Microchip MIC2297-42YML Bedienungsanleitung

Microchip Nicht kategorisiert MIC2297-42YML

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MIC2297 Evaluation Board
600kHz 42V OVP PWM White LED Driver
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
May 2008 M9999-053008-A
General Description
The MIC2297 is a 600kHz PWM White LED Driver
optimized for 6 to 10 series WLEDs. With an output
voltage of up to 42V and a guaranteed 1.2A on the internal
power switch, the MIC2297 can easily drive 10 WLEDs at
20mA continuous current. The MIC2297 features WLED
brightness control using the BRT pin and has output over
voltage protection (OVP) to protect the device in case the
WLEDs are disconnected unexpectedly. Available in a tiny
10-pin 2.5mm x 2.5mm x 0.85mm MLF
®
package, the
MIC2297 solution only requires a total of 6 external
components.
The MIC2297 operates at a default (BRT pin is open)
feedback voltage of only 200mV. The low feedback
voltage reduces the power dissipation from the external
current set resistor and increases total operating
efficiency.
When brightness control is required, the MIC2297 features
brightness control by applying a DC voltage to the BRT
pin. When applying a DC voltage to the BRT pin, the
feedback voltage is equal to the BRT voltage divided by 5.
This feature essentially increases or decreases the
feedback voltage from its default value (200mV), changing
the WLED current to control the WLED brightness.
Alternatively, a PWM signal may also be applied to the
BRT pin for brightness control. When a PWM signal (1kHz
recommended) is applied to the BRT pin, the WLEDs are
dimmed depending on the duty cycle and the peak voltage
of the signal. The PWM frequency can range from 1kHz to
1MHz. The selected PWM frequency does not affect the
WLED brightness. Assuming a 1V PWM signal is applied,
as the duty cycle decreases, the feedback voltage
decreases, thus reducing the WLED current.
Requirements
The MIC2297 evaluation board requires an input power
source that is capable of delivering greater than 1.2A at
2.5V.
Precautions
The evaluation board does not have reverse polarity
protection. Applying a negative voltage to the V
IN
terminal
may damage the device.
The MIC2297 evaluation board is tailored for a single or
dual Li-Ion input source. The input voltage should never
exceed 10V.
Getting Started
1. Connect an external supply to V
IN
. Apply desired
input voltage to the V
IN
(J1) and ground (J4)
terminals of the evaluation board, paying careful
attention to polarity and supply voltage (2.5V ≤ V
IN
≤ 10V). An ammeter may be placed between the
input supply and the V
IN
(J1) terminal to the
evaluation board to accurately monitor the input
current. The ammeter and/or power lead
resistance can reduce the voltage supplied to the
input; therefore, the supply voltage at the V
IN
(J1)
terminal should be monitored.
2. Enable/Disable the MIC2297. To enable the
MIC2297, apply a 1.5V or greater voltage signal to
the EN (J2) terminal. To disable the device, pull
the EN (J2) pin below 0.4V. The evaluation board
is configured with a jumper (JP1) from V
IN
to the
enable pin and a 10k pull-down resistor to ground
to conveniently turn the part on or off. Connecting
the jumper (JP1) will enable the MIC2297, while
removing the jumper will disable the part.
3. DCV Brightness Control. To control the
brightness with a DC voltage, see the DVC
Brightness Control section.
4. PWM Brightness Control. To control the
brightness with a PWM Signal, see the PWM
Brightness Control section.
Note: For detailed specifications, please refer to the
MIC2297 Datasheet at www.micrel.com.
Ordering Information
Part Number Description
MIC2297-42YML EV Evaluation board with the
MIC2297 42V device
Micrel, Inc. MIC2297 Evaluation Board
May 2008
2 M9999-053008-A
LED Current Setting
There are 10 WLEDs provided with the evaluation board.
Two of the WLEDs (D1 and D2) may be by-passed by
placing a jumper on JP3. The WLED current (I
LED
) is equal
to the feedback voltage (V
FB
= 200mV by default) divided
by the R3 resistance value. The evaluation board is
provided with R3 equal to 10Ω. The brightness level is
proportional to I
LED
. Programming the feedback voltage
changes the I
LED
, therefore changing the brightness level.
I
LED
= V
FB
/ R3 (1)
DCV Brightness Control
The brightness level can be set by applying a DC voltage
(BRT) to the BRT pin. When a DC voltage is applied to the
BRT pin, the feedback voltage is changed from the default
value of 200mV to:
V
FB
= BRT / 5 (2)
Assuming BRT equals 1V, then V
FB
will be 200mV and
ILED may be calculated by:
I
LED
= V
FB
/ R3
I
LED
= 200mV / 10Ω
I
LED
= 20mA
Similarly, if BRT equals 2V, then V
FB
will be 400mV and
the I
LED
may be calculated by:
I
LED
= V
FB
/ R3
I
LED
= 400mV / 10Ω
I
LED
= 40mA
The feedback voltage can be changed using the BRT pin.
Changing the feedback voltage changes the WLED
current, which will change the WLED brightness. Refer to
the Figure 1 and Figure 2 for reference.
(DC Coupled)
(50mV/div)
Feedback Volta
g
e
TIME (400µs/div)
BR
T
Volta
g
e
(DC Coupled)
(1V/div)
Figure 1. BRT = 1V, V
FB
= 200mV, I
LED
= 20mA
(DC Coupled)
(100mV/div)
Feedbac
k
Volta
g
e
TIME (400µs/div)
BR
T
Volta
g
e
(DC Coupled)
(1V/div)
Figure 2. BRT = 2V, V
FB
= 400mV, I
LED
= 40mA
PWM Brightness Control
The brightness level can also be set by applying a PWM
signal to the BRT pin. To calculate the feedback voltage
when a PWM signal is applied to the BRT pin, use the
following formula:
V
FB = VPEAK / 5 * D (3)
VPEAK is the peak of the PWM voltage and D is the duty
cycle. If VPEAK is 1V and the duty cycle is 1%, then VFB can
be calculated by:
VFB = 1V / 5 * 0.01
VFB = 2mV
The ILED can then be calculated by:
ILED = VFB / R3
ILED = 2mV / 10Ω
ILED = 200µA
Similarly, if the VPEAK is 1V and the duty cycle is 50%, then
VFB can be calculated by:
VFB = 1V / 5 * 0.5
VFB = 100mV
The ILED can then be calculated by:
ILED = VFB / R3
ILED = 100mV / 10Ω
ILED = 10mA
With PWM brightness control, the MIC2297 has great
versatility since brightness may be set anywhere from 0 to
100 percent. Refer to the following figures for reference.
Micrel, Inc. MIC2297 Evaluation Board
May 2008
3 M9999-053008-A
(DC Coupled)
(10mV/div)
Feedback Voltage
TIME (400µs/div)
BRT Voltage
(DC Coupled)
(1V/div)
Figure 3. Duty Cycle = 1%, V
FB
= 2mV, I
LED
= 200µA
(DC Coupled)
(50mV/div)
Feedbac
k
Volta
g
e
TIME (400µs/div)
BR
T
Volta
g
e
(DC Coupled)
(1V/div)
Figure 4. Duty Cycle = 20%, V
FB
= 40mV, I
LED
= 4mA
(DC Coupled)
(50mV/div)
Feedbac
k
Volta
g
e
TIME (400µs/div)
BRT Voltage
(DC Coupled)
(1V/div)
Figure 5. Duty Cycle = 50%, V
FB
= 100mV, I
LED
= 10mA
(DC Coupled)
(50mV/div)
Feedbac
k
Volta
g
e
TIME (400µs/div)
BRT Voltage
(DC Coupled)
(1V/div)
Figure 6. Duty Cycle = 80%, V
FB
= 160mV, I
LED
= 16mA
In Figure 7, when the duty cycle is equal to 100%, D
equals 1. When we set D equal to 1 in equation (3), notice
(3) becomes the same as equation (2), if we assume V
PEAK
equals BRT. Using a 100% duty cycle is the same as
applying a constant DC voltage to the BRT pin. In this
instance, Figure 7 is exactly the same as Figure 1.
(DC Coupled)
(50mV/div)
Feedback Volta
g
e
TIME (400µs/div)
BR
T
Volta
g
e
(DC Coupled)
(1V/div)
Figure 7. Duty Cycle = 100%, V
FB
= 200mV, I
LED
= 20mA

Produktspezifikationen

Marke: Microchip
Kategorie: Nicht kategorisiert
Modell: MIC2297-42YML

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