MIC2295 Datasheet, PDF (10/13 Pages) Micrel Semiconductor – High Power Density 1.2A Boost Regulator

Micrel
MIC2295
or X7R dielectric ceramic capacitors are
recommended for designs with the MIC2295. Y5V
values may be used, but to offset their tolerance
over temperature, more capacitance is required. The
following table shows the recommended ceramic
(X5R) output capacitor value vs. output voltage.
Output Voltage
<6V
<16V
<34V
Recommended Output
Capacitance
10µF
4.7µF
2.2µF
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 1uF 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 Vref 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
^VIN
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).
R₁ = R₂ _1V.2^OU₄^T_V
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. An R2 value of xx KW is suitable for most
applications
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 4.7uH to 10uH 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 4.7uF to 10uF 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
July 2004
10
M9999-052402
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