RT4805A

Copyright © 2017 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

DS4805A-02

April

2017

www.richtek.com

13

Application Information

Enable

The device can be enabled or disabled by the EN pin.

When the EN pin is higher than the threshold of

logic-high, the device starts operating with soft-start.

Once the EN pin is set at low, the device will be shut

down. In shutdown mode, the converter stops

switching, internal control circuitry is turned off, and

the load is disconnected from the input. This also

means that the output voltage can drop below the

input voltage during shutdown.

Soft-Start State

the Soft-Start period for more than 2ms, a fault

condition is declared.

Output Voltage Setting

User can select the output voltage level by VSEL and

I2C. If the VSEL pulled low, the default is 3.6V, and if

it pulled high, the default is 3.7V.

The output voltage range is from 2.85V to 4.4V.

Power Save Mode

PSM is the way to improve efficiency at light load.

When the output voltage is lower than a set threshold

voltage, the converter will operate in PSM.

It raises the output voltage with several pulses until

the loop exits PSM.

Under-Voltage Lockout

The under-voltage lockout circuit prevents the device

from operating incorrectly at low input voltages. It

prevents the converter from turning on the power

switches under undefined conditions and prevents the

battery from deep discharge. VIN voltage must be

greater than 1.7V to enable the converter. During

operation, if VIN voltage drops below 1.6V, the

converter is disabled until the supply exceeds the

UVLO rising threshold. The RT4805A automatically

restarts if the input voltage recovers to the input

voltage UVLO high level.

Thermal Shutdown

The device has a built-in temperature sensor which

monitors the internal junction temperature. If the

temperature exceeds the threshold, the device stops

operating. As soon as the IC temperature has

decreased below the threshold with a hysteresis, it

starts operating again. The built-in hysteresis is

designed

to

avoid

unstable

operation

at

IC

temperatures near the over temperature threshold.

Inductor Selection

The recommended nominal inductance value is

1.5

H.

It is recommended to use inductor with dc saturation

current

≥ 3500mA

Input Capacitor Selection

At least a 10

F input capacitor is recommended to

improve transient behavior of the regulator and EMI

behavior of the total power supply circuit for LX. And

at least a 1

F ceramic capacitor placed as close as

possible to the VIN and GND pins of the IC is

recommended.

Output Capacitor Selection

At least 22

F x 2 capacitors is recommended to

Output voltage ripple is inversely proportional to

Output capacitor is selected according to output ripple

which is calculated as :

LOAD

RIPPLE(P P)

ON

OUT

IN

ON

SW

SW

OUT

IN

LOAD

OUT

SW

OUT

RIPPLE(P P)

SW

SW

I

Vt

C

and

V

t

t

D

t

1

V

therefore :

VI

C

t

1

VV

and

1

t

f











 

 









 







