RT4841

11

DS4841-00 May 2021

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Copyright

2021 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

Application Information

The RT4841 is a 5A Synchronous Boost Converter with

Output Isolation MOS. It is a synchronous current mode

boost converter integrated with a 22V/5A isolation power

switch, covering a wide VIN range from 8.6V to 15.9V. It

performs high efficiency and fast transient responses to

generate source driver supplies for TFT LCD display. The

synchronous step-up converter uses a current mode, pulse

width modulation (PWM) topology with a built in N-

MOSFET to achieve high efficiency and fast transient

response in TFT-LCD applications. The following content

contains detailed description and information for

component selection.

Boost Regulator

The RT4841 is a synchronous current mode boost

converter integrated with a 22V/5A power switch, covering

a wide VIN range from 8.6V to 15.9V. It performs high

efficiency and fast transient responses to generate source

driver supplies for TFT LCD display . The high operation

frequency allows use of smaller components to minimize

the thickness of the LCD panel. The output voltage can be

adjusted by setting the resistive voltage-divider sensing

at the FB pin. The error amplifier varies the COMP voltage

by sensing the FB pin to regulate the output voltage. For

better stability, the slope compensation signal summed

with the current sense signal will be compared with the

COMP voltage to determine the current trip point and duty

cycle.

Soft-Start

The RT4841 provides soft-start function to minimize the

inrush current. When powered on, an internal constant

current charges an external capacitor. The rising voltage

rate on the COMP pin is limited from VSS = 0V to 1.24V

and the inductor peak current will also be limited at the

same time. When powered off, the external capacitor will

be discharged until the next soft-start time.

The soft-start function is implemented by the external

capacitor with a 4

μA constant current charging to the soft-

start capacitor. Therefore, the capacitor should be large

enough for output voltage regulation. A typical value for

soft-start capacitor is 47nF.

Output Voltage Setting

The regulated output voltage is shown as the following

equation :

OUT

REF

REF

R1 R2 R3

V

= V

x

,

R3

where V

= 1.25V (typ.)









The recommended value for R3 should be at least 10k

Ω

without some sacrificing. Place the resistive voltage divider

as close as possible to the chip to reduce noise sensitivity.

Over Current Protection

The RT4841 includes a current sensing circuitry which

monitors the inductor peak current during each ON period.

If the peak current crosses the current limit threshold of

5A (typical), the on cycle is terminated immediately and

forcing the inductor to leave charging stage and enter

discharge stage. Therefore, the inductor current can be

kept below the current limit. The next cycle resumes at

the next clock pulse.

Over Temperature Protection

The RT4841 step-up converter has thermal protection

function to prevent the chip from overheating. When the

junction temperature exceeds 150

°C, the function shuts

down the device. Once the device cools down by

approximately 15

°C, it will automatically restart to normal

operation. To guarantee continuous operation, do not

operate over the maximum junction temperature rating of

125

°C.

Short Circuit Protection

To limit the short circuit current, the IC has a voltage detect

device. If VIN power on that SWO voltage lower than 4V

after 5ms, then the IC will shut down.