ACE711E

2A, Step-Up DC/DC Regulator 20V

VER 1.2

6

Output Open Circuit

Protection

Output open circuit protection circuitry prevents IC damage as the result of output open circuit (e.g. LED

string absence). The ACE711E monitors the voltage at the SW pin and FB pin during each switching cycle.

The circuitry turns off the switch FET and shuts down the IC when both of the following conditions persist

for 8 switching clock cycles: (1) the SW voltage exceeds the VOVP threshold, which is 25V and (2) the FB

voltage is less than half of regulation voltage. As a result, the output voltage falls to the level of the input

supply. The device remains in shutdown mode until it is enabled by toggling the EN pin logic. The output

voltage plus the 200mV reference voltage cannot exceed the minimum OVP threshold or OUTPUT

VOLTAGE + 200 mV ≤ VOVP(MIN).

Shutdown

The ACE711E enters shutdown mode when the EN voltage is logic low for more than 2.5ms. During

shutdown, the input supply current for the device is less than 1μA (max). Although the internal FET does

not switch in shutdown, there is still a DC current path between the input and the output through the

inductor and Schottky diode.

For LED driving application, the minimum forward voltage of the LED array must exceed the maximum

input voltage to ensure that the LEDs remain off in shutdown. However, in the typical application with two

or more

LEDs, the forward voltage is large enough to reverse bias the Schottky and keep leakage

current low.

Output Voltage Setting

The FB voltage is regulated by a low 0.2V reference voltage. The output voltage is set externally by

using a resistor divider. The value of the R1/R2 divider is calculated using Equation 1.1:

VOUT =VFB/R2*(R1+R2)

(1.1)

Where

VOUT = output voltage, R2 = resistance between FB and GND, R1 = resistance between OUT and FB.

The output current tolerance depends on the FB accuracy and the current sensor resistor accuracy.

LED Driving Current Setting

The FB voltage is regulated by a low 0.2V reference

voltage. The LED current is set externally using a

current-sense resistor in series with the LED string. The value of the RSET is calculated using Equation

1.2:

ILED =VFB/RSET

(1.2)

Where

ILED = output current of LEDs, RSET = current sense resistor

The output current tolerance depends on the FB accuracy and the current sensor resistor accuracy.