Datasheet

7
LTC1149
LTC1149-3.3/LTC1149-5
TEST CIRCUIT
APPLICATIO S I FOR ATIO
WUU U
Typical Application Circuit
The basic LTC1149 series application circuit is shown in
Figure 1. External component selection is driven by the
input voltage and output load requirement, and begins
with the selection of R
SENSE
. Once R
SENSE
is known, C
T
and L can be chosen. Next, the power MOSFETs and D1
are selected. Finally, C
IN
and C
OUT
are selected and the
loop is compensated. The circuit shown in Figure 1 can be
configured for operation up to an input voltage of 48V. If
the application does not require greater than 15V opera-
tion, then the LTC1148 should be used.
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
The LTC1149 series current comparator has a threshold
range which extends from a minimum of 25mV/R
SENSE
to
a maximum of 150mV/R
SENSE
. The current comparator
threshold sets the peak of the inductor ripple current,
yielding a maximum output current I
MAX
equal to the peak
value less half the peak-to-peak ripple current. For proper
Burst Mode operation, I
RIPPLE(P-P)
must be less than or
equal to the minimum current comparator threshold.
Since efficiency generally increases with ripple current,
the maximum allowable ripple current is assumed, i.e.,
I
RIPPLE(P-P)
= 25mV/R
SENSE
(see C
T
and L Selection for
Operating Frequency). Solving for R
SENSE
and allowing a
margin for variations in the LTC1149 series and external
component values yields:
R
SENSE
=
100mV
I
MAX
A graph for selecting R
SENSE
versus maximum output
current is given in Figure 2. The LTC1149 series works well
with values of R
SENSE
from 0.02Ω to 0.2Ω.
The load current below which Burst Mode operation
commences, I
BURST
, and the peak short-circuit current,
I
SC(PK)
, both track I
MAX
. Once R
SENSE
has been chosen,
I
BURST
and I
SC(PK)
can be predicted from the following
equations:
I
BURST
≈
15mV
R
SENSE
I
SC(PK)
=
150mV
R
SENSE
+
PGATE
V
IN

V
CC

PDRIVE
V
CC

C
T

I
TH

SENSE
–
CAP
SHDN2
RGND
NGATE
PGND
SGND

SENSE
+
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9

LTC1149
0.047µF
1µF
0.068µF
V
IN
0.1µF
+
V
8
3300pF390pF
1000pF
+
V
9
– V
8
25k
75k
220µF
0.05Ω
+
V
10
V
OUT
+
V
15
50µH
IRF9Z34
220µF
100V
IRFZ34
MBR380
1149 TC
V
FB
/
SHDN1
1k
+
+
+