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 :
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.
DI
OUT
V
= I ESR
IN
OUT
C
f
OUT OSC
Over-Current Protection
The output capacitor, COUT, should be selected
accordingly.
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.
ΔI
L
Input Current
Inductor Current
Output Current
Time
(1-D)T
S
Output Ripple
Voltage (ac)
Time
ΔV
OUT1
Figure 1. The Output Ripple Voltage without the
Contribution of ESR
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.
Schottky Diode Selection
Schottky diodes are chosen for their low
forward-voltage drop and fast switching speed.
Over-Temperature Protection
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.
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.).
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
Copyright © 2017 Richtek Technology Corporation. All rights reserved.
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12
DS5047-02 July 2017
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
