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RT6248B 参数 Datasheet PDF下载

RT6248B图片预览
型号: RT6248B
PDF下载: 下载PDF文件 查看货源
内容描述: [暂无描述]
分类和应用:
文件页数/大小: 17 页 / 274 K
品牌: RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
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RT6248B  
current, which can be calculated using the following  
equation :  
(with little stored charge in the output capacitors), and  
low duty cycle applications (which require high inductance  
to get reasonable ripple currents with high input voltages)  
increases the size of voltage variations in response to  
very quick load changes. Typically, load changes occur  
slowly with respect to the IC's switching frequency.  
However, some modern digital loads can exhibit nearly  
instantaneous load changes and the following section  
shows how to calculate the worst-case voltage swings in  
response to very fast load steps.  
2   
VOUT  
V
IN  
VOUT  
V
IN  
IL  
12  
IRMS  
(1  
)IOUT2   
The next step is to select a proper capacitor for RMS  
current rating. One good design uses more than one  
capacitor with low Equivalent Series Resistance (ESR) in  
parallel to form a capacitor bank. The input capacitance  
value determines the input ripple voltage of the regulator.  
The input voltage ripple can be approximately calculated  
using the following equation :  
The amplitude of the ESR step up or down is a function of  
the load step and the ESR of the output capacitor :  
VESR_STEP  IOUT RESR  
I
V  
V  
V
OUT  
V
IN  
OUT  
IN  
V  
(1  
)
IN  
C
f  
IN SW  
OUT  
The amplitude of the capacitive sag is a function of the  
load step, the output capacitor value, the inductor value,  
the input-to-output voltage differential, and the maximum  
duty cycle. The maximum duty cycle during a fast transient  
is a function of the on-time and the minimum off-time since  
the ACOTTM control scheme will ramp the current using  
on-times spaced apart with minimum off-times, which is  
as fast as allowed. Calculate the approximate on-time  
(neglecting parasitics) and maximum duty cycle for a given  
input and output voltage as :  
The typical operating circuit is recommended to use two  
10μF low ESR ceramic capacitors on the input.  
Output Capacitor Selection  
The IC is optimized for ceramic output capacitors and best  
performance will be obtained by using them. The total  
output capacitance value is usually determined by the  
desired output voltage ripple level and transient response  
requirements for sag (undershoot on positive load steps)  
and soar (overshoot on negative load steps).  
VOUT  
IN fSW  
tON  
tON  
and DMAX   
Output ripple at the switching frequency is caused by the  
inductor current ripple and its effect on the output  
capacitor's ESR and stored charge. These two ripple  
components are called ESR ripple and capacitive ripple.  
Since ceramic capacitors have extremely low ESR and  
relatively little capacitance, both components are similar  
in amplitude and both should be considered if ripple is  
critical.  
V
tON+ tOFF_MIN  
The actual on-time will be slightly longer as the IC  
compensates for voltage drops in the circuit, but we can  
neglect both of these since the on-time increases  
compensations for the voltage losses. Calculate the output  
voltage SAG as :  
2
L(IOUT  
)
VSAG  
2COUT (VIN(MIN) DMAX VOUT  
)
VRIPPLE VRIPPLE(ESR) VRIPPLE(C)  
VRIPPLE(ESR)  IL RESR  
The amplitude of the capacitive SOAR is a function of the  
load step, the output capacitor value, the inductor value  
IL  
VRIPPLE(C)  
8COUT fSW  
and the output voltage :  
2
L(I  
)
In addition to voltage ripple at the switching frequency,  
the output capacitor and its ESR also affect the voltage  
sag (undershoot) and soar (overshoot) when the load steps  
up and down abruptly. The ACOT transient response is  
very quick and output transients are usually small.  
However, the combination of small ceramic output  
capacitors (with little capacitance), low output voltages  
OUT  
V
SOAR  
2C  
V  
OUT  
OUT  
Copyright 2018 Richtek Technology Corporation. All rights reserved.  
©
is a registered trademark of Richtek Technology Corporation.  
DS6248B-01 May 2018  
www.richtek.com  
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