LTC1159CS-5 Linear Technology, LTC1159CS-5 Datasheet - Page 9

• Manufacturer Part Number: LTC1159CS-5
• Description: IC SW REG STEP-DOWN 5V 16-SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1159CS-5

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 50mA
• Frequency - Switching: 250kHz
• Voltage - Input: 4 ~ 40 V
• Operating Temperature: 0°C ~ 70°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
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:
P-Ch Duty Cycle =
N-Ch Duty Cycle =
DS(ON)
, reverse transfer capacitance C
U
V
V
V
IN
OUT
IN
U
V
– V
IN
OUT
DSS
W
specification for the
GS(TH)
CC
< 4V) may be
U
voltage on
CC
RSS
Pin
CC
,
The MOSFET dissipations at maximum output current are
given by:
where is the temperature dependency of R
is a constant inversely related to the gate drive current.
Both MOSFETs have I
equation includes an additional term for transition losses,
which are highest at high input voltages. For V
high current efficiency generally improves with larger
MOSFETs, while for V
increase to the point that the use of a higher R
device with lower C
ciency. The N-channel MOSFET losses are the greatest at
high input voltage or during a short circuit when the
N-channel duty cycle is nearly 100%.
The term (1 + ) is generally given for a MOSFET in the form
of a normalized R
voltage MOSFETs. C
electrical characteristics. The constant k = 5 can be used for
the LTC1159 to estimate the relative contributions of the
two terms in the P-channel dissipation equation.
The Schottky diode D1 shown in Figure 1 only conducts
during the dead time between the conduction of the two
power MOSFETs. D1 prevents the body diode of the
N-channel MOSFET from turning on and storing charge
during the dead time, which could cost as much as 1% in
efficiency (although there are no other harmful effects if
D1 is omitted). Therefore, D1 should be selected for a
forward voltage of less than 0.6V when conducting I
P-Ch P
N-Ch P
= 0.007/ C can be used as an approximation for low
D
D
=
=
V
k(V
V
V
IN
OUT
IN
IN
V
– V
LTC1159-3.3/LTC1159-5
IN
)
2
(I
OUT
RSS
DS(ON)
MAX
(I
IN
RSS
MAX
> 20V the transition losses rapidly
(I
is usually specified in the MOSFET
2
)
R losses while the P-channel
MAX
2
) (C
actually provides higher effi-
(1 +
vs Temperature curve, but
)
RSS
2
(1 +
P
) (f)
) R
DS(ON)
N
) R
LTC1159
DS(ON)
DS(ON)
IN
+
< 20V the
DS(ON)
and k
MAX
9
.