LTC1159_15 Datasheet, PDF (9/20 Pages) Linear Technology – High Efficiency Synchronous Step-Down Switching Regulators

LTC1159
LTC1159-3.3/LTC1159-5
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 Mµ. 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_CC voltage on
the LTC1159. This voltage is typically 4.5V during start-up
and 5V to 7V during normal operation (see EXTV_CC Pin
Connection). Consequently, logic-level threshold
MOSFETs must be used in most LTC1159 family applica-
tions. The only exception is applications in which EXTV_CC
is powered from an external supply greater than 8V, in
which standard threshold MOSFETs (V_GS(TH) < 4V) may be
used. Pay close attention to the BV_DSS specification for the
MOSFETs as well; many of the logic-level MOSFETs are
limited to 30V.
Selection criteria for the power MOSFETs include the “ON”
resistance R_DS(ON), reverse transfer capacitance C_RSS,
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
=
V_OUT
V_IN
N-Ch
Duty
Cycle
=
V_IN
– V_OUT
V_IN
The MOSFET dissipations at maximum output current are
given by:
P-Ch P_D =
V_OUT
V_IN
(I_MAX)² (1 + ∂_P) R_DS(ON) +
k(V_IN)² (I_MAX) (C_RSS) (f)
N-Ch
P_D
=
V_IN
– V_OUT
V_IN
(I_MAX)²
(1
+
∂_N)
R_DS(ON)
where ∂ is the temperature dependency of R_DS(ON) and k
is a constant inversely related to the gate drive current.
Both MOSFETs have I²R losses while the P-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. For V_IN < 20V the
high current efficiency generally improves with larger
MOSFETs, while for V_IN > 20V the transition losses rapidly
increase to the point that the use of a higher R_DS(ON)
device with lower C_RSS actually provides higher effi-
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_DS(ON) vs Temperature curve, but
∂ = 0.007/°C can be used as an approximation for low
voltage MOSFETs. C_RSS is usually specified in the MOSFET
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_MAX.
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