ltc1147l Linear Technology Corporation, ltc1147l Datasheet - Page 7

• Manufacturer Part Number: ltc1147l
• Description: High Efficiency Step-down Switching Regulator Controllers -
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
The basic LTC1147 application circuit is shown in Figure
1. External component selection is driven by the load
requirement and begins with the selection of R
R
MOSFET and D1 are selected. Finally, C
selected and the loop is compensated. The circuit shown
in Figure 1 can be configured for operation up to an input
voltage of 16V. If the application requires higher input
voltage, then the synchronous switched LTC1149 should
be used. Consult factory for lower minimum input voltage
version.
R
R
The LTC1147 series current comparator has a thresh-
old range which extends from a minimum of 25mV/
R
comparator threshold sets the peak of the inductor
ripple current, yielding a maximum output current I
equal to the peak value less half the peak-to-peak ripple
current. For proper Burst Mode operation, I
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
Operating Frequency). Solving for R
a margin for variations in the LTC1147 series and
external component values yields:
A graph for selecting R
current is given in Figure 2.
The load current below in which Burst Mode operation
commences, I
I
I
SC(PK)
BURST
RIPPLE(P-P)
SENSE
SENSE
SENSE
SENSE
R
I
I
BURST
SC(PK)
SENSE
, both track I
and I
is known, C
Selection for Output Current
is chosen based on the required output current.
to a maximum of 150mV/R
= 150mV
= 100mV
= 25mV/R
SC(PK)
R
R
15mV
BURST
I
SENSE
MAX
SENSE
can be predicted from the following:
T
U
MAX
and L can be chosen. Next, the power
and the peak short-circuit current
SENSE
. Once R
SENSE
U
(see C
versus maximum output
SENSE
W
T
SENSE
and L Selection for
SENSE
has been chosen,
IN
. The current
and C
and allowing
SENSE
RIPPLE(P-P)
U
OUT
. Once
MAX
are
The LTC1147 series automatically extend 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 LTC1147 series use a constant off-time architecture
with t
Each time the P-channel 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:
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 reduce efficiency (see Efficiency Consider-
ations). The complete expression for operating frequency
SC(AVG)
C
T
OFF
=
T
(1.3)(10
to be reduced to approximately I
D
Selection for Operating Frequency
determined by an external timing capacitor C
0.20
0.15
0.10
0.05
is the drop across the Schottky diode.
T
OUT
0
is discharged by a current which is propor-
T
0
T
is reset to approximately 3.3V. During the
1
. The voltage on C
is calculated from the desired continuous
LTC1147-5/LTC1147L
Figure 2. Selecting R
OUT
4
MAXIMUM OUTPUT CURRENT (A)
)(f)
1
. Thus the inductor value must track
T
V
V
2
IN
versus frequency including the
IN
– V
+ V
3
OUT
D
LTC1147-3.3
T
SENSE
is analogous to the
4
LTC1147 • F02
MAX
5
OFF
.
sn1147 1147fds
during a
7
T
.