AP8800EV2 User Guide Issue 3

AP8800EV2 USER GUIDE
DESCRIPTION
The AP8800EV2, Figure 1, is a PCB constructed using an FR4 base for evaluating the AP8800 LED
driver with internal switch in MSOP-8L package. The evaluation board can be used to drive an
external choice of LEDs; the total forward voltage across the LEDs depending on the number and
type connected. [1]
The operating voltage for the evaluation board ranges from 8V to 28V maximum.
Higher voltage means lower supply current.
The nominal current for the evaluation board is set at 300mA with a 0R33 sense resistor, R1.
Test point CTRL provides a connection point for DC or PWM dimming and shutdown.
Note: The evaluation board does not have reverse polarity protection
Warning: with 300mA output, the connected LED will be hot and very bright
Figure 1: AP8800EV2 Evaluation Board
AP8800 DEVICE DESCRIPTION
The AP8800 is a step-down DC/DC converter designed to drive LEDs with a constant current. The
device can drive up to thirteen LEDs, depending largely on the forward voltage of the LEDs, in series
from a voltage source of 8V to 28V. Series connection of the LEDs provides identical LED currents
resulting in uniform brightness and eliminates the need for ballast resistors. The AP8800 switches at
frequencies up to 500KHz. This allows the use of small size external components, hence minimizing
the PCB area needed.
Maximum output current of AP8800 is set via an external resistor connected between the VIN and SET
input pins. Dimming is achieved by applying either a DC voltage or a PWM signal at the CTRL input
pin. An input voltage of 0.2V or lower at CTRL shuts down the output at SW and puts the device into
a low-current standby state.
The device includes the output switch and a current sense circuit, which requires an external sense
resistor to set the nominal current up to 300mA.
Issue 3 – August 2009
© Diodes Incorporated, 2009
www.diodes.com
AP8800EV2
AP8800 DEVICE FEATURES
•
•
•
•
•
•
•
•
•
DEVICE APPLICATIONS
•
•
•
•
LED driving current up to 350mA
Input voltage from 8V to 28V
High efficiency up to 92%
High switching frequency up to 500KHz
minimum
PWM/DC input for dimming control
Built-in soft-start function
Built-in output open-circuit protection
SOP-8L, MSOP-8L and DFN3030-10 are
available in “Green” Molding Compound (No
Br, Sb)
Lead Free Finish/RoHS Compliant (Note 1)
AP8800 Device Packages, Pin and Definitions
(1) SOP-8L
Commercial & industrial lighting
Small LCD panel backlighting
Appliance interior lighting
Architecture detail lighting
(2) MSOP-8L
( Top View )
( Top View )
CTRL 1
8
SET
CTRL 1
8
2
7
NC
2
7
NC
6
5
NC
RSVD
SW
6
5
NC
RSVD
SW
3
AP8800
GND 4
VIN
3
AP8800
GND 4
SOP-8L
SET
VIN
MSOP-8L
(3) DFN3030-10
( Top View )
CTRL
1
10
SET
RSVD
2
9
NC
NC
3
SW
GND
8
NC
4
7
NC
5
6
VIN
AP8800
DFN3030-10
Pin Description
Name
Description
Switch Pin. Connect inductor/freewheeling diode here. Minimize trace area at this pin to reduce
SW
EMI.
GND
GND pin
SET
Set Nominal Output Current Pin. Configure the output current of the device.
Dimming and On/Off Control Input. Input voltage of 0.2V or lower forces the device into low
current standby mode and shut off the output. A PWM signal allows the output current to be
CTRL
adjusted above or below the level set by the resistor connected to SET input pin.
The input impedance is about 200k, and if the pin is left open VCTRL = VREF
VIN
Input Supply Pin. Must be locally bypassed with a capacitor.
NC
No connection
ORDERING INFORMATION
EVALBOARD ORDER
NUMBER
AP8800EV2
DEVICE ORDER NUMBER
Package
Tape &
Reel
S (SOP8L)
-13
AP8800
M8 (MSOP-8L)
-13
FN(DFN3030-10)
-7
Device
Please note: Evaluation boards are subject
to availability and qualified leads.
Issue 3 – August 2009
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2
AP8800EV2
AP8800EV2 EVALUATION BOARD
REFERENCE DESIGN
The AP8800EV2 is an evaluation board configured to be used with the AP8800 in MSOP-8L package.
The target application is a driver for one or more series-connected LEDs for luminaires in both
commercial and automotive applications.
The maximum operating voltage for the evaluation board is 28V, and a 0R33 sense resistor R1 sets
the nominal current at 300mA.
An accurate way of determining the current, avoiding the need to insert an ammeter in the current
path, is to measure the voltage on the sense resistor. A 10K resistor and a 1uF capacitor can be used
to form a low pass filter and the voltage across the capacitor represents a more stable dc reading of
current. Using this method, 100mV represents 0.3 Amp when using a 0R33 sense resistor.
The CTRL pin connects to a low pass filter within the AP8800 chip to provide some decoupling, but
the external capacitor C2 (100nF) can be used to provide additional decoupling to reduce any high
frequency noise as well as providing soft start function.
Both DC and PWM dimming can be achieved by driving the CTRL pin. For DC dimming, the CTRL pin
may be driven between 0.3V and 1.25V. Driving the CTRL pin below 0.2V will shut down the output
current. For PWM dimming, an external open-collector NPN transistor or open-drain N-channel
MOSFET can be used to drive the CTRL pin. The PWM frequency can be around 100Hz to 1 kHz.
For better resolution, PWM capacitor C2 should be removed on the evaluation board, to give a more
accurate duty cycle.
Shorting R2 will connect the test pin CTRL to device pin CTRL if needed. The external capacitor C2
on the CTRL pin can be used to set the soft start time. The amount of soft start time achievable is
approximately 0.5ms/nF.
For other reference designs or further applications information, please refer to the AP8800 datasheet.
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AP8800EV2
AP8800 Operation
In normal operation, when voltage is applied at VIN, the AP8800’s internal NDMOS switch is turned
on. Current starts to flow through sense resistor R1, inductor L1, and the LED(s). The current ramps
up linearly. The ramp rate is determined by the input voltage VIN and inductor L1. This rising current
produces a voltage ramp across R1. The internal circuit of the AP8800 senses this voltage and
applies a proportional voltage to the input of the internal comparator. When this voltage reaches an
internally set upper threshold, the NDMOS switch is turned off. The inductor current continues to flow
through R1, L1, LED(s), Schottky diode D1, and back to the supply rail. The current decays, with the
rate of decay determined by the forward voltage drop of the LEDs and the Schottky diode. This
decaying current produces a falling voltage at R1 which is sensed by the AP8800. A voltage
proportional to the sense voltage across R1 is applied at the input of the internal comparator. When
this voltage falls to the internally set lower threshold, the NDMOS switch is turned on again. This
switch-on-and-off cycle continues to provide an average current (set by the sense resistor R1) to the
LEDs. Please refer to the datasheet for the threshold limits, AP8800 internal circuits, electrical
characteristics and parameters.
AP8800EV2 Evaluation Board - BOM
Ref
U1
Value
AP8800
D1
40V, 1A
R1
R2
C1,C4
C2
C3
0R33
0R0
1uF 50V
100nF,
100V
100uH
L1
Package
MSOP-8L
SMA
0805
0805
1206
0805
-
Part Number
AP8800M8G
Manufacturer
DIODES inc
B140
DIODES inc
NCST10FR330FTRF Generic
Generic
NMC1206X7R105K50F
Not Fitted
C0805C104K1RAC
NMC0805X7R104K100
MSS6132 100uH
NPIS53D101MTRF
NIC
NIC
Kemet
NIC
Coilcraft
NIC
Notes
DC-DC
converter
Schottky
diode
1%
10%
10%
100uH
The FR4 PCB design, with adequate copper top and bottom and plated through vias for thermal
coupling, guarantees a good thermal dissipation for the AP8800 device. Other sources of heat are the
Schottky diode, the inductor and the sense resistor, therefore care must be taken in their placement.
Warning: At 28V operation with 300mA output, the board temperature rises by around 50C
from ambient after 30 minutes of operation.
Issue 3 – August 2009
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AP8800EV2
Figure 3: Component layout and circuit board view
AP8800EV2 Connection Point Definition
Name
Description
VIN
Positive supply voltage.
GND
Supply Ground (0V).
CTRL
LED A
LED K
Internal voltage ref. pin (1.25V). This pin can be used to achieve dimming and
soft-start, and for switching the output current off.
• Leave floating for normal operation.
• See 'Circuit Features' section to achieve dimming, soft-start and for
switching the output current off.
LED A connects to
C4the external LED anode
LED K connects to the external LED cathode
AP8800EV2 Basic operation at full voltage
1. Connect external LEDs across test pins ‘LED A’ (anode) and ‘LED K’ (cathode). The number
of external LEDs that can be connected depends on their operating power and forward
voltage drop, but typically 7 x 3.2V LEDs can be connected using a 28V rail. For an external
load other than LEDs, the positive terminal of the load should be connected to the anode and
the negative to the cathode.
2. Connect VIN and GND.
Warning: The board does not have reverse battery/supply protection.
3. Set the PSU to the desired input voltage (usually between 8V and 28V)
4. Turn on the PSU. The external LEDs will illuminate and the current should be approximately
300mA
5. The switching waveform on the SW pin can be acquired using the test point J1
Warning: Do not stare at the LEDs directly.
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AP8800EV2
Circuit features
N.B. Remove power whilst changing components!
Soft-start
1.
The AP8802 has a in-built soft start function. A capacitor, C2 may be fitted to the evaluation
board to increase the soft start time by slowing the rise time of the adjust pin at start-up at the
rate of 0.5ms/nF. The board is supplied with a zero-ohm resistor in position R2. Please see
the data sheet for further details.
PWM
1. Remove the soft start capacitor C2
2. Refer to the datasheet for instructions on how to perform PWM
Switching off the output current
3. Shorting the CTRL pin to GND will cause the LED current to go to zero. Releasing this pin will
switch on the system (creating a soft-start power up sequence if the C2 capacitor is used).
Changing the LED current
1. Remove R1
2. Calculate and fit a new sense resistor, R1, the value of which is based on the required LED
current without dimming. R1 can be calculated using following equation :
R1= 0.1V / ILED where ILED = the LED current.
R1 = the sense resistors value in ohms.
0.1V is the nominal sense voltage with CTRL open circuit or set to
1.25V.
PERFORMANCE
The system efficiency depends on the sense resistor, supply voltage, switching inductor, and the
number of LEDs.
With a 28V supply and 7 LEDs, the switching frequency is typically 320 kHz and efficiency levels
>90% are achievable.
Visit our website www.diodes.com to find useful tools for circuit design and simulation.
REFERENCE
[1] AP8800 Datasheet – www.diodes.com
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AP8800EV2
INTENTIONALLY BLANK
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AP8800EV2
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