LT3433 LINER [Linear Technology], LT3433 Datasheet - Page 10

• Manufacturer Part Number: LT3433
• Description: High Voltage Step-Up/Step-Down DC/DC Converter
• Manufacturer: LINER [Linear Technology]

LT3433
APPLICATIO S I FOR ATIO
Ripple current:
Inductor Selection
The primary criterion for inductor value selection in LT3433
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.
The LT3433 maximizes available dynamic range using a
slope compensation generator that generates a continu-
ously increasing slope as duty cycle increases. The slope
compensation waveform is calibrated at 80% duty cycle to
compensate for ripple currents up to 12.5% of I
~ 60mA.
Ripple current can be calculated as:
This relation can be used to determine minimum induc-
tance sizes for various values of V
calibration:
10
L
MIN
I
I
L P P
L P P
(
(
= (V
)
)
OUT
V
12V
4V
5V
9V
OUT
V
V
+ 1.5V) • (1 – 0.8) 60mA • 200kHz)
OUT
OUT
U
2
2
L f
L f
V
V
•
•
U
F
F
O
O
•
•
1
1
OUT
DC
DC
W
using the DC = 80%
108 H
175 H
225 H
92 H
L
MIN
U
MAX
, or
Discontinuous operation occurs when the ripple current in
the inductor is greater than twice the load current (I
in buck mode, or greater than I
bridged mode. Current mode instability is not a concern
during discontinuous operation so inductor values smaller
than L
however, it must be assured that the converter never
enters continuous operation at duty cycles greater than
50% to prevent current mode instability.
Design Example
V
Using V
IN(MIN)
DC = (V
I
= [0.5A – (1/2 • 0.07)](1 – 1.1 • 0.69) = 0.112A
LOAD(MAX)
I
L
MIN
= (5V + 1.5V)/(4V + 5V + 1.5V – 0.6V – 0.5V)
= 0.69
= (V
= (5V + 1.5V) • (1 – 0.69) • (150 H • 200kHz)
= 67mA
F
= 4V, V
= 0.75V yields:
can be used. If such a small inductor is used,
OUT
OUT
= I
+ 2V
+ 2V
OUT
SW(MAX)
F
F
= 5V, L = 150 H
)/(V
) • (1 – DC) • (L • f
OUT
• (1 – 1.1 • DC)
+ V
IN
LOAD
+ 2V
F
/(1 – DC) during
0
– V
)
–1
SWH
– V
LOAD
SWL
3433ia
–1
)
)