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RT5047

Single Output LNB Supply and Control Voltage Regulator

Richtek Technology 

RT5047

where D is the duty cycle and η is the boost converter

efficiency. Finally, take ESR into consideration, the

overall output ripple voltage can be determined by the

following equation :

VOUT

=

IIN



ESR







D  IOUT

COUT  fOSC

The output capacitor, COUT, should be selected

accordingly.

Input Current

ΔIL

Inductor Current

ΔVOUT1

Output Current

Time

(1-D)TS

Output Ripple

Voltage (ac)

Time

Figure 1. The Output Ripple Voltage without the

Contribution of ESR

Schottky Diode Selection

Schottky diodes are chosen for their low

forward-voltage drop and fast switching speed.

However, when making a selection, important

parameters such as power dissipation, reverse voltage

rating, and pulsating peak current should all be taken

into consideration. A suitable Schottky diode’s reverse

voltage rating must be greater than the maximum

output voltage and its average current rating must

exceed the average output current. The chosen diode

should also have a sufficiently low leakage current level,

since it increases with temperature.

Under-Voltage Lockout (UVLO)

The UVLO circuit compares the input voltage at VIN

with the UVLO threshold (7.7V rising typically) to

ensure that the input voltage is high enough for reliable

operation. The 350mV (typ.) hysteresis prevents supply

transients from causing a shutdown. Once the input

voltage exceeds the UVLO rising threshold, start-up

begins. When the input voltage falls below the UVLO

falling threshold, all IC internal functions will be turned

off by the controller.

Over-Current Protection

The RT5047 features an over-current protection

function to prevent chip damage from high peak

currents. Both the boost converter and the linear

regulator have independent current limit. In the boost

converter, this is achieved through cycle-by-cycle

internal current limit. During the ON-period, the chip

senses the inductor current that is flowing into the LX

pin. The internal NMOS will be turned off if the peak

inductor current reaches the current-limit value of 3A

(typ.).When the linear regulator exceeds 550mA (typ.)

more than 45ms, the LNB output will be disabled.

During this period of time, if the current limit condition

disappears, the OCP will be cleared and the part

restarts. If the part is still in current limit after this time

period, the linear regulator and boost converter will

automatically disable to prevent the part from

overheating.

Short Circuit Protection

If the LNB output is shorted to ground, and more than

45ms, the RT5047 will be disabled 1.8s then enable

automatically.

Over-Temperature Protection

When the junction temperature reaches the critical

temperature (typically 140 oC), the boost converter and

the linear regulator are immediately disabled. When the

junction temperature cools down to a lower

temperature threshold specified, the RT5047 will be

allowed to restart by normal start operation.

LNB Output Voltage

The RT5047 has voltage control function on the LNB

output. This function provides 4 levels for the common

standards and compensation if the cable line has

voltage drop. These voltage levels are defined in table

1. The rise time and fall time of the VLNB is 3mS (typ.).

Copyright © 2015 Richtek Technology Corporation. All rights reserved.

www.richtek.com

12

is a registered trademark of Richtek Technology Corporation.

DS5047-00 March 2015

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