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RT5047B Fiches technique(PDF) 11 Page - Richtek Technology Corporation

No de pièce RT5047B
Description  Single Output LNB Supply and Control Voltage Regulator
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Fabricant  RICHTEK [Richtek Technology Corporation]
Site Internet  http://www.richtek.com
Logo RICHTEK - Richtek Technology Corporation

RT5047B Fiches technique(HTML) 11 Page - Richtek Technology Corporation

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RT5047B
Copyright © 2017 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS5047B-00
November
2017
www.richtek.com
11
Application Information
Boost Converter/Linear Regulator
The 5047B integrates a current-mode boost converter
and linear regulator. Use the SEL pin to control the
LNB voltage and the boost converter track is at least
greater 800mV than the LNB voltage. The boost
converter is high efficiency PWM architecture with
700kHz operation frequency. The linear regulator has
the capability to source current up to 550mA during
continuous operation. All the loop compensation,
current sensing, and slope compensation functions are
provided internally.
The RT5047B has current limiting on the boost
converter and the LNB output to protect the IC against
short circuits. The internal MOSFET will turn off when
the LX current is higher than 3A cycle-by-cycle. The
LNB output will turn off when the output current higher
than the 550mA and 6ms and turn-on after 1800ms
automatically.
Input Capacitor Selection
The input capacitor reduces voltage spikes from the
input supply and minimizes noise injection to the
converter. A 30
F capacitance is sufficient for most
applications. Nevertheless, a higher or lower value may
be used depending on the noise level from the input
supply and the input current to the converter. Note that
the voltage rating of the input capacitor must be greater
than the maximum input voltage.
Inductor Selection
The inductance depends on the maximum input current.
As a general rule, the inductor ripple current range is
20% to 40% of the maximum input current. If 40% is
selected as an example, the inductor ripple current can
be calculated according to the following equations :
OUT
OUT(MAX)
IN(MAX)
IN
RIPPLE
IN(MAX)
VI
I
=
V
I
= 0.4 I
where η is the efficiency of the converter, IIN(MAX) is
the maximum input current, and IRIPPLE is the
inductor ripple current. The input peak current can
then be obtained by adding the maximum input current
with half of the inductor ripple current as shown in the
following equation :
IPEAK = 1.2 x IIN(MAX)
note that the saturated current of the inductor must be
greater than IPEAK. The inductance can eventually be
determined according to the following equation :
  
2
2
IN
OUT
IN
OUT
OUT(MAX)
OSC
η
V
V
V
L
0.4
V
I
f
where fOSC is the switching frequency. For better
system performance, a shielded inductor is preferred to
avoid EMI problems.
Boost Output Capacitor Selection
The
RT5047B
boost
regulator
is
internally
compensated and relies on the inductor and output
capacitor value for overall loop stability. The output
capacitor is in the 30
F to 50F range with a low ESR,
as strongly recommended. The voltage rating on this
capacitor should be in the 25V to 35V range since it is
connected to the boost VOUT rail.
The output ripple voltage is an important index for
estimating chip performance. This portion consists of
two parts. One is the product of the inductor current
with the ESR of the output capacitor, while the other
part is formed by the charging and discharging process
of the output capacitor. As shown in Figure 1,
VOUT1
can
be
evaluated
based
on
the
ideal
energy
equalization. According to the definition of Q, the Q
value can be calculated as the following equation :
IN
L
OUT
IN
L
OUT
IN
OUT
OUT1
OUT
OSC
1
1
1
Q =
I
I
I
I
I
I
2
2
2
V
1
= C
V
Vf

 
  
  
 

 

 
where fOSC is the switching frequency and IL is the
inductor ripple current. Bring COUT to the left side to
estimate the value of
VOUT1 according to the following
equation :
OUT
OUT1
OUT
OSC
DI
V
=
Cf



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