AP8800EV2

Issue 3 – August 2009

www.diodes.com

© Diodes Incorporated 2009

4

AP8800 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 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.

AP8800EV2 Evaluation Board - BOM

Ref

Value

Package

Part Number

Manufacturer

Notes

U1

AP8800

MSOP-8L

AP8800M8G

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.