AP8801EV1 User Guide Datasheet by Diodes Incorporated

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Inconronnrnn Warninq: with 500mA output the connected LED will be hot and very bright
Issue 5 – August 2009 www.diodes.com
© Diodes Incorporated, 2009
AP8801EV1 USER GUIDE
DESCRIPTION
The AP8801EV1, Figure 1, is a PCB constructed using an FR4 base for evaluating the AP8801 LED
driver with internal switch in SOP-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 suggested operating voltage for the evaluation board ranges from 8V to 45V maximum.
Higher voltage means lower supply current.
The nominal current for the evaluation board is set at 460mA with a 0R43 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 500mA output, the connected LED will be hot and very bright
Figure 1: AP8801EV1 Evaluation Board
AP8801 DEVICE DESCRIPTION
The AP8801 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 48V. Series connection of the LEDs provides identical LED currents
resulting in uniform brightness and eliminates the need for ballast resistors. The AP8801 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 AP8801 is set via an external resistor connected between the V
IN
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 500mA.
_|_j_|_j CECE VCTRL VREF Vw
AP8801EV1
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AP8801 DEVICE FEATURES
LED driving current up to 500mA
Input voltage from 8V to 48V
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 are available in “Green”
Molding Compound (No Br, Sb)
Lead Free Finish/RoHS Compliant (Note 1)
DEVICE APPLICATIONS
Commercial & industrial lighting
Small LCD panel backlighting
Appliance interior lighting
Architecture detail lighting
AP8801 Device Packages, Pin and Definitions
(1) SOP-8L (2) MSOP-8L
SOP-8L
1
2
3
4 5
6
7
8
( Top View )
AP8801
NC SW
CTRL
SET
SW
V
IN
GND GND
MSOP-8L
1
2
3
4 5
6
7
8
( Top View )
AP8801
NC SW
CTRL
SET
SW
V
IN
GND GND
Pin Description
Name Description
SW Switch Pin. Connect inductor/freewheeling diode here. Minimize trace area at this pin to reduce
EMI.
GND GND pin
SET Set Nominal Output Current Pin. Configure the output current of the device.
CTRL
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
adjusted above or below the level set by the resistor connected to SET input pin.
The input impedance is about 50k, and if the pin is left open V
CTRL
= V
REF
V
IN
Input Supply Pin. Must be locally bypassed with a capacitor.
NC No connection
ORDERING INFORMATION
EVALBOARD ORDER
NUMBER
AP8801EV1
Please note: Evaluation boards are subject
to availability and qualified leads.
DEVICE ORDER NUMBER
Device Package Tape &
Reel
S (SOP8L) -13
AP8801 M8 (MSOP-8L) -13
suqaested maximum oueratinq voflaqe tor the evaluation board is 45V VIN 8V ~ 48V DC Vw CTRL R2 SET C2 C1 C3 AP8801 GND GND sw Figure 2: Schematic of the Evaluation Board
AP8801EV1
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AP8801EV1 EVALUATION BOARD
REFERENCE DESIGN
The AP8801EV1 is an evaluation board configured to be used with the AP8801 in SOP-8L package.
The target application is a driver for one or more series-connected LEDs for luminaires in both
commercial and automotive applications.
The suggested maximum operating voltage for the evaluation board is 45V, and a 0R43 sense
resistor R1 sets the nominal current at 460mA.
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, 200mV represents 0.46 Amp when using a 0R43 sense resistor.
The CTRL pin connects to a low pass filter within the AP8801 chip to provide some decoupling, but
the external capacitor C2 (100nF) is 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 1kHz.
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 sets the soft start time. The amount of soft start time achievable is approximately
0.2ms/nF.
For other reference designs or further applications information, please refer to the AP8801 datasheet.
Warninq: At 45V operation with SOOmA output the board temperature rises bv around SOC from amblent atter 30 minutes at operation.
AP8801EV1
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AP8801 OPERATION
In normal operation, when voltage is applied at V
IN
, the AP8801’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 V
IN
and inductor L1. This rising current
produces a voltage ramp across R1. The internal circuit of the AP8801 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 AP8801. 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, AP8801 internal circuits, electrical
characteristics and parameters.
AP8801EV1 Evaluation Board - BOM
Ref Value Package Part Number Manufacturer Notes
U1 AP8801 SOP-8L AP8801SG DIODES inc DC-DC
converter
D1 100V, 2A SMB B2100 DIODES inc Schottky
diode
R1 0R43 1206 SR732BTTDR430F
Generic KOA +/-1%,
+/-100ppm
R2 0R0 0805 Generic - +/-1%
C1 4.7uF 50V 1210 C1210C475K5RAC
GRM32ER71H475K
Kemet
Murata +/-10%
C2 - - Not fitted -- -
C3 100nF, 100V 0805 C0805C104K1RAC
NMC0805X7R104K100 Kemet
NIC +/-10%
C4 100nF, 100V 1206 C1206C104K1RAC
GRM319R72A104K
NMC1206X7R104K100
Kemet
Murata
NIC
+/-10%
L1 68uH - MSS1038-683ML
NPIS104F680MTRF
NPIS24H680MTRF
Coilcraft
NIC
NIC
68uH
The FR4 PCB design, with adequate copper top and bottom and plated through vias for thermal
coupling, guarantees a good thermal dissipation for the AP8801 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 45V operation with 500mA output, the board temperature rises by around 60C
from ambient after 30 minutes of operation.
Warninu: The board does not have reverse battery/sunulv urotection. Warning: Do not stare at the LEDs directly.
AP8801EV1
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Figure 3: Component layout and circuit board view
AP8801EV1 Connection Point Definition
Name Description
VIN Positive supply voltage.
GND
Supply Ground (0V).
CTRL 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 LED A connects to the external LED anode
LED K LED K connects to the external LED cathode
AP8801EV1 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 12 x 3.2V LEDs can be connected using a 45V 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 12V and 45V)
4. Turn on the PSU. The external LEDs will illuminate and the current should be approximately
500mA
5. The switching waveform on the SW pin can be acquired using the test point J1
Warning: Do not stare at the LEDs directly.
C4
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CIRCUIT FEATURES
N.B. Remove power whilst changing components!
Soft-start
1.
The AP8801 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 0.2ms/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. refer to the datasheet for the derating curve and the power dissipation capability of the
package
2. Remove R1
3. 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= 0.2V / I
LED
where I
LED
= the LED current.
R1 = the sense resistors value in ohms.
0.2V 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 45V supply and 12 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] AP8801 Datasheet – www.diodes.com
AP8801EV1
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INTENTIONALLY BLANK
AP8801EV1
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