AP8802EV2 User Guide Issue 5

AP8802EV2 USER GUIDE
DESCRIPTION
The AP8802EV2, Figure 1, is a PCB constructed using an FR4 base for evaluating the AP8802 LED
driver with internal switch in the SOP 8L-EP 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 1A with a 0R2 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 1A output, the connected LED will be hot and very bright
Figure 1: AP8802EV2 evaluation board
AP8802 DEVICE DESCRIPTION
The AP8802 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 AP8802 switches at
frequencies up to 500KHz. This allows the use of small size external components, hence minimizing
the PCB area needed.
The maximum output current of the AP8802 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 1A.
Issue 5 – August 2009
© Diodes Incorporated, 2009
www.diodes.com
AP8802EV2
AP8802 DEVICE FEATURES
•
•
•
•
•
•
•
•
•
DEVICE APPLICATIONS
•
•
•
•
LED driving current up to 1A
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, DFN3030-10 are
available in “Green” Molding Compound (No
Br, Sb)
Lead Free Finish/RoHS Compliant
Commercial & industrial lighting
Small LCD panel backlighting
Appliance interior lighting
Architecture detail lighting
AP8802 Device Packages, Pin and Definitions
(1) SOP-8L
(2) SOP-8L-EP
( Top View )
SET 1
GND 2
NC 3
( Top View )
CTRL
SET 1
7
GND
GND 2
6
5
SW
NC 3
SW
VIN 4
8
AP8802
VIN 4
SOP-8L
8
AP8802
CTRL
7
GND
6
5
SW
SW
SOP-8L-EP
(3) DFN3030-10
( Top View )
SET
1
10
CTRL
GND
2
9
GND
NC
3
8
NC
NC
4
7
SW
VIN
5
6
SW
AP8802
DFN3030-10
PIN DESCRIPTION
Name
SW
GND
SET
CTRL
VIN
NC
Description
Switch Pin. Connect inductor/freewheeling diode here. Minimize trace area at this pin to reduce
EMI.
GND pin
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
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 VCTRL = VREF
Input Supply Pin. Must be locally bypassed with a capacitor.
No connection
Issue 5 August 2009
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2
AP8802EV2
ORDERING INFORMATION
EVALBOARD ORDER
NUMBER
AP8802EV2
DEVICE ORDER NUMBER
Package
Tape &
Reel
S (SOP-8L)
-13
AP8802
SP (SOP-8L-EP)
-13
FN (DFN3030-10)
-7
Device
Please note: Evaluation boards are subject
to availability and qualified leads.
AP8802EV2 EVALUATION BOARD
REFERENCE DESIGN
The AP8802EV2 is an evaluation board configured to be used with the AP8802 in SOP-8L-EP
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 0R2 sense resistor
R1 sets the nominal current at 1A.
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 1 Amp when using a 0R2 sense resistor.
The CTRL pin connects to a low pass filter within the AP8802 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 the 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 from 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 AP8802 datasheet.
Issue 5 August 2009
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© Diodes Incorporated 2009
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AP8802EV2
VIN
8V ~ 48V DC
VIN
CTRL
R1
R2
SET
C1
C3
GND
AP8802
D1
C4
1
C2
L1
GND
SW
Figure 2: Schematic of the Evaluation Board
AP8802 OPERATION
In normal operation, when voltage is applied at VIN, the AP8802’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 rate being determined by the input voltage VIN and inductor L1. This rising current
produces a voltage ramp across R1. The internal circuit of the AP8802 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, the 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 AP8802. 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, AP8802 internal circuits, electrical
characteristics and parameters.
AP8802EV2 EVALUATION BOARD - BILL OF MATERIALS
Ref
U1
Value
AP8802
Package
SOP8LEP
Part Number
AP8802SPG
Manufacturer
DIODES inc
D1
100V, 2A
SMB
B2100
DIODES inc
R1
0R2
1206
SR732ETTDR200F
KOA
R2
C1
0R0
4.7uF 50V
0805
1210
C2
C3
C4
L1
100nF,100V
100nF 100V
68uH
0805
1206
-
Generic
C1210X475K5RAC
GRM32ER71H475KA88L
Not Fitted
Generic
Generic
MSS1038-683ML
NPIS104F680MTRF
NPIS24H680MTRF
KEMET
Murata
Coilcraft
NIC
NIC
Notes
DC-DC
converter
Schottky
diode
+/-1%,
+/-100ppm
+/-1%
X7R +/-10%
X7R +/-20%
X7R +/-20%
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 AP8802 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 1A output, the board temperature rises by around 60C from
ambient after 30 minutes of operation.
Issue 5 August 2009
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AP8802EV2
Figure 3: Component layout and circuit board view
AP8802EV2 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.
C4
LED A connects to the external LED anode
LED K connects to the external LED cathode
AP8802EV2 BASIC OPERATION AT FULL VOLTAGE
1. Connect external LEDs across the 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 1A
5. The switching waveform on the SW pin can be acquired using the test point J1
Warning: Do not stare at the LEDs directly.
Issue 5 August 2009
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AP8802EV2
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 0.2ms/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 (if it has been added by the user)
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 / ILED where ILED = 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 200kHz and efficiency levels
>90% are achievable.
Visit our website www.diodes.com to find useful tools for circuit design and simulation.
REFERENCE
[1] AP8802 Datasheet – www.diodes.com
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AP8802EV2
INTENTIONALLY BLANK
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AP8802EV2
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