Datasheet

14
LT3437
3437fc
Discontinuous operation occurs when:
I
VVV
LfV
OUT DIS
OUT IN OUT
IN
()
–
()()( )
≤
()
2
For V
OUT
= 5V, V
IN
= 8V and L = 68µH:
I
ee
A
OUT MAX()
.–
–
–
.–. .
=
()( )
()()()
==
05
58 5
2 68 6 200 3 8
0 5 0 069 0 431
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()
.–
–
–
.–. .
=
()( )
()()()
==
05
515 5
2 68 6 200 3 15
0 5 0 121 0 379
To calculate actual peak switch current in continuous
mode with a given set of conditions, use:
II
VVV
LfV
SW PK OUT
OUT IN OUT
IN
()
–
=+
()
()()( )
2
If a small inductor is chosen which results in discontinuous
mode operation over the entire load range, the maximum
load current is equal to:
I
IfLV
VVV
OUT MAX
PK IN
OUT IN OUT
()
–
=
()( )( )
()( )
2
2
2
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 LT3437 is 500mA. Unlike most current mode
converters, the LT3437 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 LT3437 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
Figure 3. LT3437 Ripple Voltage Waveform
V
OUT
10mV/DIV
100µF TANTALUM
ESR 75mΩ
V
OUT
10mV/DIV
100µF CERAMIC
V
SW
10V/DIV
V
IN
= 12V 1µs/DIV 3437 F03
V
OUT
= 3.3V
I
LOAD
= 500mA
L = 100µH
APPLICATIO S I FOR ATIO
WUU
U