Datasheet

8
LTC1159
LTC1159-3.3/LTC1159-5
MAXIMUM OUTPUT CURRENT (A)
0
R
SENSE
(Ω)
0.12
0.16
0.20
4
LTC1159 • F02
0.08
0.04
0
1
2
3
5
0.10
0.14
0.18
0.06
0.02
Figure 2. R
SENSE
vs Maximum Output Current
APPLICATIO S I FOR ATIO
WUUU
I
BURST
≈
15mV
R
SENSE
I
SC(PK)
=
150mV
R
SENSE
The LTC1159 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 LTC1159 uses a constant off-time architecture with
t
OFF
determined by an external timing capacitor C
T
. The
value of C
T
is calculated from the desired continuous mode
operating frequency, f:
C
T
=
7.8 • 10
–5
f
) )
1 –
V
OUT
V
IN
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 is given by:
f =
1
t
OFF
) )
1 –
V
OUT
V
IN
where t
OFF
= 1.3 • 10
4
• C
T
Once the frequency has been set by C
T
, the inductor L
must be chosen to provide no more than 0.025V/R
SENSE
of peak-to-peak inductor ripple current. This results in a
minimum required inductor value of:
L
MIN
= 5.1 • 10
5
• R
SENSE
• C
T
• V
REG
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 Mµ
®
cores. Actual core loss
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
2
R)
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
FREQUENCY (kHz)
0
0
C
T
CAPACITANCE (pF)
200
400
600
1400
1000
50
100
1200
800
150
200
250
V
OUT
= 5V
V
IN
= 48V
V
IN
= 12V
V
IN
= 24V
LTC1159 • F03
Figure 3. Timing Capacitor Selection
Kool Mµ is a registered trademark of Magnetics, Inc.