SP6121CN-L Datasheet, PDF (24/42 Pages) Sipex Corporation – Low Voltage, Synchronous Step Down PWM Controller

Schematic Description
SP6121
LMV321
DRQ125-47
T1
T2
D2
500kHz Synchronous PWM Controller
Low Voltage Op Amp for Current Sensing
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:
VL2 = L2 ^di = V 2 − VOUT
dt
(V2 = Secondary Switch Node Voltage)
VOUT = V 2 − L2× IOUT × Ripple × F × D^−1 (Ripple=Ripple Ratio, F=Freq, D=Duty Cycle)
L2
=
V 2 −VOUT
Ripple × IOUT × F
× D ^−1
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
VOUT = 3.8V
V2 = VIN + VOUT = 7.6V
Ripple = 40%
IOUT = 1.5A
F = 500kHz
D = 50%
(average voltage of the battery)
(average voltage of the K2 LED)
Nov 28 06 AppNote: SP6121 as a Synchronous Buck-Boost LED Driver
Page 4
© 2006 Sipex Corporation