AP8802EV2

Issue 5 August 2009

www.diodes.com

© Diodes Incorporated 2009

4

AP8802

SET

SW

GND

8V ~ 48V DC

Figure 2: Schematic of the Evaluation Board

CTRL

GND

AP8802 OPERATION

on. Current starts to flow through sense resistor R1, inductor L1, and the LED(s). The current ramps

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

Value

Package

Part Number

Manufacturer

Notes

U1

AP8802

SOP8LEP

AP8802SPG

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

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.