MIC2172_06 Datasheet, PDF (18/20 Pages) Micrel Semiconductor – 100kHz 1.25A Switching Regulators

Micrel
MIC2172/3172
Figure 13. Step-Down or Buck Regulator
mutual coupling to the feedback winding further reduces
primary current eventually turning Q1 off. The primary
windings now change state with the feedback winding
forcing Q2 on repeating the alternate half cycle exactly
as with Q1. This action produces a sinusoidal voltage
wave form; whose amplitude is proportional to the input
voltage, across T1’s primary winding which is stepped
up and capacitively coupled to the lamp.
Lamp Current Regulation
Initial ionization (lighting) of the fluorescent lamp
requires several times the ac voltage across it than is
required to sustain current through the device. The
current through the lamp is sampled and regulated by
the MIC3172 to achieve a given intensity. The MIC3172
uses L1 to maintain a constant average current through
the transistor emitters. This current controls the voltage
amplitude of the Royer oscillator and maintains the lamp
current. During the negative half cycle, lamp current is
rectified by D3. During the positive half cycle, lamp
current is rectified by D2 through R4 and R5. R3 and C5
filter the voltage dropped across R4 and R5 to the
MIC3172’s feedback pin. The MIC3172 maintains a
constant lamp current by adjusting its duty cycle to keep
the feedback voltage at 1.24V. The intensity of the lamp
is adjusted using potentiometer R5. The MIC3172
adjusts its duty cycle accordingly to bring the average
voltage across R4 and R5 back to 1.24V.
On/Off Control
Especially important for battery powered applications,
the lamp can be remotely or automatically turned off
using the MIC3172’s EN pin. The entire circuit draws
less than 1µA while shutdown.
Efficiency
To obtain maximum circuit efficiency careful selection of
Q1 and Q2 for low collector to emitter saturation voltage
is a must. Inductor L1 should be chosen for minimal core
and copper losses at the switching frequency of the
MIC3172, and T1 should be carefully constructed from
magnetic materials optimized for the output power
required at the Royer oscillator frequency. Suitable
inductors may be obtained from Coiltronics, Inc., tel:
[phone].
April 2006
18
M9999-041806
[phone]