LTC1148CS-5 Linear Technology, LTC1148CS-5 Datasheet - Page 8

• Manufacturer Part Number: LTC1148CS-5
• Description: IC SW REG STEP-DOWN 5V 14-SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1148CS-5

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 50mA
• Frequency - Switching: 250kHz
• Voltage - Input: 3.5 ~ 20 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 14-SOIC (3.9mm Width), 14-SOL
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIO S I FOR ATIO
LTC1148
LTC1148-3.3/LTC1148-5
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
known, C
MOSFETs 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 20V. If the application requires
higher input voltage, then the LTC1149 or LTC1159
should be used.
R
R
The LTC1148 series current comparator has a threshold
range which extends from a minimum of 25mV/R
a maximum of 150mV/R
threshold sets the peak of the inductor ripple current,
yielding a maximum output current I
value less half the peak-to-peak ripple current. For proper
Burst Mode operation, I
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 LTC1148 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 com-
mences (I
both track I
I
8
SC(PK)
RIPPLE(P-P)
SENSE
SENSE
R
I
I
BURST
SC(PK)
SENSE
can be predicted from the following:
Selection for Output Current
is chosen based on the required output current.
BURST
T
MAX
= 100mV
≈ 15mV
= 150mV
= 25mV/R
and L can be chosen. Next, the power
R
R
. Once R
I
) and the peak short-circuit current (I
SENSE
MAX
SENSE
U U
SENSE
SENSE
RIPPLE(P-P)
SENSE
SENSE
has been chosen, I
(See C
. The current comparator
versus maximum output
SENSE
must be less than or
T
MAX
W
and L Selection for
SENSE
. Once R
equal to the peak
IN
and C
and allowing
BURST
SENSE
U
SENSE
OUT
SC(PK)
and
are
to
is
)
The LTC1148 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 LTC1148 series uses a constant off-time architecture
with t
Each time the P-channel MOSFET switch turns on, the
voltage on C
time, C
to V
inductor L, which likewise decays at a rate proportional to
V
capacitor value.
The value of C
mode operating frequency, f:
Assumes 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:
SC(AVG)
OUT
C
OUT
T
. Thus the inductor value must track the timing
OFF
=
T
. The voltage on C
T
2.6(10
to be reduced to approximately I
is discharged by a current which is proportional
Selection for Operating Frequency
determined by an external timing capacitor C
0.20
0.15
0.10
0.05
IN
0
T
1
0
is reset to approximately 3.3V. During the off
= 2V
T
4
is calculated from the desired continuous
)f
Figure 2. Selecting R
MAXIMUM OUTPUT CURRENT (A)
OUT
1
, Figure 1 circuit.
T
2
versus frequency including the
T
is analogous to the current in
3
SENSE
4
LTC1148 • F02
MAX
5
OFF
.
during a
114835fd
T
.