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AP8802HEV2

P1-P3P4-P6P7-P8
AP8802HEV2
Issue 1 – August 2010 www.diodes.com
© Diodes Incorporated, 2010
4
R
1
AP8802H
1
L
1
D
1
V
IN
SET
SW
GND
8V ~ 60V DC
Figure 2: Schematic of the Evaluation Board
R
2
C
1
C
3
CTRL
V
IN
GND
C
2
C
4
AP8802H OPERATION
In normal operation, when voltage is applied at V
IN
, the AP8802Hs 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 V
IN
and inductor L1. This rising current
produces a voltage ramp across R1. The internal circuit of the AP8802H 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 AP8802H. 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, AP8802H internal circuits, electrical
characteristics and parameters.
AP8802HEV2 EVALUATION BOARD - BILL OF MATERIALS
Ref Value Package Part Number Manufacturer Notes
U1 AP8802H SOP8LEP AP8802HSPG DIODES inc DC-DC converter
D1 100V, 2A SMB B2100 DIODES inc Schottky diode
R1 0R2 1210 Generic +/-1%, +/-100ppm
R2 0R0 0805 Generic -
C1 10uF 100V generic
NACE100M100V6.3x8
NIC
Components
6.3mm x 8mm SMD
electrolytic 85C
C2 - - Not Fitted
-
-
C3 100nF,100V 0805 Generic
NMC0805X7R104K100
NIC
Components
X7R +/-20%
C4 100nF 100V 1206 Generic
NMC1206X7R104K100
NIC
Components
X7R +/-20%
L1 68uH - MSS1038-683ML
NPIS104F680MTRF
NPIS24H680MTRF
Coilcraft
NIC
Comps.
NIC Comps.
68uH, ~0.2R, ~1.5A
Note: if not available, components shown above may be changed without notice.
The FR4 PCB design, with adequate copper top and bottom and plated through vias for thermal
coupling, guarantees a good thermal dissipation for the AP8802H device. Other sources of heat are
the Schottky diode, the inductor and the sense resistor, therefore care must be taken in their
placement.
AP8802HEV2
Issue 1 – August 2010 www.diodes.com
© Diodes Incorporated, 2010
5
Warning: At 60V operation with 1A output, the board will become hot !
Figure 3: Component layout and circuit board view
AP8802HEV2 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
AP8802HEV2 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 16 x 3.2V LEDs can be connected using a 60V 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 (60V max.)
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.
C4
AP8802HEV2
Issue 1 – August 2010 www.diodes.com
© Diodes Incorporated, 2010
6
CIRCUIT FEATURES
N.B. Remove power whilst changing components!
Soft-start
1.
The AP8802H 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 / 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 60V supply and 16 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] AP8802H Datasheet – www.diodes.com
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