SP6121 datasheet(24/42 Pages) SIPEX

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Nov 28 06 AppNote: SP6121 as a Synchronous Buck-Boost LED Driver

Page 4

Schematic Description

SP6121

500kHz Synchronous PWM Controller

LMV321

Low Voltage Op Amp for Current Sensing

DRQ125-47

Dual Winding Coil 4.7µH

PMOS Transistor

NMOS Transistor

Zener Diode for OVP

Component Options

The first component to choose is the coil. L1 and L2 will be chosen so they have

the same value, in order to be able to use a dual winding coil (two coils on the

same core). This kind of coil is usually used in a SEPIC converter and is now a

very common product. The advantages to using a spiral winding coil versus two

single coils are reduced footprint (the package is typically the same as a single

coil) and reduced current ripple due to internal coupling.

If we state that Duty Cycle will be 50% (VIN = VOUT), we can make the

assumption that the AC current going through L1 and L2 are the same.

From the buck cell we have:

−

(V2 = Secondary Switch Node Voltage)

−

Ripple

OUT

(Ripple=Ripple Ratio, F=Freq, D=Duty Cycle)

−

Ripple

OUT

In this case, as we are powering an LED, we can set the current ripple to be high

as it does not matter for the LED lighting performance. Also this will let us

reduce the coil value and reduce PCB area. In this topology, V2 (secondary

switch node voltage), is always equal to VOUT + VIN regardless of the input and

output voltages.

VIN = 3.8V

(average voltage of the battery)

VOUT = 3.8V

(average voltage of the K2 LED)

V2 = VIN + VOUT = 7.6V

Ripple = 40%

IOUT = 1.5A

F = 500kHz

D = 50%

SP6121 Datasheet(PDF) 24 Page - Sipex Corporation

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