APW7136ACITRG datasheet(9/15 Pages) ANPEC

9 / 15 page

Rev. A.2 - Jan., 2008

APW7136A/B/C

www.anpec.com.tw

Function Descriptions

Main Control Loop

The APW 7136 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 0.25V 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 (2.2V, 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, startup begins. When

the input voltage falls below the UVLO falling threshold,

the controller turns off the converter.

Soft-Start

The APW7136 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 7136 monitors the inductor current, flowing

through the N-channel MOSFET, and limits the current

peak at current-limit level to prevent loads and the

APW7136 from damages during overload conditions.

Over-Temperature Protection (OTP)

The over-temperature circuit limits the junction tempera-

ture of the APW7136. 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

regulate the output voltage again after the junction tem-

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

hysteresis lowers the average Junction Temperature (T

during continuous thermal overload conditions, increas-

ing the lifetime of the device.

Enable/Shutdown

Driving EN to ground places the APW7136 in shutdown

mode. When 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 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, the converter stops switching

and prevents the output voltage from rising. The converter

can work again when the OVP voltage falls below the

falling of 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.

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