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AP8800EV1

P1-P3P4-P6P7-P8
AP8800EV1
Issue 3 – August 2009 www.diodes.com
© Diodes Incorporated 2009
4
AP8800 Operation
In normal operation, when voltage is applied at V
IN
, 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 V
IN
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.
AP8800EV1 Evaluation Board - BOM
Ref Value Package Part Number Manufacturer Notes
U1 AP8800 SOP-8L AP8800SG DIODES inc DC-DC
converter
D1 40V, 1A SMA B140 DIODES inc Schottky
diode
R1 0R33 0805 NCST10FR330FTRF Generic NIC 1%
R2 0R0 0805 Generic - -
C1,C4
1uF 50V 1206 NMC1206X7R105K50F NIC 10%
C2 - - Not Fitted - -
C3 100nF,
100V
0805 C0805C104K1RAC
NMC0805X7R104K100
Kemet
NIC
10%
L1 100uH - MSS6132 100uH
NPIS53D101MTRF
Coilcraft
NIC
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.
AP8800EV1
Issue 3 – August 2009 www.diodes.com
© Diodes Incorporated 2009
5
Figure 3: Component layout and circuit board view
AP8800EV1 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
AP8800EV1 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 waveforms on the SW pin can be acquired using the test point J1
Warning: Do not stare at the LEDs directly.
AP8800EV1
Issue 3 – August 2009 www.diodes.com
© Diodes Incorporated 2009
6
Circuit features
N.B. Remove power whilst changing components!
Soft-start
1.
The AP8800 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 / I
LED
where I
LED
= 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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