MIC3263 Datasheet, PDF (12/32 Pages) Micrel Semiconductor – Six-Channel WLED Driver for Backlighting Applications with Flicker-Free Dimming

Micrel, Inc.
MIC3263
V(IO) Too High
If the collector voltage V(IO1–IO6) is greater than 1.2V,
then the CRV loop will slowly discharge (lower the voltage)
the CRV capacitor. Since the CRV capacitor is used as the
reference voltage for the boost error amp the boost voltage
(V_OUT) will decrease. With lower V_OUT, V(IO) also
decreases. Discharging of CRV continues until V(IO) is
1.2V.
V(IO) Too Low
If the collector voltage V(IO1-IO6) is less than 1.2V the
CRV loop will slowly charge (increase the voltage) the
CRV. Since CRV is used as the reference voltage for the
boost converter’s error amp the boost voltage (V_OUT) will
increase. With higher V_OUT, V(IO) also increases. Charging
of CRV continues until V(IO) is about 1.2V.
These control loops operate as described above during
DR high pulses. When DR is low the booster is off and the
last state of the CRV charge or discharge will continue
until the next DR pulse. If the external PWM Dimming
pulse (DRC) is removed, the internal dimming pulse (DR)
will continue dimming at the same dimming level before
the signal at DRC was removed and the CRV loop will
keep operating normally. If external PWM DIM is 0% then
charge/discharge states will discontinue and CRV will no
longer be charged or discharged. CRV will slowly
discharge through the circuitry connected to it
Boost Controller Operation
The MIC3263 uses a peak current-mode boost controller
in its boost stage. The boost converter is a pulse width
modulation (PWM) controller and operates thus. 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 bipolar switch
on. Current flows from V_IN through the inductor (L) and
through the internal power switch and 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 (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_OVP).
The OVP pin is used as the voltage feedback to the error
amplifier. In this way the output voltage is regulated. If
V_OVP drops, V_EA increases and therefore the power switch
remains on longer so that V_CS can increase to the level of
V_EA. The reverse occurs when V_OVP increases.
The output voltage is always higher than the input voltage.
The external CRV (see C7 in Typical Application
illustration) is used as the reference voltage to the boost
error amp.
The boost regulated output voltage is:
Equation 1
VOUT
=
CRV
×
(R1+ R2)
R2
The MIC3263 is designed for a wide input voltage range,
from 6V to 40V. As a peak current-mode controller, the
MIC3263 provides the benefits of superior line transient
response as well as an easier to design compensation.
MIC3263 provides several protection features, including:
• Current Limit (I_LIMIT) ─ Current sensing for
over current and overload protection
• Over-Voltage Protection (OVP) ─ output
over-voltage protection to prevent
operation above a safe upper limit
• The boost stage is on (switching) during a
high DR pulse and is off (not switching)
when the DR pulse is low.
Application Information
At Start Up
At start up, a switch connects 1.8V to the CRV. The
feedback resistor divider (R1 and R2) is calculated to
achieve the approximate boost output voltage with a V_CRV
of 1.8V.
Example:
•
•
•
•
8 LEDs at 3.5V each = 28V
VIO = 1.2V
V_OUT = 29.3V estimate
Set R divider to: R1 =150k R2 = 9.88k
The CRV control loop will charge/discharge CRV until the
correct boost voltage appears at the output.
Case 1
If 29.3V is too high to properly forward bias the LED
channel at the ISET current level, then the current amp
loop will decrease the drive to the bipolar transistor and
V(IO) will increase and the CRV control loop will decrease
CRV and the boost output voltage (V_OUT) will decrease.
January 2010
12
M9999-012110