LTC3812-5_15 Datasheet, PDF (17/34 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3812-5
APPLICATIONS INFORMATION
MINIMUM OFF-TIME AND DROPOUT OPERATION
The minimum off-time t_OFF(MIN) is the smallest amount of
time that the LTC3812-5 is capable of turning on the bot-
tom MOSFET, tripping the current comparator and turning
the MOSFET back off. This time is generally about 250ns.
The minimum off-time limit imposes a maximum duty
cycle of t_ON/(t_ON + t_OFF(MIN)). If the maximum duty cycle
is reached, due to a dropping input voltage for example,
then the output will drop out of regulation. The minimum
input voltage to avoid dropout is:
V_IN(MIN)
=
V_OUT
t_ON
+ t_OFF(MIN)
t_ON
A plot of maximum duty cycle vs frequency is shown in
Figure 8.
INDUCTOR SELECTION
Given the desired input and output voltages, the induc-
tor value and operating frequency determine the ripple
current:
I_L
=


V_OUT
fL



 ¹

V_OUT
V_IN


Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Highest efficiency operation is obtained at low
frequency with small ripple current. However, achieving
this requires a large inductor. There is a tradeoff between
component size, efficiency and operating frequency.
A reasonable starting point is to choose a ripple current
that is about 40% of I_OUT(MAX). The largest ripple current
occurs at the highest V_IN. To guarantee that ripple current
does not exceed a specified maximum, the inductance
should be chosen according to:
L
=



V_OUT
f I_L(MAX)




 ¹


V_OUT
V_IN(MAX)



Once the 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. A variety of inductors designed for
high current, low voltage applications are available from
manufacturers such as Sumida, Panasonic, Coiltronics,
Coilcraft and Toko.
SCHOTTKY DIODE D1 SELECTION
The Schottky diode D1 shown in the front page schematic
conducts during the dead time between the conduction of
the power MOSFET switches. It is intended to prevent the
body diode of the bottom MOSFET from turning on and
storing charge during the dead time, which can cause a
modest (about 1%) efficiency loss. The diode can be rated
for about one half to one fifth of the full load current since
2.0
1.5
DROPOUT
REGION
1.0
0.5
0
0
0.25
0.50
0.75
1.0
DUTY CYCLE (V_OUT/V_IN)
38125 F08
Figure 8. Maximum Switching Frequency vs Duty Cycle
38125fc
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