Datasheet

LT3434
12
3434fb
APPLICATIO S I FOR ATIO
WUUU
For high frequency switchers the ripple current slew rate
is also relevant and can be calculated from:
di
dt
V
L
IN
=
Peak-to-peak output ripple voltage is the sum of a triwave
created by peak-to-peak ripple current times ESR and a
square wave created by parasitic inductance (ESL) and
ripple current slew rate. Capacitive reactance is assumed
to be small compared to ESR or ESL.
V I ESR ESL
di
dt
RIPPLE
=
()( )
+
()
P-P
Example: with V
IN
= 12V, V
OUT
= 3.3V, L = 33µH, ESR =
0.08Ω, ESL = 10nH:
I
P-P
=
()( )
()
−
()()
=
==
33 12 33
12 33 6 200 3
0 362
12
33 5
363 5
.–.
.
.–
.
ee
A
di
dt e
e
V
RIPPLE
= (0.362A)(0.08) + (10e – 9)(363e3)
= 0.0289 + 0.003 = 32mV
P-P
MAXIMUM OUTPUT LOAD CURRENT
Maximum load current for a buck converter is limited by
the maximum switch current rating (I
PK
). The minimum
peak current rating for the LT3434 is 3A. Unlike most
current mode converters, the LT3434 maximum switch
current limit does not fall off at high duty cycles. Most
current mode converters suffer a drop off of peak switch
current for duty cycles above 50%. This is due to the
effects of slope compensation required to prevent
subharmonic oscillations in current mode converters.
(For detailed analysis, see Application Note 19.)
The LT3434 is able to maintain peak switch current limit
over the full duty cycle range by using patented circuitry to
cancel the effects of slope compensation on peak switch
current without affecting the frequency compensation it
provides.
Maximum load current would be equal to maximum
switch current for an infinitely large inductor, but with
finite inductor size, maximum load current is reduced by
one-half peak-to-peak inductor current. The following
formula assumes continuous mode operation, implying
that the term on the right (I
P-P
/2) is less than I
OUT
.
II
VVV
LfV
I
I
OUT MAX PK
OUT IN OUT
IN
PK
P
()
–
–
–=
()( )
()()( )
=
2
-P
2
Discontinuous operation occurs when:
I
VVV
LfV
OUT DIS
OUT IN OUT
IN
()
–
()()( )
≤
()
2
For V
OUT
= 5V, V
IN
= 8V and L = 20µH:
I
ee
A
OUT MAX()
–
–
–
–. .
=
()( )
()()()
==
3
58 5
2 20 6 200 3 8
3 0 24 2 76
Note that there is less load current available at the higher
input voltage because inductor ripple current increases. At
V
IN
= 15V, duty cycle is 33% and for the same set of
conditions:
I
ee
A
OUT MAX()
–
–
–
–. .
=
()( )
()()()
==
3
515 5
2 20 6 200 3 15
3 0 42 2 58
Figure 3. LT3434 Ripple Voltage Waveform
V
OUT
10mV/DIV
100µF
75mΩ TANTALUM
V
OUT
10mV/DIV
100µF CERAMIC
I
SW
10V/DIV
I
LOAD
= 2A 1µs/DIV 3434 F03