APW7208 datasheet(8/16 Pages) ANPEC

8 / 16 page

Rev. A.6 - Aug., 2011

APW7208

www.anpec.com.tw

Function Description

Main Control Loop

The APW 7208 is a constant frequency current-mode

switching regulator. During normal operation, the inter-

nal N-channel power MOSFET is turned on each cycle

when the oscillator sets an internal RS latch and turned

off when an internal comparator (ICMP) resets the latch.

The peak inductor current at which ICMP resets the RS

latch is controlled by the voltage on the COMP node, which

is the output of the error amplifier (EAMP). An external

resistive divider connected between V

OUT and ground al-

lows the EAMP to receive an output feedback voltage V

at FB pin. When the load current increases, it causes a

slightly decrease in V

FB relative to the 104mV reference,

which in turn causes the COMP voltage to increase until

the average inductor current matches the new load

current.

VIN Under-Voltage Lockout (UVLO)

The Under-Voltage Lockout (UVLO) circuit compares the

input voltage at VIN with the UVLO threshold (2.3V rising,

typical) to ensure the input voltage is high enough for

reliable operation. The 100mV (typ) hysteresis prevents

supply transients from causing a restart. Once the input

voltage exceeds the UVLO rising threshold, start-up

begins. When the input voltage falls below the UVLO fall-

ing threshold, the controller turns off the converter.

Soft-Start

The APW7208 has a built-in soft-start to control the N-

channel MOSFET current rise during start-up. During soft-

start, an internal ramp, connected to one of the inverting

inputs, raise up to replace the output voltage of error am-

plifier until the ramp voltage reaches the V

COMP.

Current-Limit Protection

The APW 7208 monitors the inductor current, flowing

through the N-channel MOSFET, and limits the current

peak at current-limit level to prevent loads and the

APW7208 from damages during overload or short-circuit

conditions.

Over-Temperature Protection (OTP)

The over-temperature circuit limits the junction tempera-

ture of the APW7208. When the junction temperature ex-

ceeds 150οC, a thermal sensor turns off the power

MOSFET, allowing the devices to cool. The thermal sen-

sor allows the converters to start a soft-start process and

to regulate the output voltage again after the junction tem-

perature cools by 40οC. The OTP is designed with a 40οC

hysteresis to lower the average Junction Temperature

J) during continuous thermal overload conditions, in-

creasing the lifetime of the device.

Enable/Shutdown

Driving EN to the ground places the APW7208 in shut-

down mode. W hen in shutdown, the internal power

MOSFET turns off, all internal circuitry shuts down and

the quiescnet supply current reduces to 1

µA maximum.

Open-LED Protection

In driving LED applications, the feedback voltage on the

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 LX

and the output voltages from exceeding their maximum

voltage ratings. When the voltage on the OVP pin rises

above the OVP threshold (28V typical), the converter stops

switching and prevents the output voltage from rising.

The converter can work again when the falling OVP volt-

age falls below the OVP voltage threshold.

This pin also could be used as a digital input allowing

brightness control using a PWM signal from 100Hz to

100kHz. The 0% duty cycle of PWM signal corresponds to

zero LEDs current and 100% corresponds to full one.

Seggestion dimming duty range is from 7% to 100%.

APW7208 Datasheet(PDF) 8 Page - Anpec Electronics Coropration

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