MIC2619 Datasheet, PDF (11/15 Pages) Micrel Semiconductor – 1.2MHz PWM Boost Converter with OVP

Micrel, Inc.
saturate the inductor. Peak current can be calculated as
follows:
I_PEAK
=
⎡
⎢^IOUT
⎣
+
V_OUT
⎜⎛
⎝
1
− V_OUT
2×f ×
⋅ V_IN
L
⎟⎠⎞⎥⎦⎤
As shown by the previous calculation, the peak inductor
current is inversely proportional to the switching
frequency and the inductance; the lower the switching
frequency or the inductance the higher the peak current.
As input voltage increases the peak current also
increases.
The size of the inductor depends on the requirements of
the application.
DC resistance (DCR) is also important. While DCR is
inversely proportional to size, DCR can represent a
significant efficiency loss. Refer to the Efficiency
Considerations.
To maintain stability, increasing inductor size will have to
be met with an increase in output capacitance. This is
due to the unavoidable “right half plane zero” effect for
the continuous current boost converter topology. The
frequency at which the right half plane zero occurs can
be calculated as follows:
Frequency =
V_IN ²
V_OUT ⋅ L ⋅ I_OUT ⋅ 2π
The right half plane zero has the undesirable effect of
increasing gain, while decreasing phase. This requires
that the loop gain is rolled off before this has significant
effect on the total loop response. This can be
accomplished by either reducing inductance (increasing
RHPZ frequency) or increasing the output capacitor
value (decreasing loop gain).
MIC2619
Diode Selection
The MIC2619 requires an external diode for operation. A
Schottky 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.
Soft-start
Feed-forward capacitors can be used to provide soft-
start for the MIC2619. Figure 2 shows a typical circuit
for soft-start applications. Typically a 0.22nF feed-
forward capacitor will yield 5ms in rise time.
Figure 2. Soft-start Circuit
Feedback resistors
The MIC2619 utilizes a feedback pin to compare the
output to an internal reference. The output voltage is
adjusted by selecting the appropriate feedback resistor
network values. Using the evaluation board schematic
as a reference, the desired output voltage can be
calculated as follows:
V_OUT
=
V_REF
⋅ _{⎜⎜⎝⎛}
R₄
R₅
+ 1 _{⎟⎟⎠⎞}
Where V_REF is equal to 1.265V. Over-voltage Protection
uses the same equation as the feedback pin.
V_OVP
= V_REF
⋅
⎜⎜⎝⎛
R₁
R₂
+ 1 _{⎟⎟⎠⎞}
March 2010
11
M9999-030410-A