RT9535B datasheet(13/18 Pages) RICHTEK

13 / 18 page

RT9535B

is a registered trademark of Richtek Technology Corporation.

DS9535B-04

February

2016

www.richtek.com

Application Information

Input and Output Capacitors

In the typical application circuit, the input capacitor (C2)

is assumed to absorb all input switching ripple current

in the converter, so it must have adequate ripple

current rating. Typically, at high charging currents, the

converter will operate in continuous conduction mode.

In this case, the RMS current IRMSIN of the input

capacitor C2 can be estimated by the equation:

RMSIN

BATT

= I

D D



Where IBATT is the battery charge current and D is the

duty cycle. In worst case, the RMS ripple current will be

equal to one half of output charging current at 50% duty

cycle. For example, IBATT = 2A, the maximum RMS

current will be 1A. A low-ESR ceramic capacitor such

as X7R or X5R is preferred for the input-decoupling

capacitor and should be placed to the drain of the

high-side MOSFET and source of

the low-side

MOSFET as close as possible. The voltage rating of

the capacitor must be higher than the normal input

voltage level. Above 20

F capacitance is suggested for

typical of 2A charging current.

The output capacitor (CBATT) is also assumed to

absorb output switching current ripple. The general

formula for capacitor current is :

BATT

VIN

RMSCB

osc

3 L1 f











 

For example, VVIN = 19V, VBATT = 8.4V, L1 = 10H,

and fOSC = 475kHz, IRMS = 0.15A.

EMI considerations usually make it desirable to

minimize ripple current in the battery leads. Beads or

inductors may be added to increase battery impedance

at the 475kHz switching frequency. Switching ripple

current splits between the battery and the output

capacitor depending on the ESR of the output capacitor

and the battery impedance. If the ESR of COUT is 0.2

and the battery impedance is raised to 4

 with a bead

or inductor, only 5% of the ripple current will flow in the

battery.

Inductor

The inductor value will be changed for more or less

current ripple. The higher the inductance, the lower the

current ripple will be. As the physical size is kept the

same, typically, higher inductance will result in higher

series resistance and lower saturation current. A good

tradeoff is to choose the inductor so that the current

ripple is approximately 30% to 50% of the full-scale

charge current. The inductor value is calculated as :





BATT

VIN

BATT

VIN

OSC

L1 =



 Δ

Where

IL is the inductor current ripple. For example,

VVIN = 19V, choose the inductor current ripple to be

40% of the full-scale charge current in the typical

application circuit for 2A, 2-cell battery charger,

IL =

0.8A, VBATT = 8.4V, calculate L1 to be 12H. So

choose L1 to be 10

H which is close to 12H.

Soft-Start and Under-Voltage Lockout

The soft-start is controlled by the voltage rise time at

VC pin. There are internal soft-start and external

soft-start in the RT9535B. With a 1

F capacitor, time to

reach full charge current is about 60ms and it is

assumed that input voltage to the charger will reach full

value in less than 60ms. The capacitor can be

increased if longer input start-up times are needed.

For the RT9535B, it provides Under-Voltage Lockout

(UVLO) protection. If LDO output voltage is lower than

3.9V, the internal top side power FET and input power

FET M1 will be cut off. This will protect the adapter from

entering a quasi “latch” state where the adapter output

stays in a current limited state at reduced output

voltage.

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