Copyright ANPEC Electronics Corp.
Rev. A.6 - Aug., 2011
APW7208
www.anpec.com.tw8
Function Description
Main Control Loop
The APW7208 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 VOUT and ground al-
lows the EAMP to receive an output feedback voltage VFB
at FB pin. When the load current increases, it causes a
slightly decrease in VFB 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 VCOMP.
Current-Limit Protection
The APW7208 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
(TJ) 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. When in shutdown, the internal power
MOSFET turns off, all internal circuitry shuts down and
the quiescnet supply current reduces to 1µA 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%.