el7571c ETC-unknow, el7571c Datasheet - Page 9

• Manufacturer Part Number: el7571c
• Description: Programmable Controller
• Manufacturer: ETC-unknow

tion. The following table lists some of the differences
between the various configurations:
Converter Topologies
Circuit schematics and Bills of Material (BOMs) for the
various topologies are provided at the end of this data
sheet. If your application requirements differ from the
included samples, the following design guide lines
should be used to select the key component values.
Refer to the front page connection diagram for compo-
nent locations.
Output Inductor, L
Two key converter requirements are used to determine
inductor value:
• I
• I
Although many factors influence the choice of the
inductor value, including efficiency, transient response
and ripple current, one practical way of sizing the induc-
tor is to select a value which maintains continuous mode
operation, i.e. inductor current positive for all condi-
tions. This is desirable to optimize load regulation and
light load transient response. When the minimum induc-
tor ripple current just reaches zero and with the mean
ripple current set to I
twice I
inductor value is given by:
5V only Non-synchronous
5V only Synchronous
5V &12V Non-synchronous
5V & 12V Synchronous
12V only Synchronous
which the converter enters the discontinuous mode of
operation (refer to Elantec application note #18 for a
detailed discussion of discontinuous mode)
MIN
MAX
L
1 MIN
- minimum output current; the current level at
MAX
- maximum output current
Topology
, independent of duty cycle. The minimum
=
---------------------------------------------------- -
V
IN
–
V
1
PEAK
OUT
MIN
1
, peak inductor ripple current is
T
ON
Connection
=
Diagram
Diagram
figure 1
figure 2
figure 3
figure 4
-------------------------------------------------------- -
V
V
IN
IN
–
F
V
S W
OUT
2
V
I
Efficiency
MIN
OUT
92%
95%
92%
95%
92%
9
where:
Since inductance value tends to decrease with current,
ripple current will generally be greater than 21
higher output current.
Once the minimum output inductance is determined, an
off the shelf inductor with current rating greater than the
maximum DC output required can be selected. Pulse
Engineering and Coil Craft are two manufactures of
high current inductors. For converter designers who
want to design their own high current inductors, for
experimental purposes or to further reduce costs, we rec-
ommend the Micrometals Powered Iron Cores data
sheet and applications note as a good reference and start-
ing point.
Current Sense Resistor, R
Inductor current is monitored indirectly via a low value
resistor R1. The voltage developed across the current
sense resistor is used to set the maximum operating cur-
rent, the current reversal threshold and the system load
regulation. To ensure reliable system operation it is
important to sense the actual voltage drop across the
resistor. Accordingly a four wire Kelvin connection
should be made to the controller current sense inputs.
highest
higher
lowest
Cost
high
low
I
T
V
F
V
I
PEAK
MIN
SW
ON
IN
OUT
= input voltage
= switching frequency
= top switch on time
= minimum load
= output voltage
= peak ripple current
Complexity
Programmable PWM Controller
highest
lowest
higher
high
low
Input Noise
1
high
high
high
high
high
EL7571C
Transient
Response
good
good
good
good
best
MIN
at