LTC1159IS Linear Technology, LTC1159IS Datasheet - Page 8

• Manufacturer Part Number: LTC1159IS
• Description: IC SW REG SYNC STEP-DOWN 16-SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1159IS

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: Adjustable
• Current - Output: 50mA
• Frequency - Switching: 250kHz
• Voltage - Input: 4 ~ 40 V
• Operating Temperature: -40°C ~ 85°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
LTC1159
LTC1159-3.3/LTC1159-5
The LTC1159 automatically extends t
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
reduced to approximately I
L and C
The LTC1159 uses a constant off-time architecture with
t
value of C
operating frequency, f:
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 is given by:
8
OFF
C
f =
I
I
BURST
SC(PK)
T
determined by an external timing capacitor C
= 7.8 • 10
t
OFF
T
1
Figure 2. R
T
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
Selection for Operating Frequency
is calculated from the desired continuous mode
= 150mV
0
1 –
0
R
f
R
15mV
SENSE
SENSE
–5
V
MAXIMUM OUTPUT CURRENT (A)
V
SENSE
OUT
1
U
IN
1 –
vs Maximum Output Current
T
2
versus frequency including the
U
V
V
MAX
OUT
IN
.
3
W
4
LTC1159 • F02
OFF
during a short
5
SC(AVG)
U
T
to be
. The
where t
Once the frequency has been set by C
must be chosen to provide no more than 0.025V/R
of peak-to-peak inductor ripple current. This results in a
minimum required inductor value of:
As the inductor value is increased from the minimum value,
the ESR requirements for the output capacitor are eased at
the expense of efficiency. If too small an inductor is used,
the LTC1159 may not enter Burst Mode operation and
efficiency will be severely degraded at low currents.
Inductor Core Selection
Once the minimum value for L is known, the type of
inductor must be selected. High efficiency converters
generally cannot afford the core loss found in low cost
powdered iron cores, forcing the use of more expensive
ferrite, molypermalloy or Kool M
is independent of core size for a fixed inductor value, but
it is very dependent on the inductance selected. As induc-
tance increases, core losses go down but copper (I
losses will increase.
Ferrite designs have very low core loss, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design cur-
rent is exceeded. This results in an abrupt increase in
Kool M is a registered trademark of Magnetics, Inc.
L
MIN
OFF
= 5.1 • 10
1400
1200
1000
800
600
400
200
= 1.3 • 10
0
Figure 3. Timing Capacitor Selection
0
5
50
• R
4
• C
SENSE
FREQUENCY (kHz)
100
T
V
IN
• C
= 48V
150
T
• V
®
V
V
IN
IN
cores. Actual core loss
= 12V
V
REG
= 24V
200
OUT
LTC1159 • F03
= 5V
T
, the inductor L
250
SENSE
2
R)