LTC1159 Linear Technology, LTC1159 Datasheet - Page 8

• Manufacturer Part Number: LTC1159
• Description: High Efficiency Synchronous Step-Down Switching Regulators
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC1159/LTC1159-3.3/LTC1159-5
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
Kool M is a registered trademark of Magnetics, Inc.
8
where t
L
f =
MIN
t
OFF
= 5.1 10
1
1400
1200
1000
800
600
400
200
OFF
0
Figure 3. Timing Capacitor Selection
1 –
0
= 1.3 10
V
V
50
OUT
5
IN
U U
FREQUENCY (kHz)
R
100
4
SENSE
V
IN
C
= 48V
T
150
V
V
C
®
IN
IN
cores. Actual core loss
T
= 12V
V
= 24V
200
W
OUT
LTC1159 • F03
V
= 5V
T
REG
, the inductor L
250
U
SENSE
2
R)
inductance collapses abruptly when the peak design cur-
rent is exceeded. This results in an abrupt increase in
inductor ripple current and consequent output voltage
ripple which can cause Burst Mode operation to be falsely
triggered in the LTC1159. Do not allow the core to saturate!
Molypermalloy (from Magnetics, Inc.) is a low loss core
material for toroids, but it is more expensive than ferrite.
A reasonable compromise from the same manufacturer is
Kool M . Toroids are very space efficient, especially when
you can use several layers of wire. Because they generally
lack a bobbin, mounting is more difficult. However, new
surface mount designs available from Coiltronics do not
increase the height significantly.
Power MOSFET Selection
Two external power MOSFETs must be selected for use
with the LTC1159: a P-channel MOSFET for the main
switch and an N-channel MOSFET for the synchronous
switch.
The peak-to-peak drive levels are set by the V
the LTC1159. This voltage is typically 4.5V during start-up
and 5V to 7V during normal operation (see EXTV
Connection). Consequently, logic-level threshold
MOSFETs must be used in most LTC1159 family applica-
tions . The only exception is applications in which EXTV
is powered from an external supply greater than 8V, in
which standard threshold MOSFETs (V
used. Pay close attention to the BV
MOSFETs as well; many of the logic-level MOSFETs are
limited to 30V.
Selection criteria for the power MOSFETs include the “ON”
resistance R
input voltage, and maximum output current. When the
LTC1159 is operating in continuous mode, the duty cycle
for the P-channel MOSFET is given by:
The MOSFET dissipations at maximum output current are
given by:
P-Ch Duty Cycle =
N-Ch Duty Cycle =
DS(ON)
, reverse transfer capacitance C
V
V
V
OUT
IN
IN
V
– V
IN
OUT
DSS
specification for the
GS(TH)
CC
< 4V) may be
voltage on
CC
RSS
Pin
CC
,