Micrel
MIC2295
April 2005 10
M9999-042605
(408) 955-1690
Diode Selection
The MIC2295 requires an external diode for operation. A
Schottkey diode is recommended for most applications
due to their lower forward voltage drop and reverse
recovery time. Ensure the diode selected can deliver the
peak inductor current and the maximum reverse voltage is
rated greater than the output voltage.
Input Capacitor
A minimum 1µF ceramic capacitor is recommended for
designing with the MIC2295. Increasing input capacitance
will improve performance and greater noise immunity on
the source. The input capacitor should be as close as
possible to the inductor and the MIC2295, with short traces
for good noise performance.
Feedback Resistors
The MIC2295 utilizes a feedback pin to compare the
output to an internal reference. The output voltage is
adjusted by selecting the appropriate feedback resistor
values. The desired output voltage can be calculated as
follows;
V
OUT
= V
REF
×
R1
R2
+1
⎝
⎜
⎠
⎟
Where V
REF
is equal to 1.24V.
Duty-Cycle
The MIC2295 is a general-purpose step up DC-DC
converter. The maximum difference between the input
voltage and the output voltage is limited by the maximum
duty-cycle (D
max
) of the converter. In the case of MIC2295,
D
MAX
= 85%. The actual duty cycle for a given application
can be calculated as follows:
D = 1−
V
IN
V
OUT
The actual duty-cycle, D, cannot surpass the maximum
rated duty-cycle, D
max
.
Output Voltage Setting
The following equation can be used to select the feedback
resistors R1 and R2 (see figure 1).
R
1
= R
2
⋅
V
OUT
1.24V
−1
⎣
⎢
⎦
⎥
A high value of R2 can increase the whole system
efficiency, but the feedback pin input current (I
FB
) of the gm
operation amplifier will affect the output voltage. The R2
resistor value must be less than or equal to 5kΩ (R2 ≤ 5
kΩ).
Inductor Selection
In MIC2295, the switch current limit is 1.2A. The selected
inductor should handle at least 1.2A current without
saturating. The inductor should have a low DC resistor to
minimize power losses. The inductor’s value can be 4.7µH
to 10µH for most applications.
Capacitor Selection
Multi-layer ceramic capacitors are the best choice for input
and output capacitors. They offer extremely low ESR,
allowing very low ripple, and are available in very small,
cost effective packages. X5R dielectrics are preferred. A
4.7µF to 10µF output capacitor is suitable for most
applications.
Diode Selection
For maximum efficiency, Schottky diode is recommended
for use with MIC2295. An optimal component selection can
be made by choosing the appropriate reverse blocking
voltage rating and the average forward current rating for a
given application. For the case of maximum output voltage
(34V) and maximum output current capability, a 40V / 1A
Schottky diode should be used.
Open-Circuit Protection
For MLF package option of MIC2295, there is an output
over-voltage protection function that clamps the output to
below 34V in fault conditions. Possible fault conditions
may include: if the device is configured in a constant
current mode of operation and the load opens, or if in the
standard application the feedback resistors are
disconnected from the circuit. In these cases the FB pin
will pull to ground, causing the MIC2295 to switch with a
high duty-cycle. As a result, the output voltage will climb
out of regulation, causing the SW pin to exceed its
maximum voltage rating and possibly damaging the IC and
the external components. To ensure the highest level of
safety, the MIC2295 has a dedicated pin, OVP, to monitor
and clamp the output voltage in over-voltage conditions.
The OVP function is offered in the 2mm x 2mm MLF
-
8L
package option
only. To disable OVP function, tie the
OVP pin to ground
