MIC3223 Datasheet, PDF (10/24 Pages) Micrel Semiconductor – High Power Boost LED Driver with Integrated FET

Micrel, Inc.
Functional Description
A constant current output converter is the preferred
method for driving LEDs. Small variations in current
have a minimal effect on the light output, whereas small
variations in voltage have a significant impact on light
output. The MIC3223 LED driver is specifically designed
to operate as a constant current LED Driver.
The MIC3223 is designed to operate as a boost
converter, where the output voltage is greater than the
input voltage. This configuration allows for the design of
driving multiple LEDs in series to help maintain color and
brightness. The MIC3223 can also be configured as a
SEPIC converter, where the output voltage can be either
above or below the input voltage.
The MIC3223 has an input voltage range, from 4.5V and
20V, to address a diverse range of applications. In
addition, the LED current can be programmed to a wide
range of values through the use of an external resistor.
This provides design flexibility in adjusting the current for
a particular application need.
The MIC3223 features a low impedance gate driver
capable of switching large MOSFETs. This low
impedance provides higher operating efficiency.
The MIC3223 can control the brightness of the LEDs via
its PWM dimming capability. Applying a PWM signal (up
to 20kHz) to the DIM_IN pin allows for control of the
brightness of the LEDs.
The MIC3223 boost converter employs peak current
mode control. Peak current mode control offers
advantages over voltage mode control in the following
manner. Current mode control can achieve a superior
line transient performance compared to voltage mode
control and is easier to compensate than voltage mode
control, thus allowing for a less complex control loop
stability design. Page 9 of this datasheet shows the
functional block diagram.
Boost Converter operation
The boost converter is a peak current mode pulse width
modulation (PWM) converter and operates as follows. A
flip-flop (FF) is set on the leading edge of the clock
cycle. When the FF is set, a gate driver drives the power
FET on. Current flows from V_IN through the inductor (L)
and through the power switch and also through the
current sense resistor to PGND. The voltage across the
current sense resistor is added to a slope compensation
ramp (needed for stability). The sum of the current sense
voltage and the slope compensation voltages (called
V_CS) is fed into the positive terminal of the PWM
comparator. The other input to the PWM comparator is
the error amp output (called V_EA). The error amp’s
negative input is the feedback voltage (V_FB). V_FB is the
voltage across R_ADJ (R5). In this way the output LED
MIC3223
current is regulated. If V_FB drops, V_EA increases and
therefore the power FET remains on longer so that V_CS
can increase to the level of V_EA. The reverse occurs
when V_FB increases.
PWM Dimming
This control process just described occurs during each
DIM_IN pulse and when ever DIM_IN is high. When
DIM_IN is low, the boost converter will no longer switch
and the output voltage will drop. For high dimming ratios
use an external PWM Dimming switch as shown in the
Typical Application. When the dim pulse is on the
external switch is on and circuit operates in the closed
loop control mode as described. When the DIM_IN is low
the boost converter does not switch and the external
switch is open and no LED current can flow and the
output voltage does not droop. When DIM_IN goes high
the external switch is driven on and LED current flows.
The output voltage remains the same (about the same)
during each on and off DIM_IN pulse.
PWM Dimming can also be used in the Test Circuit in
applications that do not require high dimming ratios. In
the Test Circuit, the load is not removed from the output
voltage between DIM_IN pulses and will therefore drain
the output capacitors. The voltage that the output will
discharge to is determined by the sum of the V_F (forward
voltage drops of the LEDs). When V_OUT can no longer
forward bias the LEDs, then the LED current will stop
and the output capacitors will stop discharging. During
the next DIM_IN pulse V_OUT has to charge back up
before the full LED current will flow. For applications that
do not require high dimming ratios.
January 2010
10
M9999-011510-A