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RT5047

Single Output LNB Supply and Control Voltage Regulator

Richtek Technology 

Application Information

Boost Converter/Linear Regulator

The 5047 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 RT5047 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 45ms 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 :

IIN(MAX)

=

VOUT IOUT(MAX)

  VIN

IRIPPLE = 0.4 IIN(MAX)

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

Copyright © 2015 Richtek Technology Corporation. All rights reserved.

DS5047-00 March 2015

RT5047

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 :

L



η VIN 2  VOUT  VIN 

0.4   VOUT 2 IOUT(MAX)fOSC

where fOSC is the switching frequency. For better

system performance, a shielded inductor is preferred to

avoid EMI problems.

Boost Output Capacitor Selection

The RT5047 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 :

Q=

1

2



IIN



1

2

IL

 IOUT





IIN



1

2

IL

 IOUT





VIN

VOUT



1

fOSC

= COUT  VOUT1

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 :

VOUT1

=

D  IOUT

  COUT  fOSC

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