SP6121 datasheet(22/42 Pages) SIPEX

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

Page 2

and S2 open (On Time). Phase 2 is S1 open and S2 closed (Off Time). The Duty

Cycle will be TON/TOFF. C1 is an energy transfer capacitor and is used to provide

a positive voltage on V2 so the buck cell can regulate the appropriate VOUT.

During On Time,

I1 current is stored into L1 and also provided to L2 through C1.

During Off Time, V2 equals GND and any remaining current in L2 still delivers to

the load. At the same time, the voltage across C1 is equal to V1 and so becomes

negative as current is still flowing across L1. VC1 = (V2-V1) and is positive.

During the following on-time, this VC1 is added on top of VIN and so generates a

positive voltage V2 to the buck cell (V2 = Vin + VC1).

Figure 1

The Duty Cycle of this converter will be :

VOUT / VIN = D / (1-D)

D =

_______1_______

[ ( VIN/VOUT) + 1]

So typically, if VIN equals VOUT the Duty Cycle will be 50%.

Design Considerations

Our need here is to power a Luxeon K2 at 1.5A from a one cell Li

Ion battery

pack. The K2 is an ultra-bright LED that has a VF from 3.6 to 4.3V depending on

IF and temperature. The battery will have a voltage from 3.4V to 4.2V. Also

depending on charge level, battery output impedance can vary and so induce a

loss depending on output current. Due to these conditions, a step up/step down

topology is definitely required to get a constant current into the LED regardless of

VIN

VOUT

SP6121 Datasheet(PDF) 22 Page - Sipex Corporation