LT3437 LINER [Linear Technology], LT3437 Datasheet - Page 12

• Manufacturer Part Number: LT3437
• Description: High Voltage 500mA, 200kHz Step-Down Switching Regulator with 100uA Quiescent Current
• Manufacturer: LINER [Linear Technology]

LT3437
APPLICATIO S I FOR ATIO
MAXIMUM OUTPUT LOAD CURRENT
Maximum load current for a buck converter is limited by
the maximum switch current rating (I
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
12
I
OUT MAX
100µF TANTALUM
100µF CERAMIC
(
ESR 75mΩ
10mV/DIV
10mV/DIV
10V/DIV
Figure 3. LT3437 Ripple Voltage Waveform
)
V
V
V
=
OUT
OUT
SW
I
PK
V
V
I
L = 100µH
LOAD
IN
OUT
= 12V
–
U
= 3.3V
= 500mA
( )(
V
OUT
2
U
( )( )( )
L f V
P-P
V
IN
/2) is less than I
1µs/DIV
–
IN
V
W
OUT
PK
)
=
). The current
I
PK
U
OUT
–
3437 F03
I
P
.
2
-P
Discontinuous operation occurs when:
For V
Note that there is less load current available at the higher
input voltage because inductor ripple current increases. At
V
conditions:
To calculate actual peak switch current in continuous
mode with a given set of conditions, use:
If a small inductor is chosen which results in discontinuous
mode operation over the entire load range, the maximum
load current is equal to:
CHOOSING THE INDUCTOR
For most applications the output inductor will fall in the
range of 68µH to 220µH. Lower values are chosen to
reduce physical size of the inductor. Higher values allow
IN
I
I
I
I
I
OUT DIS
OUT MAX
OUT MAX
SW PK
OUT MAX
= 15V, duty cycle is 33% and for the same set of
OUT
(
(
(
(
(
)
= 5V, V
)
=
)
)
)
≤
I
=
=
=
=
=
OUT
V
0 5 0 069 0 431
0 5 0 121 0 379
0 5
0 5
2
OUT
. – .
. – .
. –
. –
( )(
IN
2
+
I
V
PK
( )( )(
OUT
L f V
= 8V and L = 68µH:
(
V
V
2
2 68
2 68
OUT
IN
2
(
(
2
( )( )( )
–
( )( )( )
f L V
V
L f V
(
IN
e
e
V
V
IN
=
( )( )
( )(
IN
OUT
=
–
–
5 15 5
5 8 5
)
–
6 200 3 8
6 200 3 15
–
V
.
.
)(
)(
IN
)
OUT
V
IN
OUT
–
–
A
A
)
e
e
)
)
)( )
)( )
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