MIC3205 Datasheet, PDF (11/23 Pages) Micrel Semiconductor – High-Brightness LED Driver Controller with Fixed-Frequency Hysteretic Control

Micrel, Inc.
MIC3205
Functional Description
The MIC3205 is a hysteretic step-down driver that
regulates the LED current with a patent pending
frequency regulation scheme. This scheme maintains a
fixed operating frequency over a wide input voltage
range.
Theory of Operation
The device operates from a 4.5V to 40V input MOSFET
voltage. At turn-on, after the VIN input voltage crosses
4.5V, the DRV pin is pulled high to turn on an external
MOSFET. The inductor and series LED current builds up
linearly. This rising current results in a rising differential
voltage across the current sense resistor (R_CS). When
this differential voltage reaches an upper threshold, the
DRV pin is pulled low, the MOSFET turns off, and the
Schottky diode takes over and returns the series LEDs
and inductor current to V_IN. Then, the current through the
inductor and series LEDs starts to decrease. This
decreasing current results in a decreasing differential
voltage across R_CS. When this differential voltage
reaches a lower threshold, the DRV pin is pulled high,
the MOSFET is turned on, and the cycle repeats. The
average of the CS pin voltage is 200mV below V_IN
voltage. This is the average current sense threshold
(∆V_CS). Thus, the CS pin voltage switches about V_IN –
200mV with a peak-to-peak hysteresis that is the product
of the peak-to-peak inductor current times the current
sense resistor (R_CS). The average LED current is set by
R_CS, as explained in the “Application Information”
section.
MIC3205 dynamically adjusts hysteresis to
accommodate fixed-frequency operation. Average
frequency is programmed using an external capacitor
connected to the CTIMER pin, as explained in the
“Frequency of Operation” subsection in the “Application
Information” section. The internal frequency regulator
dynamically adjusts the inductor current hysteresis every
eight switching cycles to make the average switching
frequency a constant. If the instantaneous frequency is
higher than the programmed average value, the
hysteresis is increased to lower the frequency and vice
versa. In other hysteretic control systems, current sense
hysteresis is constant and frequency can change with
input voltage, inductor value, series LEDs voltage drop,
or LED current. However, with this patent pending
frequency regulation scheme, the MIC3205 changes
inductor current hysteresis and keeps the frequency
fixed even upon changing input voltage, inductor value,
series LEDs voltage drop, or LED current.
The MIC3205 has an on-board 5V regulator, which is for
internal use only. Connect a 4.7µF capacitor on VCC pin
to analog ground.
The MIC3205 has an EN pin that gives the flexibility to
enable and disable the output with logic high and low
signals. The maximum EN voltage is V_IN.
Figure 2. Theory of Operation
LED Dimming
The MIC3205 LED driver can control the brightness of the
LED string through the use of pulse width modulated (PWM)
dimming. A DIM pin is provided, which can turn on and off
the LEDs if EN is in an active-high state. This DIM pin
controls the brightness of the LED by varying the duty cycle
of DIM pin from 1% to 99%.
An input signal from DC up to 20kHz can be applied to the
DIM pin (see “Typical Application”) to pulse the LED string
on and off. A logic signal can be applied on the DIM pin for
dimming, independent of input voltage (V_IN). Using PWM
dimming signals above 120Hz is recommended to avoid any
recognizable flicker by the human eye. Maximum allowable
dimming frequency is 2% of operating frequency that is set
by the external capacitor on the CTIMER pin (see
“Frequency of Operation”). See “Functional Characteristics”
on page 9 for PWM dimming waveforms. Maximum DIM
voltage is V_IN.
PWM dimming is the preferred way to dim an LED to prevent
color/wavelength shifting. Color/wavelength shifting occurs
with analog dimming. By using PWM dimming, the output
current level remains constant during each DIM pulse. The
hysteretic buck converter switches only when the DIM pin is
high. When the DIM pin is low, no LED current flows and the
DRV pin is low turning the MOSFET off.
October 2012
11
M9999-102312-A