EM8800AG datasheet(7/8 Pages) EXCELLIANCE

7 / 8 page

2011/09/06

Rev.A.3

EM8800A

Functional Description

EM8800A is a constant off time current mode

controller. The PWMD pin enables the user to use

a PWM signal to modulate the switching pattern

and adjust the average luminosity. The LD/SS pin

enables the user to set peak current by adjust the

voltage of this pin. A capacitor connected to the

LD/SS pin sets the soft start period.

Supply Voltage

The VCC pin provides the bias supply of EM8800A

control circuit, as well as MOSFET’s gate. A

minimum 0.1uF ceramic capacitor is recommended

to bypass the supply voltage.

Power ON Reset

To let EM8800A start to operation, VCC voltage

must be higher than its POR voltage. Typical POR

voltage is 9.75V.

Start Up Delay

When VCC reaches POR threshold, start-up-delay

timer begins to count. When start-up-delay is

finished, the current source of LD/SS pin is enabled

and the soft-start sequence begins.

Soft Start

A capacitor connected to the LD/SS pin sets the

soft start period.

Dimming

The device is able to perform both linear and PWM

dimming. Two separate inputs are provided (LD/SS

pin for linear dimming, PWMD pin for PWM

dimming).

The

two

functions

can

used

simultaneously and independently to one another.

Linear dimming can be performed by applying a dc

voltage between 0.3V and 3V. This voltage is

internally processed, generating the reference level

VCS_LD. The relationship between VLD and VCS_LD

is:

VCS_LD=VLD/3, when VLD>0.3V

VCS_LD=0.1V, when VLD<=0.3V

PWM dimming can be achieved by driving the

PWMD pin with a low frequency square wave

signal. When the PWMD signal is lower than 0.8V,

switching is inhibited. When the PWMD signal is

higher than 2V, the switching is enabled.

Constant Off-Time Generator

The off-time generator begins at the end of each

on time. Once the off time generator has elapsed,

it begins the next GATE pulse. The off-time is

programmed by connecting a resistor from the

TOFF pin to ground.

Protections and Fault Flag

In case the LED string fails short, the inductor

current will rise cycle by cycle without control. To

prevent the current from exceeding levels that

might lead to catastrophic failure of the power

MOSFET, EM8800A detects if VCS exceeds 1.5V. If

this occurs for eight consecutive switching cycles, a

latched shutdown function is tripped: the device is

stopped in an idle state and it is necessary to

recycle its supply voltage to make it restart.

In case the LED string fails open, the current will

drop to zero and the inductor current will never

reach the programmed level. To detect this

situation, an internal timer checks the ON time of

the power MOSFET and, if it exceeds 30us for eight

consecutive switching cycles, a latched shutdown

function is tripped: the device is stopped in an idle

state and it is necessary to recycle its supply

voltage to make it restart.

In either case, the activation of the protection

functions is signaled by FAULT pin asserted high,

becoming a 5V voltage source able to provide up to

5mA source current.

EM8800AG データシート(PDF) 7 Page - Excelliance MOS Corp.

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