LD5805GS datasheet(14/18 Pages) LEADTREND

14 / 18 page

LD5805

06/15/2015

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD5805-DS-01 June 2015

capacitor determine the loop stability. The inductor and

output capacitor are chosen based on the performance,

size and cost. The compensation resistor and capacitor

at COMP are selected to optimize control loop stability.

Thermal protection

Thermal protection limits total power dissipation in this

device. When the junction temperature reaches 150

C

approximately, the thermal sensor signals the shutdown

logic turning off this device. The thermal sensor will turn

this device on again after the IC

’s junction temperature

cools by 20

C.

PCB Layout Guideline

It’s recommended to separate the high frequency

switching current from the low-level control signals in

layout. The high switching current (MOSFET, inductor,

LDR driver and LED return ends) may disturb the other

low-level signals in the feedback loop and protection

circuitry. As a result, it may cause the control function to

behave abnormally. To avoid these issues, a few

guidelines are recommended for the PCB layout, as

below.

1. VCC bypass capacitor connected with signal ground

must be placed as close as possible to the IC. The

traces between capacitor and VCC pin should be

kept as short as possible to avoid noise interference.

2. VIN bypass capacitor connected with signal ground

must be placed as close as possible to the IC. The

traces between capacitor and VIN pin should be kept

as short as possible to avoid noise interference.

3. Use broader traces for VIN, VOUT and power

ground. Components such as the power MOSFET

and decoupling capacitors connected to VIN, VOUT

and power ground, have high input/output current.

To minimize power loss in these traces, the

resistance of traces should be kept as low as

possible.

4. Use broader traces between power MOSFET drain,

inductor and diode. There is high current in these

traces. To minimize power loss in these traces, the

resistance of traces should be kept as low as

possible.

5. Keep the LDR drive traces short and board between

the IC driver output, LDR pin, and the power

MOSFET.

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