MIC2299_11 Datasheet, PDF (8/14 Pages) Micrel Semiconductor – 3.5A Minimum, 2MHz High Brightness LED Driver

Micrel, Inc.
Functional Description
The MIC2299 is a constant frequency, pulse-width-
modulated (PWM) peak current-mode step-up regulator.
The MIC2299 simplified control scheme is illustrated in
the block diagram in Figure 2. A reference voltage is fed
into the PWM engine where the duty cycle output of the
constant frequency PWM engine is computed from the
error, or difference, between the REF and FB voltages.
The PWM engine encompasses the necessary circuit
blocks to implement a current-mode boost switch-mode
power supply. The necessary circuit blocks include, but
are not limited to, an oscillator/ramp generator, slope
compensation ramp generator, g_m error amplifier, current
amplifier, PWM comparator, and drive logic for the
internal bipolar power transistor.
Inside the PWM engine the oscillator functions as a
trigger for the PWM comparator that turns on the bipolar
power transistor and resets the slope compensation
ramp generator. The current amplifier is used to
measure the power transistor’s current by amplifying the
voltage signal from the CS+ and CS- inputs from the
sense resistor connected to the emitter of the bipolar
power transistor. The output of the current amplifier is
summed with the output of the slope compensation ramp
generator where the result is connected to one of the
inputs of the PWM comparator.
The g_m error amplifier measures the white LED current
through the external sense resistor and amplifies the
error between the detected voltage signal from the
feedback, or FB pin and the internal reference voltage.
The output of the g_m error amplifier provides the voltage
loop signal that is fed to the other input of the PWM
comparator. When the current loop signal exceeds the
voltage loop signal the PWM comparator turns off the
power transistor. The next oscillator/clock period initiates
the next switching cycle, maintaining the constant
frequency current-mode PWM control. The white LED
current is set by the feedback resistor (the resistor
connected from the feedback pin to ground):
I _LED
=
200mV
R_FB
The enable pin shuts down the output switching and
disables control circuitry to reduce input current to
leakage levels. Enable pin input current is zero at zero
volts.
DC-to-DC PWM Boost Conversion
The MIC2299 is a constant-frequency boost converter. It
operates by taking a DC input voltage and regulating a
higher DC output voltage. Figure 3 shows a typical
circuit. Boost regulation is achieved by turning on an
internal switch, which draws current through the inductor
(L1). When the switch turns off, the inductor’s magnetic
field collapses. This causes the current to be discharged
into the output capacitor through an external Schottky
October 2007
MIC2299
diode (D1). Waveform 5 in Functional characteristics
shows Input Voltage ripple, Output Voltage ripple, SW
Voltage, and Inductor Current for 900mA LED current.
Regulation is achieved by modulating the pulse width i.e.
pulse-width modulation (PWM).
Figure 3. Typical Application Circuit
Duty Cycle Considerations
Duty cycle refers to the switch on-to-off time ratio and
can be calculated as follows for a boost regulator:
D = 1− ^Vin
V_out
However, at light loads the inductor will completely
discharge before the end of a switching cycle. The
current in the inductor reaches 0A before the end of the
switching cycle. This is known as discontinuous
conduction mode (DCM). DCM occurs when:
I _out
< ^Vin
V_out
⋅ ^{I peak}
2
Where
I _peak
=
(V_out
L
− V_in
⋅f
)
⋅
⎜⎜⎝⎛
V_in
V_out
⎟⎟⎠⎞
In DCM, the duty cycle is smaller than in continuous
conduction mode. In DCM the duty cycle is given by:
f⋅
D=
2 ⋅ L ⋅ I_out ⋅ (V_out − V_in )
V_in
The duty cycle required for voltage conversion should be
less than the maximum duty cycle of 90%. Also, in light
load conditions where the input voltage is close to the
output voltage, the minimum duty cycle can cause pulse
skipping. This is due to the energy stored in the inductor
causing the output to overshoot slightly over the
regulated output voltage. During the next cycle, the error
amplifier detects the output as being high and skips the
following pulse. This effect can be reduced by increasing
the minimum load or by increasing the inductor value.
Increasing the inductor value also reduces the peak
current.
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M9999-101907-B