RT4723 datasheet(11/15 Pages) RICHTEK

11 / 15 page

RT4723

is a registered trademark of Richtek Technology Corporation.

DS4723-00

April

2016

www.richtek.com

Application Information

The RT4723 is a highly integrated Boost, LDO and

inverting charge pump to generate positive and

negative output voltages for AMLOED bias. It can

support input voltage range from 2.5V to 4.6V and the

output current up to 30mA. The VOP positive output

voltage is generated from the LDO supplied from a

synchronous Boost converter, and VOP is set at a

typical value of 4.6V. The Boost converter output also

drives an inverting charge pump controller to generate

VON negative output voltage which is set at a typical

value of

2.4V. The negative output voltage can be

programmed

through

the

dedicated

which

implements single wire protocol and the available

voltage range is from

0.6V to 2.4V with 100mV per

step.

Input Capacitor Selection

Input ceramic capacitor with 4.7

F capacitance is

suggested for applications. For better voltage filtering,

select ceramic capacitors with low ESR, X5R and X7R

types are suitable because of their wider voltage and

temperature ranges.

Boost Inductor Selection

The inductance depends on the maximum input current.

As a general rule, the inductor ripple current range is

20% to 40% of the maximum input current. If 40% is

selected as an example, the inductor ripple current can

be calculated according to the following equations :

OUT

OUT(MAX)

IN(MAX)

I = 0.4 I







here η is the efficiency of the VOP Boost converter,

IIN(MAX) is the maximum input current, and IL is the

inductor ripple current. The input peak current can then

be obtained by adding the maximum input current with

half of the inductor ripple current as shown in the

following equation :

IPEAK = 1.2 x IIN(MAX)

Note that the saturated current of the inductor must be

greater than IPEAK.

The

inductance

can

eventually

determined

according to the following equation :

  







OUT

OUT(MAX)

OSC

0.4









where fOSC is the switching frequency. For better

system performance, a shielded inductor is preferred to

avoid EMI problems.

Boost Output Capacitor Selection

The output ripple voltage is an important index for

estimating IC performance. This portion consists of two

parts. One is the product of ripple current with the ESR

of the output capacitor, while the other part is formed

by the charging and discharging process of the output

capacitor. As shown in Figure 1,

VOUT1 can be

evaluated based on the ideal energy equalization.

According to the definition of Q, the

VOUT1 value can

be calculated as the following equation :

OUT

OUT1

SOC

OUT

OUT1

SOC

OUT

Q = I

= C

 

 







where fOSC is the switching frequency and D is the duty

cycle.

Finally, taking ESR into consideration, the overall

output ripple voltage can be determined by the

following equation :

OUT

ESR

OUT1

ESR

OSC

OUT

= V

+ V

= V







where

VESR = ICrms x RCESR

The output capacitor, COUT, should be selected

accordingly.

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