LTC3717-1 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC3717-1
Linear Technology
'LTC3717-1' PDF : 20 Pages View PDF
LTC3717-1
APPLICATIO S I FOR ATIO
VOUT
RVON1
30k
RVON2
100k
RC
CC
CVON
0.01µF
VON
LTC3717-1
ITH
(3a)
VOUT
INTVCC
RVON1
3k
RVON2
10k 10k
Q1
2N5087
CVON
0.01µF
RC
CC
VON
LTC3717-1
ITH
37171 F03
(3b)
Figure 3. Adjusting Frequency Shift with Load Current Changes
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.
Inductor L1 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 cores 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. High efficiency converters generally cannot af-
ford 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 manu-
facturers such as Sumida, Panasonic, Coiltronics, Coil-
craft and Toko.
Schottky Diode D1, D2 Selection
The Schottky diodes, D1 and D2, shown in Figure 1
conduct during the dead time between the conduction of
the power MOSFET switches. It is intended to prevent the
body diodes of the top and bottom MOSFETs from turning
on and storing charge during the dead time, which can
cause a modest (about 1%) efficiency loss. The diodes can
be rated for about one half to one fifth of the full load current
since they are on for only a fraction of the duty cycle. In
order for the diode to be effective, the inductance between
it and the bottom MOSFET must be as small as possible,
mandating that these components be placed adjacently.
The diodes can be omitted 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
commonly used for design because even significant
deviations do not offer much relief. Note that ripple
current ratings from capacitor manufacturers are often
Kool Mµ is a registered trademark of Magnetics, Inc.
sn37171 37171fs
11
Share Link: 