RT5047BF

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is a registered trademark of Richtek Technology Corporation.

www.richtek.com

DS5047BF-00

December

2017

12

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 :

OUT

OUT

IN

OUT

OSC

DI

V

= I

ESR

Cf













accordingly.

Time

Time

Inductor Current

Output Current

Output Ripple

Voltage (ac)

Input Current

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 s

uitable 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 RT5047BF 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.

(1) Boost

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.).

(2) LNB

When the linear regulator exceeds 550mA (typ.)

more than 6ms, 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

6ms, the RT5047BF will be disabled 1800ms then

enable automatically.

Over-Temperature Protection

When the junction temperature reaches the critical

temperature (typically 140°C), the boost converter and

the linear regulator are immediately disabled. When the

junction

temperature

cools

down

to

a

lower

temperature threshold specified, the RT5047BF will be

allowed to restart by normal start operation.

FAULT

FAULT is a open-drain input. If it is 0, it stands for fault

occurred. The FAULT provide the information to show

over-current and short circuit protection of the system.