LTC3813 Datasheet, PDF (18/32 Pages) Linear Technology – 100V Current Mode Synchronous Step-Up Controller

LTC3813
APPLICATIONS INFORMATION
The required inductance can then be calculated to be:
L = ^{VIN(MIN) • DMAX}
f •
I_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
+
I_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.
where the first term is due to the bulk capacitance and
second term due to the ESR.
For many designs it is possible to choose a single capacitor
type that satisfies both the ESR and bulk C requirements
for the design. In certain demanding applications, however,
the ripple voltage can be improved significantly by con-
necting two or more types of capacitors in parallel. For
example, using a low ESR ceramic capacitor can minimize
the ESR step, while an electrolytic capacitor can be used
to supply the required bulk C.
L
D
V_IN
SW
V_OUT
C_OUT R_L
8a. Circuit Diagram
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


18
I_L
I_IN
8b. Inductor and Input Currents
I_SW
t_ON
8c. Switch Current
I_D
t_OFF
I_O
8d. Diode and Output Currents
V_OUT
(AC)
ΔV_COUT
ΔV_ESR
RINGING DUE TO
TOTAL INDUCTANCE
(BOARD + CAP)
8e. Output Voltage Ripple Waveform 3813 F08
Figure 8. Switching Waveforms for a Boost Converter
3813fb