RT4813

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DS4813-00

April

2016

14

Input Capacitor Selection

At least two capacitor and capacitance is 22

F with

rating voltage is 16V for DC bias 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











 

 









 









voltage and maximum output load.

Output Discharge Function

With the EN pin set to low, the VOUT pin is internally

connected to GND for 10ms by an internal discharge

N-MOSFET switch. After the 10ms, IC will be

true-shut down.

This feature prevents residual charge voltages on

capacitor connected to VOUT pins, which may impact

proper power up of the system.

Valley Current Limit

RT4813 employs a valley-current limit detection

scheme to sense inductor current during the off-time.

When the loading current is increased such that the

loading is above the valley current limit threshold, the

off-time is increased until the current is decreased to

valley-current threshold. Next on-time begins after

current is decreased to valley-current threshold.

output voltage decreases when further loading

current

increase.

The

current

limit function is

implemented by the scheme, refer to Figure 2.

Average Output Current Limit

RT4813 features the average output current limit to

protect the output terminal. When the load current is

over the limit, output current will be clamped.

Valley Current Limit

Inductor Current

V

D

IN

I =

L

Lf



f

Figure 2. Inductor Currents In Current Limit Operation