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APW7071CI-TRG

Step-Up Converter for 4 Series White LEDs Driver

Anpec Electronics 

APW7071

Function Description

Operation

The APW7071 operates in a pulse frequency modulation

(PFM) scheme with constant peak current control. The

operation can be understood by referring to the Block

Diagram. The converter keeps monitoring the output volt-

age through the resistor-divider connected with FB, GND,

and VOUT. When the feedback voltage on FB falls below

the reference voltage (typical 0.25V), the internal switch

turns on and the inductor current ramps up. The switch

turns off if the inductor current reaches the internal peak

current limit (400mA typical). The second criterion that

turns off the switch is the maximum on-time control. As

the switch is off, the external Schottky diode forwards bias,

so that the current is delivered to the output. The switch

remains off for a minimum of 400ns (typical), and it

wouldn’t be turned on again until the feedback voltage

drops below the reference voltage. This regulation

scheme allows a wider selection range for the inductor

and output capacitor.

Over Voltage Protection

In driving LED applications, the feedback voltage on FB

pin falls down if one of the LEDs, in series, is failed.

Meanwhile, the converter unceasingly boosts the output

voltage like a open-loop operation. Therefore, an over-

voltage protection (OVP), monitoring the output voltage

via OVP pin, is integrated into the chip to prevent the SW

and the output voltages from exceeding their maximum

voltage ratings. When the voltage on the OVP pin rises

above the OVP threshold (17V typical), the converter stops

switching and prevents the output voltage from rising.

The converter can work again when the OVP voltage falls

below the OVP voltage threshold.

Shutdown

Driving SHDN to ground places the APW7071 in

shutdown. When in shutdown, the internal power

MOSFET turns off, all internal circuitry shuts down and

the quiescent supply current of VIN reduces to <0.1µA

(typical).

Soft-Start

The APW7071 limits this inrush current by increasing the

current limit at start-up.

Under Voltage Lockout

Transients cause system damage or failure when pow-

ering on or undergoing instantaneous glitches in the sup-

ply voltage. Then, the undervoltage lockout circuit turns

the main switch off to prevent malfunction at low input

voltage.

Copyright © ANPEC Electronics Corp.

8

Rev. A.3 - Jul., 2008

www.anpec.com.tw

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