AL1791/1792/1793/1794

Document number:37957 Rev. 3 - 2

5 of 18

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September 2015

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AL1791/1792/1793/1794

Analog Dimming Control

With the same PWMx pins, analog dimming can be facilitated to control the brightness of each LED channel. When no PWM signal with frequency

ranging from 500Hz and above during startup is present at PWMx pins, the device is determined in analog dimming mode. In this mode, the

signals at PWMx pins are interpreted as analog dimming signals. The meaningful dimming voltage should range from 0.4V to 1.5V. When the

voltage is 0.4V or below, the current-sink turns off the LED string. When the voltage is 1.5V or above, it turns the current level on to 100%. For any

voltage between 0.4V to 1.5V, the current level is linearly prorated to its full scale.

Undervoltage Lockout

The Undervoltage lockout function (UVLO) guarantees that the device is initially off during start-up. The UVLO functions only when the device is

enabled. Even if the AL179x is enabled, the device is not turned ON until the power supply has reached 6.3V. Whenever the input voltage falls

below approximately 6V, the device is turned off. The UVLO circuit has a hysteresis of 300mV. During UVLO, a FAULTB status will be reported.

Over-Temperature Protection

Thermal protection prevents the IC from damage when the die temperature exceeds safe margins. The AL1791/1792/1793/1794 features Over-

Temperature Protection (OTP). When the junction temperature reaches +160°C, the device will enter thermal shutdown mode in which all current

sinks are shut down and no current flows to the LED strings. The normal current level can be automatically restored to its set current from OTP by

triggering a Soft Start when the temperature lowers down below +100°C. Thermal Shut Mode will also be reported as a FAULTB.

LED Open and Short Protection

AL179x detects the LED status by continuously monitoring the current and voltage status of the LED strings. When an abnormal condition occurs

(LED Open or LED Short), FAULTB is asserted low. In the case of LED Short, all output channels will be turned off. In the case of LED Open, all

non-affected channels remain in the normal operating condition.

Fault Reporting

Fault reporting serves as a good communication channel between LED drivers and intelligent MCU unit. AL179x detects and reports the FAULTB

status upon the occurrence of Thermal Shutdown in OTP, LED Short, LED Open, and UVLO. FAULTB pin is an open-drain output design. Multiple

devices can share a single FAULTB signal by connecting all FAULTB pins together with an external pull-up resistor. Any FAULTB assertion from

any device pulls the signal down and informs MCU of the faulty condition.

LED Power Good Reporting

voltage headroom (VLED_REG). On the other hand, excessive voltage headroom provided needs to be absorbed by AL179x, which increases

heat dissipation to complicate thermal management. Power Good reporting serves as a mechanism to detect the LED power supply condition and

adjusts the supply voltage to LEDs through MCU in order to minimize unnecessary overdrive voltage to the regulator and optimize the efficiency.

any channel is lower than or close to the dropout voltage required by the regulator (VLED_REG) while the channel is enabled with PWMx and EN,

LEDPG pin will be pulled down to ground. During PWMx low period, internal LEDPG will not report insufficient headroom for that channel. Internal

LED Power Good logics are ANDed together before brought out to the pin. Any channel’s insufficient headroom condition will trigger a logic Low at

LEDPG pin. This signal can then be used by MCU to wisely adjust the LED power and thus the voltages to the LEDx pins to minimize power

consumption on AL179x. LEDPG needs to be connected with an external pull-up resistor. Multiple AL179x devices can also share a single pull-up

resistor to create a unified LEDPG signal.