ltc1143l-adj Linear Technology Corporation, ltc1143l-adj Datasheet - Page 9

• Manufacturer Part Number: ltc1143l-adj
• Description: Dual High Efficiency So-16 Step-down Switching Regulator Controllers
• Manufacturer: Linear Technology Corporation

APPLICATIONS
The load current below which Burst Mode operation
commences, I
I
I
The LTC1143 series automatically extends t
short circuit to allow sufficient time for the inductor
current to decay between switch cycles. The resulting
I
L and C
Each regulator section of the LTC1143 series uses a
constant off-time architecture with t
external timing capacitor C
MOSFET switch turns on the voltage on C
approximately 3.3V. During the off-time, C
by a current that is proportional to V
is analogous to the current in inductor L, which likewise
decays at a rate proportional to V
value must track the timing capacitor value.
The value of C
mode operating frequency:
SC(PK)
BURST
SC(AVG)
I
I
BURST
SC(PK)
, both track I
and I
T
to be reduced to approximately I
0.20
0.15
0.10
0.05
Selection for Operating Frequency
= 150mV
0
SC(PK)
0
R
R
15mV
T
BURST
SENSE
SENSE
is calculated from the desired continuous
Figure 2. Selecting R
MAXIMUM OUTPUT CURRENT (A)
1
can be predicted from the following:
U
MAX
, and the peak short circuit current,
. Once R
2
INFORMATION
U
T
. Each time the P-channel
3
SENSE
OUT
OUT
W
SENSE
OFF
4
. Thus the inductor
. The voltage on C
has been chosen,
determined by an
1143 F02
MAX
T
5
is discharged
T
OFF
.
is reset to
U
during a
T
where V
A graph for selecting C
effects of input voltage is given in Figure 3.
As the operating frequency is increased the gate charge
losses will be higher, reducing efficiency (see Efficiency
Considerations). The complete expression for operating
frequency of the circuit in Figure 1 is given by:
where:
V
is the measured output voltage. Thus V
regulation.
Note that as V
When the input-to-output voltage differential drops below
1.5V for a particular section, the LTC1143 series reduces
t
C
OFF
T
REG
. This prevents audible operation prior to dropout.
C
f
t
OFF
in that section by increasing the discharge current in
T
is the desired output voltage (i.e., 5V, 3.3V). V
t
OFF
D
1 3 10
1
1000
1 3 10
is the drop across the diode.
800
600
400
200
0
1
1
Figure 3. Timing Capacitor Value
0
V
4
IN
IN
V
4
V
= 7V
OUT
50
f
decreases, the frequency decreases.
IN
C
V
T
LTC1143/LTC1143L
IN
V
100
FREQUENCY (kHz)
T
IN
V
V
versus frequency including the
V
OUT
REG
IN
V
= 12V
150
OUT
V
D
V
LTC1143L-ADJ
SENSE
200
= V
V
OUT
IN
250
= 10V
LTC1143 F03
= 5V
REG
300
/V
OUT
= 1 in
9
OUT