Datasheet

LT3435
12
3435fa
Figure 3. LT3435 Ripple Voltage Waveform
V
OUT
20mV/DIV
AC-COUPLED
C
OUT
= 100µF
TANTALUM
ESR 100mΩ
V
OUT
20mV/DIV
AC-COUPLED
C
OUT
= 100µF
CERAMIC
V
SW
5V/DIV
V
IN
= 12V 500ns/DIV 3435 F03
V
OUT
= 3.3V
I
LOAD
= 1A
L = 15µH
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 = 15µH, ESR =
0.08Ω, ESL = 10nH:
I
P-P
=
()( )
()
−
()()
=
==
33 12 33
12 15 6 500 3
0 319
12
15 6
08 6
.–.
.
–
.
ee
A
di
dt e
e
V
RIPPLE
= (0.319A)(0.08) + (10e – 9)(0.8e6)
= 0.026 + 0.008 = 34mV
P-P
MAXIMUM OUTPUT LOAD CURRENT
Maximum load current for a buck converter is limited by
the maximum switch current rating (I
PK
). The current
rating for the LT3435 is 3A. Unlike most current mode
converters, the LT3435 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 oscilla-
tions in current mode converters. (For detailed analysis,
see Application Note 19.)
The LT3435 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 = 15µH:
I
ee
A
OUT MAX()
–
–
–
–. .
=
()( )
()()()
==
3
58 5
2 15 6 500 3 8
3 0 125 2 875
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 15 6 500 3 15
3 0 22 2 88
APPLICATIO S I FOR ATIO
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