LTC3814-5_15 Datasheet, PDF (15/30 Pages) Linear Technology – 60V Current Mode Synchronous Step-Up Controller

LTC3814-5
APPLICATIONS INFORMATION
Minimum On-Time and Dropout Operation
The minimum on-time t_ON(MIN) is the smallest amount of
time that the LTC3814-5 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
MOSFET back off. This time is generally about 350ns. The
minimum on-time limit imposes a minimum duty cycle
of t_ON(MIN)/(t_ON(MIN) + t_OFF). If the minimum duty cycle is
reached, due to a rising input voltage for example, then
the output will rise out of regulation. The maximum input
voltage to avoid dropout is:
V_{IN(MAX )}
=
V_OUT
t_OFF
t_ON(MIN) +
t_OFF
A plot of maximum duty cycle vs switching frequency is
shown in Figure 7.
2.0
1.5
DROPOUT
REGION
1.0
0.5
0
0
0.25
0.50
0.75
1.0
V_IN/V_OUT
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Figure 7. Maximum Switching Frequency vs Duty Cycle
Inductor Selection
An inductor should be chosen that can carry the maximum
input DC current which occurs at the minimum input volt-
age. The peak-to-peak ripple current is set by the inductance
and a good starting point is to choose a ripple current of
at least 40% of its maximum value:
ΔI_L
=
40%
•
I_{O(MAX )}
1− D_MAX
The required inductance can then be calculated to be:
L = ^{VIN(MIN) • DMAX}
f • ΔI_L
The required saturation of the inductor should be chosen
to be greater than the peak inductor current:
I_L(SAT)
≥
I_{O(MAX )}
1− D_MAX
+
ΔI_L
2
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 synchronous 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 it is on for only a fraction of the duty cycle. The peak
reverse voltage that the diode must withstand is equal to
the regulator output voltage. In order for the diode to be
effective, the inductance between it and the synchronous
MOSFET must be as small as possible, mandating that
these components be placed adjacently. The diode can
be omitted if the efficiency loss is tolerable.
Output Capacitor Selection
In a boost converter, the output capacitor requirements
are demanding due to the fact that the current waveform
is pulsed. The choice of component(s) is driven by the
acceptable ripple voltage which is affected by the ESR,
ESL and bulk capacitance as shown in Figure 8e. The total
output ripple voltage is:

V_OUT
= I_O(MAX)


f
1
• C_OUT
+
ESR
1– D_MAX


where the first term is due to the bulk capacitance and
second term due to the ESR.
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