MIC2297 Datasheet, PDF (9/12 Pages) Micrel Semiconductor – 40V PWM Boost Regulator White LED Driver

Micrel
MIC2297
Functional Description
The MIC2297 is a constant frequency, PWM current mode
boost regulator. The MIC2297 uses peak current mode
control. The block diagram is shown above. The MIC2297
is composed of an oscillator, slope compensation ramp
generator, current amplifier, gm error amplifier, PWM
generator, and a 1.2A bipolar output transistor. The
oscillator generates a 600kHz clock. The clock’s two
functions are to trigger the PWM generator that turns on
the output transistor and to reset the slope compensation
ramp generator. The current amplifier is used to measure
the switch current by amplifying the voltage signal from the
internal sense resistor. The output of the current amplifier
is summed with the output of the slope compensation
ramp generator. This summed current-loop signal is fed to
one of the inputs of the PWM generator.
The gm error amplifier measures the LED current through
the external sense resistor and amplifies the error between
the detected signal and the 200mV reference voltage. The
output of the gm error amplifier provides the voltage-loop
signal that is fed to the other input of the PWM generator.
When the current-loop signal exceeds the voltage-loop
signal, the PWM generator turns off the bipolar output
transistor. The next clock period initiates the next switching
cycle, maintaining the constant frequency current-mode
PWM control. The LED current is set by the feedback
resistor:
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 MIC2297 is a constant-frequency boost converter. It
operates by taking a DC input voltage and regulating a
higher DC output voltage. Figure 2 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 diode
(D1). Waveforms 1 and 2 show Output Voltage ripple, SW
Voltage, and Indicator Current for 5mA and 20mA LED
current respectively. Voltage regulation is achieved by
modulating the pulse width or pulse-width modulation
(PWM).
6.8µH-22µH
1-Ce l l
Li Ion
3V to 4.2V
1µF
1µF
VIN
SW
EN
OVP
MIC2297
BRT -42BM L
REF
FB
AGND PGND COMP
0.1µF
0.47µF
/50V
10
Figure 2. 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
<
V_in
V_out
⋅
I _peak
2
Where
( ) _{I peak =}
V_out − V_in
L⋅ 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 95%. 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 reduces peak current.
May 2005
9
M9999-050305
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