WS9841 datasheet(6/8 Pages) ETC2

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WS9841 Product Description

WINSEMI MICROELECTRONICS

Over Voltage Protection

The FB pin is used to detection the output voltage for

over voltage protection (OVP). The FB pin threshold is 1V.

The resistor divider of the FB pin can be set as:

OVP

FBH

FBL





Where,

RFBL is the lower resistor of the resistor divider

RFBH is the higher resistor of the resistor divider

VOVP is the over voltage protection set value.

For maximizing the system efficiency, the lower resistor

is recommended to be around 5KΩ.

Protection Function

The WS9841 offers rich protection functions to improve

the system reliability, including LED open/short protection,

CS resistor short protection, VCC under voltage protection,

thermal regulation.

When the LED is open circuit, the system will trigger the

over voltage protection and stop switching.

When the LED short circuit is detected, the system

works at low frequency (4kHz), and the CS pin turn off

threshold is reduced to 200mV. So the system power

consumption is very low. At some catastrophic fault condition,

such as CS resistor shorted or inductor saturated, the

internal fast fault detection circuit will be triggered, the

system stops switching immediately.

After the system enters into fault condition, the VCC

voltage will decrease until it reaches the UVLO threshold,

then the system will re-start again. If the fault condition is

removed, the system will recover to normal operation.

Thermal Regulation

The WS9841 integrates thermal regulation function.

When the system is over temperature, the output current is

gradually reduced; the output power and thermal dissipation

are also reduced. The system temperature is regulated and

the system reliability is improved. The thermal regulation

temperature is set to 150℃ internally.

PCB Layout

The following rules should be followed in WS9841 PCB

layout:

Bypass Capacitor

The bypass capacitor on VCC pin should be as close as

possible to the VCC Pin and GND pin.

FB Pin

The feedback resistor divider should be as close as

possible to the FB pin, and the trace must keeps away from

dynamic node of the inductor and Drain pin trace, otherwise

the FB pin OVP function might have risk to be mis-triggered by

the system noise.

VLN Pin

The VLN pin resistor divider and capacitor should be as

close as possible to the VLN pin, and the VLN node should be

away from the high voltage switching nodes.

Ground Path

The power ground path for current sense resistor should

be short and wide, and it should be as close as possible to the

IC GND pin. The IC signal ground for FB resistors should be

connected to the IC GND pin with short traces and should be

away from the power ground path.

The Area of Power Loop

The area of main current loop should be as small as

possible to reduce EMI radiation, such as the power MOSFET,

the output diode and the bus capacitor loop.

DRAIN Pin

To increase the copper area of DRAIN pin for better

thermal dissipation. However, too large copper area may

compromise the EMI performance.

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