AP8802EV3 Datasheet AP8802EV3 | P4-P6

AP8802EV3

Issue 6 – May 2010 www.diodes.com

AP8802 OPERATION

In normal operation, when voltage is applied at V

, 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 ramp rate is determined by the input voltage V

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, 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.

AP8802EV3 EVALUATION BOARD - BILL OF MATERIALS

Ref Value Package Part Number Manufacturer Notes

U1 AP8802 DFN3030 AP8802FNG DIODES inc DC-DC

converter

D1 100V, 2A SMB B2100 DIODES inc Schottky

diode

R1 0R2 1206 SR732ETTDR200F KOA +/-

1%,

+/-100ppm

R2 0R0 0805 Generic - +/-1%

C1 4.7uF 50V 1210 C1210X475K5RAC

GRM32ER71H475KA88L

KEMET Murata X7R +/-10%

C2 - - Not Fitted - -

C3 100nF,100V

0805 Generic

X7R +/-20%

C4 100nF 100V

1206 Generic - X7R +/-20%

L1 68uH - MSS1038-683ML

NPIS104F680MTRF

NPIS24H680MTRF

Coilcraft

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 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.

AP8802EV3

Issue 6 – May 2010 www.diodes.com

AP8802EV3 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

AP8802EV3 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

5. The switching waveform on the SW pin can be acquired using the test point J1

Warning: Do not stare at the LEDs directly.

AP8802EV3

Issue 6 – May 2010 www.diodes.com

CIRCUIT FEATURES

N.B. Remove power whilst changing components!

Soft-start

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

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 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

P1-P3 P4-P6 P7-P8

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