LTC1266 Linear Technology, LTC1266 Datasheet - Page 9

• Manufacturer Part Number: LTC1266
• Description: Synchronous Regulator Controller for N- or P-Channel MOSFETs
• Manufacturer: Linear Technology

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