RT4539AWSC datasheet(32/39 Pages) RICHTEK

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RT4539A

is a registered trademark of Richtek Technology Corporation

www.richtek.com

DS4539A-01

December 2022

Boost Inductor Selection

The value of the inductance, L, can be approximated by

the following equation, where the transition is from

Discontinuous Conduction Mode (DCM) to Continuous

Conduction Mode (CCM):

OUT

OSC

OUT

D (1 D) V

× −

××

The duty cycle, D, can be calculated by the following

equation :

OUT

−

Where VOUT is the maximum output voltage, VIN is the

minimum input voltage, fOSC is the operating frequency,

and IOUT is the sum of current from all LED strings. The

boost converter operates in DCM over the entire input

voltage range when the inductor value is less than this

value, L. With an inductance greater than L, the

converter operates in CCM at the minimum input

voltage and may be discontinuous at higher voltages.

The inductor must be selected with a saturated current

rating that is greater than the peak current as provided

by the following equation :

OUT

OSC

PEAK

××

where η is the efficiency of the power converter.

Boost Diode Selection

The Schottky diode is a good choice for any

asynchronous boost converter with its small forward

voltage. However, when selecting a Schottky diode,

important parameters such as power dissipation,

reverse voltage rating, and pulsating peak current must

all be taken into consideration. A suitable Schottky

diode's reverse voltage rating must be greater than the

maximum output voltage, and its average current rating

must exceed the average output current.

Boost Output Capacitor Selection

Output ripple voltage is an important index for

estimating the performance. This portion consists of

two parts, one is the product of IIN and ESR of output

capacitor, the other part is formed by charging and

discharging process of output capacitor. As shown in

Figure 11, ΔVOUT1 can be evaluated based on the ideal

energy equalization. According to the definition of Q,

the Q value can be calculated by the following

equation :

OUT

OUT1

OUT

OSC





 

= ×

− ∆−

 





 





×∆

where fOSC is the switching frequency and ΔIL is the

inductor ripple current. Move COUT to the left side to

estimate the value of ΔVOUT1 by the following

equation :

OUT

OUT1

OUT

OSC

∆=

Where D is the duty cycle and η is the boost

converter efficiency. Finally, taking ESR into

account, the overall output ripple voltage can be

determined by the following equation :

OUT

ESR

OUT

OSC

∆

= ∆=

Where ∆VESR = ∆IC x RESR = IPEAK x RESR

Time

Inductor Current

Output Current

Output Ripple

Voltage (ac)

(1-D)TS

VOUT1

Input Current

Figure 11. The output ripple voltage without the

contribution of ESR

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