LT3433
W U U
U
APPLICATIO S I FOR ATIO
Discontinuousoperationoccurswhentheripplecurrentin
Ripple current:
the inductor is greater than twice the load current (ILOAD
)
V
OUT
+ 2V • 1− DC
in buck mode, or greater than ILOAD/(1 – DC) during
bridged mode. Current mode instability is not a concern
duringdiscontinuousoperationsoinductorvaluessmaller
than LMIN can be used. If such a small inductor is used,
however, it must be assured that the converter never
enters continuous operation at duty cycles greater than
50% to prevent current mode instability.
(
=
F) (
)
∆IL(P−P)
L• fO
Inductor Selection
TheprimarycriterionforinductorvalueselectioninLT3433
applications is the ripple current created in that inductor.
Design considerations for ripple current are the amount of
output ripple and the ability of the internal slope compen-
sation waveform to prevent current mode instability.
Design Example
V
IN(MIN) = 4V, VOUT = 5V, L = 150µH
The LT3433 maximizes available dynamic range using a
slope compensation generator that generates a continu-
ously increasing slope as duty cycle increases. The slope
compensationwaveformiscalibratedat80%dutycycleto
compensate for ripple currents up to 12.5% of IMAX, or
~60mA.
Using VF = 0.75V yields:
DC = (VOUT + 2VF)/(VOUT + VIN + 2VF – VSWH – VSWL
= (5V + 1.5V)/(4V + 5V + 1.5V – 0.6V – 0.5V)
= 0.69
∆IL = (VOUT + 2VF) • (1 – DC) • (L • f0)–1
= (5V + 1.5V) • (1 – 0.69) • (150µH • 200kHz)–1
= 67mA
)
Ripple current can be calculated as:
V
OUT
+ 2V • 1− DC
(
=
F) (
)
∆IL(P−P)
L• fO
ILOAD(MAX) = ISW(MAX) • (1 – 1.1 • DC)
= [0.5A – (1/2 • 0.07)](1 – 1.1 • 0.69) = 0.112A
This relation can be used to determine minimum induc-
tance sizes for various values of VOUT using the DC = 80%
calibration:
LMIN = (VOUT + 1.5V) • (1 – 0.8) 60mA • 200kHz)
V
OUT
L
MIN
4V
5V
92µH
108µH
175µH
225µH
9V
12V
3433ia
10
Linear [ Linear ]
