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LTC1148L Datasheet

  • LTC1148L

  • Linear Technology [High Efficiency Synchronous Step-Down Sw...

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LTC1148
LTC1148-3.3/LTC1148-5
APPLICATIO S I FOR ATIO
The basic LTC1148 series application circuit (fixed
output versions) is shown in Figure 1. External compo-
nent selection is driven by the load requirement, and
begins with the selection of R
SENSE
. Once R
SENSE
is
known, C
T
and L can be chosen. Next, the power
MOSFETs and D1 are selected. Finally, C
IN
and C
OUT
are
selected and the loop is compensated. The circuit
shown in Figure 1 can be configured for operation up to
an input voltage of 20V. If the application requires
higher input voltage, then the LTC1149 or LTC1159
should be used.
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
The LTC1148 series current comparator has a threshold
range which extends from a minimum of 25mV/R
SENSE
to
a maximum of 150mV/R
SENSE
. The current comparator
threshold sets the peak of the inductor ripple current,
yielding a maximum 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 LTC1148 series and
external component values yields:
R
SENSE
= 100mV
I
MAX
R
SENSE
(鈩?
A graph for selecting R
SENSE
versus maximum output
current is given in Figure 2.
The load current below which Burst Mode operation com-
mences (I
BURST
) and the peak short-circuit current (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:
I
BURST
鈮?/div>
15mV
R
SENSE
I
SC(PK)
= 150mV
R
SENSE
8
U
0.20
0.15
0.10
0.05
0
0
1
3
4
2
MAXIMUM OUTPUT CURRENT (A)
5
LTC1148 鈥?F02
W
U U
Figure 2. Selecting R
SENSE
The LTC1148 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 LTC1148 series uses a constant off-time architecture
with t
OFF
determined by an external timing capacitor C
T
.
Each time the P-channel 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 proportional
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
versus frequency including the
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:

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