Datasheet

9
LTC1266
LTC1266-3.3/LTC1266-5
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
The LTC1266 series current comparator has a threshold
range which extends from a minimum of 25mV/R
SENSE
(when Burst Mode operation is enabled) to a maximum of
155mV/R
SENSE
. The current comparator threshold sets
the peak of the inductor ripple current, yielding a maxi-
mum 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 LTC1266 series and
external component values yields:
R
SENSE
=
100mV
I
MAX
A graph for selecting R
SENSE
vs maximum output
current is given in Figure 2.
I
BURST
≈
15mV
R
SENSE
I
SC(PK)
=
155mV
R
SENSE
The LTC1266 series automatically extends t
OFF
during a
short circuit to allow sufficient time for the inductor
current to decay between switch cycles. The resulting
ripple current causes the average short-circuit current
I
SC(AVG)
to be reduced to approximately I
MAX
.
L and C
T
Selection for Operating Frequency
The LTC1266 series uses a constant off-time architecture
with t
OFF
determined by an external timing capacitor C
T
.
Each time the topside MOSFET switch turns on, the
voltage on C
T
is reset to approximately 3.3V. During the
off-time, C
T
is discharged by a current which is propor-
tional to V
OUT
. The voltage on C
T
is analogous to the
current in inductor L, which likewise decays at a rate
proportional to V
OUT
. Thus the inductor value must track
the timing capacitor value.
The value of C
T
is calculated from the desired continuous
mode operating frequency, f:
C
T
=
1
2.6 • 10
4
•
f
assumes V
IN
= 2V
OUT
, (Figure 1 circuit).
A graph for selecting C
T
vs frequency including the effects
of input voltage is given in Figure 3.
Figure 2. Selecting R
SENSE
The load current, below which Burst Mode operation
commences, (I
BURST
), and the peak short-circuit cur-
rent, (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:
MAXIMUM OUTPUT CURRENT (A)
0
R
SENSE
(mΩ)
75
100
1266 F02
50
25
0
4
8
2
6
10
Figure 3. Timing Capacitor Value
FREQUENCY (kHz)
0
0
CAPACITANCE (pF)
200
400
100 200 300
400
1266 F03
600
800
500
V
IN
= 12V
V
IN
= 5V
V
OUT
= 3.3V
APPLICATIO S I FOR ATIO
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