MIC3230 Datasheet, PDF (17/20 Pages) Micrel Semiconductor – Constant Current Boost Controller for Driving High Power LEDs

Micrel, Inc.
OVP-Over voltage protection
Set OVP higher than the maximum output voltage by at
least one volt. To find the resistor divider values for
OVP use Equation 3 and set the OVP=30V and
R8=100kΩ:
R9 = ^{100kΩ ×1.245} = 4.33kΩ
30 − 1.245
PCB Layout
1. All typologies of DC-to-DC converters have a reverse
recovery current (RRC) of the flyback or (freewheeling)
diode. Even a Schottky diode, which is advertised as
having zero RRC, it really is not zero. The RRC of the
freewheeling diode in a boost converter is even greater
than in the Buck converter. This is because the output
voltage is higher than the input voltage and the diode
has to charge up to –V_OUT during each on-time pulse and
then discharge to V_F during the off-time.
MIC3230/1/2
2. Even though the RRC is very short (tens of
nanoseconds) the peak currents are high (multiple
amperes). The high RRC causes a voltage drop on the
ground trace of the PCB and if the converter control IC is
referenced to this voltage drop, the output regulation will
suffer.
3. It is important to connect the IC’s reference to the
same point as the output capacitors to avoid the voltage
drop caused by RRC. This is also called a star
connection or single point grounding.
4. Feedback trace: The high impedance traces of the
FB should be short.
January 2009
17
M9999-011409-A