MIC2619 Datasheet, PDF (10/15 Pages) Micrel Semiconductor – 1.2MHz PWM Boost Converter with OVP

Micrel, Inc.
Application Information
DC-to-DC PWM Boost Conversion
The MIC2619 is a constant-frequency boost converter. It
can convert a low DC input voltage to a higher DC
output voltage. Figure 1 shows a typical circuit. Boost
regulation is achieved by turning on an internal switch,
which draws current through the inductor. When the
switch turns off, the inductor’s magnetic field collapses.
This causes the current to be discharged into the output
capacitor through an external Schottky diode (D1). The
Functional Characteristics show Input Voltage ripple,
Output Voltage ripple, SW Voltage, and Inductor Current
for 10mA load current. Regulation is achieved by
modulating the pulse width i.e., pulse-width modulation
(PWM).
Figure 1. Typical Application Circuit
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
However at light loads, the inductor will completely
discharge before the end of a switching cycle. The
current in the inductor reaches 0A before the end of the
switching cycle. This is known as discontinuous
conduction mode (DCM). DCM occurs when:
I_OUT
<
V_IN
V_OUT
⋅ ^IPEAK
2
Where
( ) _{IPEAK =}
V_OUT − V_IN
L⋅f
⋅
⎜⎜⎝⎛
V_IN
V_OUT
⎟⎟⎠⎞
In DCM, the duty cycle is smaller than in continuous
conduction mode. In DCM the duty cycle is given by:
f⋅
D=
( ) 2 ⋅ L ⋅ I_OUT ⋅ V_OUT − V_IN
V_IN
MIC2619
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 slightly overshoot 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 also reduces the peak
current.
Input Capacitors
A 1µF ceramic capacitor is recommended on the VIN pin
for bypassing. Increasing input capacitance will improve
performance and provide greater noise immunity. The
input capacitor should be as close as possible to the
inductor and the MIC2619, with short traces for good
noise performance.
X5R or X7R dielectrics are recommended for the input
capacitor. Y5V dielectrics lose most of their capacitance
over temperature and are therefore not recommended.
Also, tantalum and electrolytic capacitors alone are not
recommended because of their reduced RMS current
handling, reliability, and ESR increases.
Output Capacitors
Output capacitor selection is also a trade-off between
performance, size, and cost. The minimum
recommended output capacitor is 1µF. Increasing output
capacitance will lead to an improved transient response
but also an increase in size and cost. X5R or X7R
dielectrics are recommended for the output capacitor.
Y5V dielectrics lose most of their capacitance over
temperature and are therefore not recommended.
Inductor
Inductor selection will be determined by the following
(not necessarily in order of importance);
• Inductance
• Rated current value
• Size requirements
• DC resistance (DCR)
The MIC2619 was designed for use with a 10µH
inductor. Proper selection should ensure the inductor
can handle the maximum average and peak currents
required by the load. Maximum current ratings of the
inductor are generally given in two methods; permissible
DC current and saturation current. Permissible DC
current can be rated either for a 40°C temperature rise
or a 10 to 20% loss in inductance. Ensure the inductor
selected can handle the maximum operating current.
When saturation current is specified, make sure that
there is enough margin so that the peak current will not
March 2010
10
M9999-030410-A