RT9758 datasheet(34/61 Pages) RICHTEK

34 / 61 page

RT9758

is a registered trademark of Richtek Technology Corporation.

www.richtek.com

DS9758-03

May 2023

Application Information

Richtek’s component specification does not include the following information in the Application Information section.

Thereby no warranty is given regarding its validity and accuracy. Customers should take responsibility to verify their

own designs and reserve suitable design margin to ensure the functional suitability of their components and systems.

Operation Principle

The cap divider topology relies on a smart wall adapter

to control the voltage and current of input in order to

charge. Based on the cap divider topology, the 4

MOSFETs (Q1~Q4) are used to charge and discharge

flying capacitor (CFLY) alternately. The simplified circuit

of cap divider is shown in Figure 1.

In period 1: When Q1 and Q3 are turned on and Q2 and

Q4 are turned off, the CFLY and COUT are in series

with VBUS. The BUS current is supplied to COUT

directly. During this period, the voltage of CFLY can be

expressed as equation 1 :

VCFLY = VBUS - VOUT ---- (1)

In period 2: When Q1 and Q3 are turned off and Q2 and

Q4 are turned on, the CFLY and COUT are in parallel.

The current of COUT is only supplied by CFLY. During

this period, the voltage of CFLY can be expressed as

equation 2 :

VCFLY = VOUT ---- (2)

If the equation 2 is substituted into equation 1, the

equation 1 can be expressed as equation 3 :

VOUT = VBUS / 2 ---- (3)

If the power dissipation of topology is ignored, the

output power can be expressed as equation 4 :

VOUT x IOUT = VBUS x IBUS ---- (4)

If the equation 3 is substituted into equation 4, the IOUT

can be expressed as equation 5:

IOUT = 2 x IBUS ---- (5)

According to the equations above, the output voltage is

half of the input voltage and the output current is twice

the input current in cap divider topology. For the

efficiency and output ripple improvement in application,

the dual phase cap divider topology with phase shift 180

degrees between phases are built in the RT9758.

COUT

CFLY

VBUS

CFLY

VBUS

IBUS

IOUT

COUT

IOUT

VBUS

VCFLY

Period 1

Period 2

COUT

Figure 1. Simplified Circuit of Cap Divider

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