LTC3778 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC3778' PDF : 24 Pages View PDF
LTC3778
APPLICATIO S I FOR ATIO
load current increases. By lengthening the on-time slightly
as current increases, constant frequency operation can be
maintained. This is accomplished with a resistive divider
from the ITH pin to the VON pin and VOUT. The values
required will depend on the parasitic resistances in the
specific application. A good starting point is to feed about
25% of the voltage change at the ITH pin to the VON pin as
shown in Figure 3a. Place capacitance on the VON pin to
filter out the ITH variations at the switching frequency. The
resistor load on ITH reduces the DC gain of the error amp
and degrades load regulation, which can be avoided by
using the PNP emitter follower of Figure 3b.
VOUT
RVON1
30k
RVON2
100k
RC
CC
CVON
0.01µF
VON
LTC3778
ITH
3778 F03a
(3a)
VOUT
INTVCC
RVON1
3k
RVON2
10k 10k
Q1
2N5087
CVON
0.01µF
RC
CC
VON
LTC3778
ITH
3778 F03b
(3b)
Figure 3. Correcting Frequency Shift with Load Current Changes
Inductor Selection
Given the desired input and output voltages, the inductor
value and operating frequency determine the ripple
current:
∆IL
=
VOUT
fL
1−
VOUT
VIN
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 IOUT(MAX). The largest ripple current
occurs at the highest VIN. To guarantee that ripple current
does not exceed a specified maximum, the inductance
should be chosen according to:
L
=
f
VOUT
∆IL(MAX)
1−
VOUT
VIN(MAX)
Once the value for L is known, the type of inductor must be
selected. 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 Figure 1 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 it is on for only
a fraction of the duty cycle. In order for the diode to be
effective, the inductance between it and the bottom MOS-
FET must be as small as possible, mandating that these
components be placed adjacently. The diode can be omit-
ted if the efficiency loss is tolerable.
CIN and COUT Selection
The input capacitance CIN is required to filter the square
wave current at the drain of the top MOSFET. Use a low
ESR capacitor sized to handle the maximum RMS current.
IRMS
≅
IOUT(MAX)
VOUT
VIN
VIN – 1
VOUT
This formula has a maximum at VIN = 2VOUT, where
IRMS = IOUT(MAX)/ 2. This simple worst-case condition is
3778f
12
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