MIC2291 Datasheet, PDF (6/9 Pages) Micrel Semiconductor – 1.2A PWM Boost Regulator Photo Flash LED Driver

MIC2291
Applications Information
DC to DC PWM Boost Conversion
The MIC2291 is a constant frequency boost converter. It
operates by taking a DC input voltage and regulating current
through series LED’s by monitoring voltage across the sense
resistor (R2). LED current regulation is achieved by turning
on an internal switch, which draws current through the
inductor (L1). When the switch turns off, the inductor’s
magnetic field collapses, causing the current to be dis-
charged into the output capacitor through an external schottkey
diode (D1). Regulation is then achieved by pulse width
modulation (PWM) to maintain a constant voltage on the FB
pin. This in turn provides constant LED current.
D1
1A/40V
V_IN
10µH
Schottky
V_OUT
MIC2291-34BML
1-Cell
Li Ion
GND
VIN SW
EN OVP
FB
GND
3xLED
R2
C2
1µF
GND
Figure 2. DC to DC PWM Boost Conversion
Duty Cycle Considerations
Duty cycle refers to the switch on-to-off time ratio and can be
calculated as follows for a boost regulator;
D = 1− ^VIN
V_OUT
The duty cycle required for voltage conversion should be less
than the maximum duty cycle of 85%. Also, in light load
conditions where the input voltage is close to the output
voltage, the minimum duty cycle can cause pulse skipping.
This is due to the energy stored in the inductor causing the
output to overshoot slightly over the regulated output voltage.
During the next cycle, the error amplifier detects the output as
being high and skips the following pulse. This effect can be
reduced by increasing the minimum load or by increasing the
inductor value. Increasing the inductor value reduces peak
current, which in turn reduces energy transfer in each cycle.
Over Voltage Protection
For MLF package of MIC2291, there is an over voltage
protection function. If the feedback resistors are discon-
nected from the circuit or the feedback pin is shorted to
ground, the feedback pin will fall to ground potential. This will
cause the MIC2291 to switch at full duty-cycle in an attempt
to maintain the feedback voltage. As a result the output
voltage will climb out of control. This may cause the switch
Micrel
node voltage to exceed its maximum voltage rating, possibly
damaging the IC and the external components. To ensure
the highest level of protection, the MIC2291 OVP pin will shut
the switch off when an over-voltage condition is detected
saving itself and other sensitive circuitry downstream.
Component Selection
Inductor
Inductor selection is a balance between efficiency, stability,
cost, size and rated current. For most applications a 10uH is
the recommended inductor value. It is usually a good balance
between these considerations.
Efficiency is affected by inductance value in that larger
inductance values reduce the peak to peak ripple current.
This has an effect of reducing both the DC losses and the
transition losses. There is also a secondary effect of an
inductors DC resistance (DCR). The DCR of an inductor will
be higher for more inductance in the same package size. This
is due to the longer windings required for an increase in
inductance. Since the majority of input current (minus the
MIC2291 operating current) is passed through the inductor,
higher DCR inductors will reduce efficiency.
Also, 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;
frhpz
=
V_OUT
×
V_IN2
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 in-
creasing the output capacitor value (decreasing loop gain).
Output Capacitor
A 1µF or greater output capacitor is sufficient for most
designs. An X5R or X7R dielectric ceramic capacitors are
recommended for designs with the MIC2291. Y5V values
may be used, but to offset their tolerance over temperature,
more capacitance is required.
Diode Selection
The MIC2291 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, the maximum output 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 MIC2291. Increasing input capacitance
will improve performance and greater noise immunity on the
source. The input capacitor should be as close as possible to
M9999-081104
6
August, 2004