic, etc.) must not be
used for the flying capacitors, as they could become reverse-
biased upon start-up of the LM2797/98.
INPUT CAPACITOR
The input capacitor (C
IN) is a reservoir of charge that aids a
quick transfer of charge from the supply to the flying capaci-
tors during the charge phase of operation. The input capaci-
tor helps to keep the input voltage from drooping at the start
of the charge phase when the flying capacitor is connected
to the input, and helps to filter noise on the input pin that
could adversely affect sensitive internal analog circuitry bi-
ased off the input line. An X7R/X5R ceramic capacitor is
recommended for use. As a general recommendation, the
input capacitor should be chosen to match the output capaci-
tor.
POWER DISSIPATION
LM2797/98 power dissipation will, typically, not be much of a
concern in most applications. Derating to accommodate self-
heating will rarely be required due to the high efficiency of
the part. Peak power dissipation (P
D) of all LM2797/98 op-
tions is seen with the LM2798-1.5 operating at V
IN = 5.5V
and I
OUT = 120mA (conditions limited to valid operating
ratings). Under these conditions, the power efficiency (E) of
the LM2798-1.5 is 54% (typ.). Assuming a typical junction-
to-ambient thermal resistance (
θ
JA) for the MSOP package
of 220C/Watt, the junction temperature (T
J) of the part is
calculated below for a part operating at the maximum rated
ambient temperature (T
A) of 85C.
P
D =PIN -POUT
=(P
OUT/E)-POUT
= [(1/E) - 1] x P
OUT
= [(1/64%) - 1] x 1.5V x 120mW
= 153mW
T
J =TA =(PD x
θ
JA)
= 85C + (.153W x 220C/W)
=119C
Even under these peak power dissipation and ambient tem-
perature conditions, the junction temperature of the LM2798-
1.5 is below the maximum operating rating of 125C.
As an additional note, the ambient temperature operating
rating range listed in the specifications is provided merely for
convenience. The LM2797/98 may be operated outside this
rating, so long as the junction temperature of the device
does not exceed the maximum operating rating of 125C.
LM2797/LM2798
www.national.com
12
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM2797MMX-1.8 | 功能描述:IC REG BUCK SWITCHED CAP 10MSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 稳压器 - DC DC 开关稳压器 系列:- 产品培训模块:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 标准包装:1 系列:- 类型:降压（降压） 输出类型:固定 输出数:1 输出电压:3.3V 输入电压:4.5 V ~ 24 V PWM 型:- 频率 - 开关:- 电流 - 输出:125mA 同步整流器:无 工作温度:-40°C ~ 85°C 安装类型:表面贴装 封装/外壳:SOT-23-6 包装:Digi-Reel® 供应商设备封装:SOT-6 其它名称:MAX1836EUT33#TG16DKR |
| LM2798 | 制造商:NSC 制造商全称:National Semiconductor 功能描述:120mA High Efficiency Step-Down Switched Capacitor Voltage Converter with Voltage Monitoring |
| LM2798C-1.5 WAF | 制造商:Texas Instruments 功能描述: |
| LM2798C-1.8 WAF | 制造商:Texas Instruments 功能描述: |
| LM2798C-2.0 WAF | 制造商:Texas Instruments 功能描述: |
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