LTC1266CS Linear Technology, LTC1266CS Datasheet - Page 9

• Manufacturer Part Number: LTC1266CS
• Description: IC REG CNTRLR N/PCH MOSFET16SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck), Step-Up (Boost)r

Specifications of LTC1266CS

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: Adjustable
• Current - Output: 50mA
• Frequency - Switching: 400kHz
• Voltage - Input: 3.5 ~ 18 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

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
I
. The current comparator threshold sets
MAX
BURST
Figure 2. Selecting R
and I
MAXIMUM OUTPUT CURRENT (A)
2
U
MAX
), and the peak short-circuit cur-
SC(PK)
SENSE
must be less than or equal to the
4
U
equal to the peak value less half
SENSE
MAX
(see C
can be predicted from the
6
. Once R
vs maximum output
W
SENSE
T
8
and L Selection for
SENSE
1266 F02
SENSE
10
and allowing
U
has been
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
2.6 • 10
to be reduced to approximately I
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
1
= 2V
Figure 3. Timing Capacitor Value
15mV
R
T
is reset to approximately 3.3V. During the
. The voltage on C
LTC1266-3.3/LTC1266-5
SENSE
SENSE
is calculated from the desired continuous
V
4
IN
OUT
OUT
• f
= 5V
100
. Thus the inductor value must track
, (Figure 1 circuit).
T
FREQUENCY (kHz)
vs frequency including the effects
200
V
IN
= 12V
300
T
V
OUT
400
is analogous to the
= 3.3V
1266 F03
MAX
LTC1266
500
OFF
.
during a
9
T
.