Microchip MIC3263YML Handleiding

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MIC3263 Evaluation Board
Six-Channel WLED Driver for
Backlighting Applications with
Flicker-Free Dimming
Bringing the Power to Lightâ„¢
MLF and MicroLeadFrame are registered trademarks of Amkor Technology, Inc.
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
March 2010 M9999-032410-B
General Description
This evaluation board (EVB) is designed for the MIC3263.
The MIC3263 is a high-efficiency Pulse Width Modulation
(PWM) boost switching regulator that is optimized for
constant-current WLED driver backlighting applications.
The MIC3263 drives six channels of up to ten low-power
WLEDs per channel. Each channel is matched in current
to within ±3% for constant brightness across the screen
and can be programmed from 15mA to 30mA.
The MIC3263 provides a very flexible dimming control
scheme with high accuracy and noise immunity. The
dimming frequency can be set to any value between
100Hz and 20kHz by an external resistor. The dimming
ratio is determined by the duty-cycle of a dimming ratio
control input signal and can be set to one of 16 levels with
a minimum ratio of 1%. The LED dimming current is set by
an external resistor to allow programming of LED current
between 15mA and 30mA.
The dimming ratio of the MIC3263 is fixed to 16
logarithmic levels to better match the sensitivity of the
human eye. Each of the dimming levels has hysteresis to
avoid skipping between levels resulting in Flicker-Free
operation.
The MIC3263 has a programmable PWM switching
frequency from 400kHz to 1.8MHz to allow small inductor
sizes. The 6V to 40V wide-input voltage range of MIC3263
allows direct operation from 6V or high cell count Li-Ion
batteries commonly found in notebook computers.
Precautions
The MIC3263 evaluation board operates as a boost
converter, which requires the output voltage to be greater
than the input voltage. Connect a PWM dimming signal to
the DRC pin. The DRC pin needs an input signal to
operate.
Requirements
1. Voltage source capable of supplying 10W
2. Load: Six LED strings
3. Scope
4. Function generator for PWM Dimming
Getting Started
1. Set the LED current of all six channels by one resistor
RISET (noted as R9 on the EVB). Use 2kΩ for 30mA
and 3kΩ for 20mA. Use the following equation to find
RISET:
LED
60
R =
ISET I
Ω
2. Connect six strings of LEDs from LED out to LED-
RTN. Each one of the six LED strings needs to
connect to one of the LED RTN. The cathode end of
each LED string connects to one of the six LED RTN
pins. The anode end of all the LED strings connects to
LED OUT (VOUT). The number of LEDs in each string
should be from 4 to 10.
3. Connect 12V (or other input voltage) to VIN and GND.
4. Use a current probe to measure the load current and
monitor the switch node with a scope to view the
switching waveform
5. PWM Dimming:
a. Connect a function generator to the
dimming input (DRC pin) 0V to 5V at
100Hz to 40kHz.
b. Adjust the duty-cycle of the function
generator for PWM dimming. The
MIC3263 converts the duty-cycle at the
DRC to one of sixteen dimming ratios
(DR) according to Table 1.
c. Follow the power-on sequence to ensure
all channels turn on.
d. The EVB is set for 30mA LED current. For
a different LED current, change R9 using
the given equation.
Micrel, Inc. MIC3263 Evaluation Board
March 2010 2 M9999-032410-B
Power-On Sequence
VIN needs to be present before PWM pulses are applied
to the DRC pin. Some channels may not turn on if the
power-on sequence is not followed. This may be the
case if the circuits that monitor the IO pins see transients
during the turn on-time and may interpret voltage spikes
during turn-on as a fault, preventing that channel from
turning on. When a channel is off, its IO pin is at high
impedance.
It is best to follow the sequence:
1. VIN
2. PWM dimming at DRC
3. Enable high
MODE
This selects the internal PWM dimming frequency range.
When MODE is low, the PWM dimming frequency range
is 100Hz to 2kHz and it selects low-frequency (LF)
mode. When MODE is high, the PWMD frequency range
is 1.5kHz to 20kHz and it selects high-frequency (HF)
mode.
DFS
DFS stands for Dimming Frequency Select. The
dimming frequency of the LEDs is different than the input
dimming frequency at the DRC input. The MIC3263 uses
an internal dimming frequency. This internal dimming
frequency is programmable by an external resistor to
ground RDFS (R8).
When using internal dimming the range is determined by
the MODE pin and the actual frequency is determined by
RDFS. Connect a resistor-to-ground to select a dimming
frequency.
Use the following equations to determine the value for
RDFS:
RDFS(kΩ) = −20 × fDIM(kHz) + 432 (HF Mode)
RDFS(kΩ) = −335 × fDIM(kHz) + 433 (LF Mode)
For direct dimming control, connect DFS to VDD and use
the MODE pin for the input dimming pulse. This method
by passes the internal dimming control and allows for
dimming control by the external PWM pulse.
OVP
This is the over-voltage protection monitor. Also this is
the feedback signal that connects to the error amplifier
input. The output voltage that the OVP will trigger is set
according to the following equation. Using the values for
this example gives a max output voltage of:
VOVP = 2.4 × (R1+R2)/R1 = 38V
Internal PWM Dimming
The duty-cycle of the PWM pulse applied to the DRC
input is converted to 16 log levels. This logarithmic
dimming is a unique feature of the MIC3263 which better
matches the sensitivity of the human eye compared to
linear dimming. The DRC duty-cycle to DR duty-cycle
conversion is shown in Table 2.
DRC Duty
Cycle
PWM Dimming Ratio (DR)
DR = 10(N −
−
−
−− 1) / 7
N
%
%
%
%% %
%
%
%%
0 0 0
1 6.25 1.0
2 12.5 1.4
3 18.75 1.9
4 25 2.7
5 31.25 3.7
6 37.5 5.2
7 43.75 7.2
8 50 10
9 56.25 14
10 62.5 19
11 68.75 27
12 75 37
13 81.25 52
14 87.5 72
15 93.75 100
Table 1. Dimming Ratio
Ordering Information
Order Part Number Description
MIC3263YML EV Boost Evaluation Board
Micrel, Inc. MIC3263 Evaluation Board
March 2010 3 M9999-032410-B
Figure 1. Schematic Diagram

Product specificaties

Merk: Microchip
Categorie: Niet gecategoriseerd
Model: MIC3263YML

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