MIC2282 Datasheet, PDF (5/10 Pages) Micrel Semiconductor – Single-Cell Ultra Low EMI Boost LED Driver

Micrel, Inc.
Functional Diagram
V_BATT
VIN
0.22V
Reference
Oscillator
MIC2282
Driver
SW
MIC2282
V_OUT
SNS
GND
Adjustable Voltage with External Components
Functional Description
The MIC2282 boost LED driver has a gated oscillator
architecture designed to operate from a single cell input
voltage as low as 0.9V and provide a high-efficiency
adjustable regulated output voltage. One advantage of
this architecture is that the output switch is disabled
whenever the output voltage is above the feedback
comparator threshold thereby greatly reducing quiescent
current and improving efficiency, especially at low output
currents.
The comparator senses the output voltage through an
external resistor and compares it to the internal
reference voltage. When the voltage at the inverting
input of the comparator is below 0.22V, the comparator
output is high and the output of the oscillator is allowed
to pass through the AND gate to the output driver and
output switch. The output switch then turns on and off
storing energy in the inductor. When the output switch is
on (low) energy is stored in the inductor; when the switch
is off (high) the stored energy is dumped into the output
capacitor which causes the output voltage to rise.
When the output voltage is high enough to cause the
comparator output to be low (inverting input voltage is
above 0.22V) the AND gate is disabled and the output
switch remains off (high). The output switch remains
disabled until the output voltage falls low enough to
cause the comparator output to go high.
approximately 3 times larger than the input voltage.
Other output voltages are also easily generated with a
slight drop in efficiency. The fixed oscillator frequency is
set to 20kHz.
Current Limit
Current limit for the MIC2282 functions by modifying the
oscillator duty cycle and frequency. When current
exceeds 1.1A, the duty cycle is reduced (switch on-time
is reduced, off-time is unaffected) and the corresponding
frequency is increased. In this way less time is available
for the inductor current to build up while maintaining the
same discharge time. The onset of current limit is soft
rather than abrupt but sufficient to protect the inductor
and output switch from damage. Certain combinations of
input voltage, output voltage and load current can cause
the inductor to go into a continuous mode of operation.
This is what happens when the inductor current can not
fall to zero and occurs when:
duty cycle ≤ ^{VOUT + VDIODE − VIN}
V_OUT + V_DIODE − V_SAT
Application Information
Oscillator Duty Cycle and Frequency
The oscillator duty cycle is set to 67% which is optimized
to provide maximum load current for output voltages
October 2009
5
M9999-102309