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参数资料
| 型号： | LM22674QMRX-ADJ |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 0.9 A SWITCHING REGULATOR, 600 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | PLASTIC, PSOP-8 |
| 文件页数： | 14/14页 |
| 文件大小： | 560K |
| 代理商： | LM22674QMRX-ADJ |
Generally, calculation as well as simulation can only aid in
selecting good power stage components. A good design prac-
tice is to test for stability with load transient tests or loop
measurement tests. Application note AN-1889 shows how to
easily perform a loop transfer function measurement with only
an oscilloscope and a function generator.
Application Information
EXTERNAL COMPONENTS
The following design procedures can be used to design a non-
synchronous buck converter with the LM22674.
Inductor
The inductor value is determined based on the load current,
ripple current, and the minimum and maximum input voltage.
To keep the application in continuous current conduction
mode (CCM), the maximum ripple current, I
RIPPLE , should be
less than twice the minimum load current.
The general rule of keeping the inductor current peak-to-peak
ripple around 30% of the nominal output current is a good
compromise between excessive output voltage ripple and ex-
cessive component size and cost. When selecting the induc-
tor ripple current ensure that the peak current is below the
minimum current limit as given in the Electrical Characteris-
tics section. Using this value of ripple current, the value of
inductor, L, is calculated using the following formula:
where F is the switching frequency which is 500 kHz (typical).
This procedure provides a guide to select the value of the
inductor L. The nearest standard value will then be used in
the circuit.
Increasing the inductance will generally slow down the tran-
sient response but reduce the output voltage ripple amplitude.
Reducing the inductance will generally improve the transient
response but increase the output voltage ripple.
The inductor must be rated for the peak current, I
PK+, to pre-
vent saturation. During normal loading conditions, the peak
current occurs at maximum load current plus maximum ripple.
Under an overload condition as well as during load transients,
the peak current is limited to 700 mA typical (900 mA maxi-
mum). This requires that the inductor be selected such that it
can run at the maximum current limit and not only the steady
state current.
Depending on inductor manufacturer, the saturation rating is
defined as the current necessary for the inductance to reduce
by 30% at 20°C. In typical designs the inductor will run at
higher temperatures. If the inductor is not rated for enough
current, it might saturate and due to the propagation delay of
the current limit circuitry, the power supply may get damaged.
Input Capacitor
Good quality input capacitors are necessary to limit the ripple
voltage at the VIN pin while supplying most of the switch cur-
rent during on-time. When the switch turns on, the current into
the VIN pin steps to the peak value, then drops to zero at turn-
off. The average current into VIN during switch on-time is the
load current. The input capacitance should be selected for
RMS current, I
RMS, and minimum ripple voltage. A good ap-
proximation for the required ripple current rating necessary is
I
RMS > IOUT / 2.
Quality ceramic capacitors with a low ESR should be selected
for the input filter. To allow for capacitor tolerances and volt-
age effects, multiple capacitors may be used in parallel. If step
input voltage transients are expected near the maximum rat-
ing of the LM22674, a careful evaluation of ringing and pos-
sible voltage spikes at the VIN pin should be completed. An
additional damping network or input voltage clamp may be
required in these cases.
Usually putting a higher ESR electrolytic input capacitor in
parallel to the low ESR bypass capacitor will help to reduce
excessive voltages during a line transient and will also move
the resonance frequency of the input filter away from the reg-
ulator bandwidth.
Output Capacitor
The output capacitor can limit the output ripple voltage and
provide a source of charge for transient loading conditions.
Multiple capacitors can be placed in parallel. Very low ESR
capacitors such as ceramic capacitors reduce the output rip-
ple voltage and noise spikes, while higher value capacitors in
parallel provide large bulk capacitance for transient loading
and unloading. Therefore, a combination of parallel capaci-
tors, a single low ESR SP or Poscap capacitor, or a high value
of ceramic capacitor provides the best overall performance.
Output capacitor selection depends on application conditions
as well as ripple and transient requirements. Typically a value
of at least 100 F is recommended. An approximation for the
output voltage ripple is:
In applications with Vout less than 3.3V, where input voltage
may fall below the operating minimum of 4.5V, it is critical that
low ESR output capacitors are selected. This will limit poten-
tial output voltage overshoots as the input voltage falls below
device normal operation range.
If the switching frequency is set higher than 500 kHz, the ca-
pacitance value may not be reduced accordingly due to sta-
bility requirements. The internal compensation is optimized
for circuits with a 500 kHz switching frequency. See the in-
ternal compensation section for more details.
Cboot Capacitor
The bootstrap capacitor between the BOOT pin and the SW
pin supplies the gate current to turn on the N-channel MOS-
FET. The recommended value of this capacitor is 10 nF and
should be a good quality, low ESR ceramic capacitor.
It is possible to put a small resistor in series with the Cboot
capacitor to slow down the turn-on transition time of the in-
ternal N-channel MOSFET. Resistors in the range of 10
to
50
can slow down the transition time. This can reduce EMI
of a switched mode power supply circuit. Using such a series
resistor is not recommended for every design since it will in-
crease the switching losses of the application and makes
thermal considerations more challenging.
Resistor Divider
For the -5.0 option no resistor divider is required for 5V output
voltage. The output voltage should be directly connected to
the FB pin. Output voltages above 5V can use the -5.0 option
with a resistor divider as an alternative to the -ADJ option.
This may offer improved loop bandwidth in some applications.
See the Internal Compensation section for more details.
For the -ADJ option no resistor divider is required for 1.285V
output voltage. The output voltage should be directly con-
9
www.national.com
LM22674/LM22674Q
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM22674QMRX-ADJ/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 |
| LM22675 | 制造商:NSC 制造商全称:National Semiconductor 功能描述:1A SIMPLE SWITCHER?, Step-Down Voltage Regulator with Precision Enable |
| LM22675MR-5.0 | 制造商:NSC 制造商全称:National Semiconductor 功能描述:1A SIMPLE SWITCHER?, Step-Down Voltage Regulator with Precision Enable |
| LM22675MR-5.0/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 |
| LM22675MR-ADJ | 制造商:Texas Instruments 功能描述:REGULATOR, ADJ. VOLTAGE, 1A 8PSOP 制造商:Texas Instruments 功能描述:REGULATOR, ADJ. VOLTAGE, 1A 8PSOP; Primary Input Voltage:42V; No. of Outputs:1; Output Current:1A; Voltage Regulator Case Style:SOP; No. of Pins:8; Operating Temperature Min:-40C; Operating Temperature Max:125C; SVHC:No SVHC ;RoHS Compliant: Yes |
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